Sample records for remagnetized volcanic rocks

  1. Archaeological calibration of remagnetized volcanic rocks from pottery firing kilns in Cuentepec, Morelos, Mexico

    L. M. Alva-Valdivia; J. A. González-Rangel; A. M. Soler-Arechalde; S. L. López-Varela; H. López-Loera


    Ethnoarchaeological research at the site of Cuentepec, Mexico includes experimental pottery dating in which social knowledge is obtained from archaeometric techniques. At Cuentepec, open kilns are used for firing pottery. Samples from volcanic rocks in the kilns were taken to verify the reliability of the magnetic direction in these rocks as compared with data from Teoloyucan Geomagnetic Observatory, near Mexico City. In the laboratory, forty-seven cores from eight hand oriented rock samples ...

  2. Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India-Asia collision?

    Huang, Wentao; Dupont-Nivet, Guillaume|info:eu-repo/dai/nl/313092559; Lippert, Peter C.; van Hinsbergen, Douwe J J; Dekkers, Mark J.|info:eu-repo/dai/nl/073463744; Guo, Zhaojie; Waldrip, Ross; Li, Xiaochun; Zhang, Xiaoran; Liu, Dongdong; Kapp, Paul


    Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic

  3. Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India-Asia collision?

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; van Hinsbergen, Douwe J J; Dekkers, Mark J.; Guo, Zhaojie; Waldrip, Ross; Li, Xiaochun; Zhang, Xiaoran; Liu, Dongdong; Kapp, Paul


    Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic

  4. Uranium series, volcanic rocks

    Vazquez, Jorge A.


    Application of U-series dating to volcanic rocks provides unique and valuable information about the absolute timing of crystallization and differentiation of magmas prior to eruption. The 238U–230Th and 230Th-226Ra methods are the most commonly employed for dating the crystallization of mafic to silicic magmas that erupt at volcanoes. Dates derived from the U–Th and Ra–Th methods reflect crystallization because diffusion of these elements at magmatic temperatures is sluggish (Cherniak 2010) and diffusive re-equilibration is insignificant over the timescales (less than or equal to 10^5 years) typically associated with pre-eruptive storage of nearly all magma compositions (Cooper and Reid 2008). Other dating methods based on elements that diffuse rapidly at magmatic temperatures, such as the 40Ar/39Ar and (U–Th)/He methods, yield dates for the cooling of magma at the time of eruption. Disequilibrium of some short-lived daughters of the uranium series such as 210Po may be fractionated by saturation of a volatile phase and can be employed to date magmatic gas loss that is synchronous with volcanic eruption (e.g., Rubin et al. 1994).

  5. Volcanic Rocks and Features

    National Oceanic and Atmospheric Administration, Department of Commerce — Volcanoes have contributed significantly to the formation of the surface of our planet. Volcanism produced the crust we live on and most of the air we breathe. The...

  6. Geochemical Characteristics and Metallogenesis of Volcanic Rocks as Exemplified by Volcanic Rocks in Ertix,Xinjiang

    刘铁庚; 叶霖


    Volcanic rocks in Ertix,Xinjiang,occurring in the collision zone between the Siberia Plate and the Junggar Plate,are distributed along the Eritix River Valley in northern Xinjiang.The volcanic rocks were dated at Late Paleozoic and can be divided into the spilite-keratophyre series and the basalt-andesite series.The spilite-keratophyre series volcanic rocks occur in the Altay orogenic belt at the southwest margin of the Siberia Plate.In addition to sodic volcanic rocks.There are also associated potassic-sodic volcanic rocks and potassic volcanic rocks.The potassic-sodic volcanic rocks occur at the bottom of the eruption cycle and control the distribution of Pb and Zn deposits.The potassic volcanic rocks occur at the top of the eruption cycle and are associated with Au and Cu mineralizations.The sodic volcanic rocks occur in the middle stage of eruption cycle and control the occurrence of Cu(Zn) deposits.The basalt-andesite series volcanic rocks distributed in the North Junggar orogenic belt at the north margin of the Junggar-Kazakstan Plate belong to the potassic sodic volcain rocks.The volcanic rocks distributed along the Ulungur fault are relatively rich in sodium and poor in potassium and are predominated by Cu mineralization and associated with Au mineralization.Those volcanic rocks distributed along the Ertix fault are relatively rich in K and poor in Na,with Au mineralization being dominant.

  7. Cenozoic volcanic rocks of Saudi Arabia

    Coleman, R.G.; Gregory, R.T.; Brown, G.F.


    The Cenozoic volcanic rocks of Saudi Arabia cover about 90,000 km2, one of the largest areas of alkali olivine basalt in the world. These volcanic rocks are in 13 separate fields near the eastern coast of the Red Sea and in the western Arabian Peninsula highlands from Syria southward to the Yemen Arab Republic.

  8. End-member modelling as an aid to diagnose remagnetization: A brief review

    Dekkers, M.J.


    Remagnetization of a palaeomagnetic signal is difficult to recognize independently of directional information. The situation becomes more complex when remagnetized rocks pass palaeomagnetic field tests, for example when the remagnetization of a rock sequence has occurred before folding. It is eviden

  9. Volcanic rock properties control sector collapse events

    Hughes, Amy; Kendrick, Jackie; Lavallée, Yan; Hornby, Adrian; Di Toro, Giulio


    Volcanoes constructed by superimposed layers of varying volcanic materials are inherently unstable structures. The heterogeneity of weak and strong layers consisting of ash, tephra and lavas, each with varying coherencies, porosities, crystallinities, glass content and ultimately, strength, can promote volcanic flank and sector collapses. These volcanoes often exist in areas with complex regional tectonics adding to instability caused by heterogeneity, flank overburden, magma movement and emplacement in addition to hydrothermal alteration and anomalous geothermal gradients. Recent studies conducted on the faulting properties of volcanic rocks at variable slip rates show the rate-weakening dependence of the friction coefficients (up to 90% reduction)[1], caused by a wide range of factors such as the generation of gouge and frictional melt lubrication [2]. Experimental data from experiments conducted on volcanic products suggests that frictional melt occurs at slip rates similar to those of plug flow in volcanic conduits [1] and the bases of mass material movements such as debris avalanches from volcanic flanks [3]. In volcanic rock, the generation of frictional heat may prompt the remobilisation of interstitial glass below melting temperatures due to passing of the glass transition temperature at ˜650-750 ˚C [4]. In addition, the crushing of pores in high porosity samples can lead to increased comminution and strain localisation along slip surfaces. Here we present the results of friction tests on both high density, glass rich samples from Santaguito (Guatemala) and synthetic glass samples with varying porosities (0-25%) to better understand frictional properties underlying volcanic collapse events. 1. Kendrick, J.E., et al., Extreme frictional processes in the volcanic conduit of Mount St. Helens (USA) during the 2004-2008 eruption. J. Structural Geology, 2012. 2. Di Toro, G., et al., Fault lubrication during earthquakes. Nature, 2011. 471(7339): p. 494-498. 3

  10. Volcanic Rocks As Targets For Astrobiology Missions

    Banerjee, N.


    Almost two decades of study highlight the importance of terrestrial subaqueous volcanic rocks as microbial habitats, particularly in glass produced by the quenching of basaltic lava upon contact with water. On Earth, microbes rapidly begin colonizing glassy surfaces along fractures and cracks exposed to water. Microbial colonization of basaltic glass leads to enhanced alteration through production of characteristic granular and/or tubular bioalteration textures. Infilling of formerly hollow alteration textures by minerals enable their preservation through geologic time. Basaltic rocks are a major component of the Martian crust and are widespread on other solar system bodies. A variety of lines of evidence strongly suggest the long-term existence of abundant liquid water on ancient Mars. Recent orbiter, lander and rover missions have found evidence for the presence of transient liquid water on Mars, perhaps persisting to the present day. Many other solar system bodies, notably Europa, Enceladus and other icy satellites, may contain (or have once hosted) subaqueous basaltic glasses. The record of terrestrial glass bioalteration has been interpreted to extend back ~3.5 billion years and is widespread in modern oceanic crust and its ancient metamorphic equivalents. The terrestrial record of glass bioalteration strongly suggests that glassy or formerly glassy basaltic rocks on extraterrestrial bodies that have interacted with liquid water are high-value targets for astrobiological exploration.

  11. Basement faults and volcanic rock distributions in the Ordos Basin


    Volcanic rocks in the Ordos Basin are of mainly two types: one in the basin and the other along the margin of the basin. Besides those along the margin, the marginal volcanic rocks also include the volcanic rocks in the Yinshanian orogenic belt north of the basin. Based on the latest collection of gravitational and aeromagnetic data, here we interpret basement faults in the Ordos Basin and its peripheral region, compare the faults derived from aeromagnetic data with those from seismic data, and identify the geological ages of the fault development. Two aeromagnetic anomaly zones exist in the NE-trending faults of the southern basin, and they are in the volcanic basement formed in pre-Paleozoic. These NE-trending faults are the channel of volcanic material upwelling in the early age (Archean-Neoproterozoic), where igneous rocks and sedimentary rocks stack successively on both sides of the continental nucleus. In the Cambrian, the basin interior is relatively stable, but in the Late Paleozoic and Mesozoic, the basin margin underwent a number of volcanic activities, accompanied by the formation of nearly north-south and east-west basement faults in the basin periphery and resulting in accumulation of great amount of volcanic materials. Volcanic tuff from the basin periphery is discovered in the central basin and volcanic materials are exposed in the margins of the basin. According to the source-reservoir-cap rock configuration, the basin peripheral igneous traps formed in the Indosinian-Early Yanshanian and Late Hercynian are favorable exploration objectives, and the volcanic rocks in the central basin are the future target of exploration.

  12. Strength and deformation properties of volcanic rocks in Iceland

    Foged, Niels Nielsen; Andreassen, Katrine Alling


    rock from Iceland has been the topic for rock mechanical studies carried out by Ice-landic guest students at the Department of Civil Engineering at the Technical University of Den-mark over a number of years in cooperation with University of Iceland, Vegagerðin (The Icelandic Road Directorate......) and Landsvirkjun (The National Power Company of Iceland). These projects involve engineering geological properties of volcanic rock in Iceland, rock mechanical testing and parameter evaluation. Upscaling to rock mass properties and modelling using Q- or GSI-methods have been studied by the students......Tunnelling work and preinvestigations for road traces require knowledge of the strength and de-formation properties of the rock material involved. This paper presents results related to tunnel-ling for Icelandic water power plants and road tunnels from a number of regions in Iceland. The volcanic...

  13. Integrated Geophysical Techniques for Exploring Deep Volcanic Rock Reservoir

    LiuXuejun; UDechun; ZhangChangjiang; RanXuefeng


    The Carboniferous and Pre-Carboniferous formations in Ludong, Zhungar basin, contain favorable oil/gas reservoirs. The Carboniferous formations, however, are complex in structure and exhibit lateral variations in lithology. Seismic reflections from Pre-Triassic formations are poor and the volcanic reservoirs are very difficult to identify. The analysis of physical properties concluded that the major targets in this region, i.e., the top of the Jurassic and Carboniferous formations, provide distinct density interfaces. The basic, intermediate and acid volcanic rocks were also different in density,resulting in distinguishable gravity anomalies. The differences in magnetism in this region existed not only between the volcanic rocks and clastic sedimentary rocks but also among volcanic rocks with different compositions. All formations and volcanic rocks of different lithologies presented high and low resistance interbeds, which are characterized by regional trends.The modeling study demonstrated that non-seismic integrated geophysical techniques should be feasible in this region, especiaUy the high-precision gravity/magnetic methods combined with long offset transient electromagnetic sounding.

  14. Fluorine geochemistry in volcanic rock series

    Stecher, Ole


    A new analytical procedure has been established in order to determine low fluorine concentrations (30–100 ppm F) in igneous rocks, and the method has also proven successful for higher concentrations (100–4000 ppm F). Fluorine has been measured in a series of olivine tholeiites from the Reykjanes...... Peninsula, a tholeiite to rhyolitic rock series from Kerlingarfjöll, central Iceland, and an alkaline rock series from Jan Mayen that ranges from ankaramites to trachytes. Fluorine is not appreciably degassed during extrusion and appears to be insensitive to slight weathering. The olivine tholeiites from...... the Reykjanes Peninsula have F contents of 30–300 ppm and exhibit linear increases proportional to the incompatible elements K, P, and Ti. Such incompatible behaviour for F has been confirmed for the less evolved rocks of the other series. The tholeiites from Kerlingarfjöll (100–2000 ppm F) show a linear...

  15. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; van Hinsbergen, Douwe J. J.; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-Pb zircon dates of 52 Ma for the upper Linzizong, and 40Ar/39Ar dates ( 51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of 5°N, compared to the upper part ( 20°N) and to underlying Cretaceous strata ( 20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by 52 Ma at 20°N.

  16. Applications of Terrestrial Remote Sensing to Volcanic Rock Masses

    Dewit, M.; Williams-Jones, G.; Stead, D.; Kremsater, R.; So, M.; Francioni, M.


    Remote sensing methods are widely used in geological applications today. The physical properties of rock such as composition, texture and structure have previously been difficult to accurately quantify through remote sensing, however, new research in the fields of terrestrial LiDAR and infrared thermography has proven useful in the differentiation of lithology in sedimentary outcrops. This study focuses on the application of these methods, in conjunction with digital photogrammetry, to a number of volcanic rock masses in the Garibaldi Volcanic Belt (GVB) and Chilcotin Group (CG) of British Columbia. The GVB is a chain of volcanoes and related features extending through southwestern British Columbia and is the northern extension of the Cascade Volcanic Arc. The CG is an assemblage of Neogene-aged lavas covering nearly 36,500 km2 in central British Columbia. We integrate infrared chronothermography, which enables the characterization of temporal change in the thermal signature, laser waveform attributes such as amplitude and intensity, and digital photogrammetry, in order to distinguish between a range of rock types, lithologies and structures. This data is compared to laboratory experiments on field samples and ground-truth information collected by classical geological and geotechnical methods. Our research clearly shows that it is possible to remotely map, in 3D, otherwise inaccessible volcanic rock masses.

  17. Pore Structure of Cement Pastes Blended with Volcanic Rock

    YU Lehua; ZHOU Shuangxi; LI Liling


    The pore parameters of cement pastes blended with volcanic rock at the curing age of 1, 28 and 90 d were de-termined by a mercury intrusion porosimetry. The pore structure of the pastes was characterized through the analysis of porosity, average pore diameter, the most probable pore aperture, pore size distribution, as well as total pore volume. For the improvement of mechanical property and durability of cement-based material, the correlation of the formed pore structure with hydration time and replacement level of volcanic rock for cement was revealed. The results indicate that volcanic rock can diminish porosity and reduce pore size in cement paste when curing time prolongs, which is particu-larly prominent with replacement level of less than 20% in late period. The more harmful pores (i.e., capillary pore) are gradually transformed into harmless pore (i.e., gel pores or micropore), even fully filled and disappeared when hydration products increase. The pore structure of the cement paste is thus refined. The beneficial effect of volcanic rock on the pore structure of cement paste could enhance the mechanical property and durability of cement-based material.

  18. The evolution of pore connectivity in volcanic rocks

    Colombier, Mathieu; Wadsworth, Fabian B.; Gurioli, Lucia; Scheu, Bettina; Kueppers, Ulrich; Di Muro, Andrea; Dingwell, Donald B.


    Pore connectivity is a measure of the fraction of pore space (vesicles, voids or cracks) in a material that is interconnected on the system length scale. Pore connectivity is fundamentally related to permeability, which has been shown to control magma outgassing and the explosive potential of magma during ascent in the shallowest part of the crust. Here, we compile a database of connectivity and porosity from published sources and supplement this with additional measurements, using natural volcanic rocks produced in a broad range of eruptive styles and with a range of bulk composition. The database comprises 2715 pairs of connectivity C and porosity ϕ values for rocks from 35 volcanoes as well as 116 products of experimental work. For 535 volcanic rock samples, the permeability k was also measured. Data from experimental studies constrain the general features of the relationship between C and ϕ associated with both vesiculation and densification processes, which can then be used to interpret natural data. To a first order, we show that a suite of rocks originating from effusive eruptive behaviour can be distinguished from rocks originating from explosive eruptive behaviour using C and ϕ. We observe that on this basis, a particularly clear distinction can be made between scoria formed in fire-fountains and that formed in Strombolian activity. With increasing ϕ, the onset of connectivity occurs at the percolation threshold ϕc which in turn can be hugely variable. We demonstrate that C is an excellent metric for constraining ϕc in suites of porous rocks formed in a common process and discuss the range of ϕc values recorded in volcanic rocks. The percolation threshold is key to understanding the onset of permeability, outgassing and compaction in shallow magmas. We show that this threshold is dramatically different in rocks formed during densification processes than in rocks formed in vesiculating processes and propose that this value is the biggest factor in

  19. Western Alborz Volcanic Rocks, a new Geochemical Viewpoint

    Ghorbani, M.


    Volcanic and pyroclastic rocks of Eocene age comprise vast outcrops of Alborz Mountain Range, a fold-thrusted structural unit extending across northern Iran for 2000 km in a curvilinear pattern. In his account of structural evolution of Iranian plateau, Berberian (1983; p. 55) ascribed these rocks to a subduction-type magmatism. Based on a tectonostratigraphic study, these rocks are attributed to an arc-type magmatism (Alavi; 1996, p. 29). Recently a new data set of major and trace element (including REE) analyses of volcanic rocks from western Alborz, some 50 km west of city of Qazvin, has been made available (Asiabanha, 2001). Careful examination of the data (i.e., those of basic-intermediate rocks) in present study revealed, for the first time, some geochemical characteristics which have important implications on the geodynamic synthesis of this structural unit. The rocks contain 50-60 wt% SiO2. They lie in the midalkaline-to-subalkaline domain of TAS diagram (Middlemost, 1997; p.216) and fall in the calcalkaline field of AFM diagram. The volcanic rocks display two distinct chondrite-normalized REE patterns, one is MREE-depleted while the other is a rather smooth uniform M-HREE pattern. These are called MREE-depleted and smooth M-HREE series. Basic rocks from the latter contain higher silica than the former (>53 vs. >50 wt%), yet they show lower incompatible elements (e.g., K and Rb) and HFSE contents. These features can not be explained by differentiation and might be interpreted as implying the involvement of two source regions. Chondrite-normalized trace element patterns of the MREE-depleted series is more akin to the island arc calcalkaline (IACA) basic rocks than the basic rocks from any other tectonic settings. However, island arc products, known for being depleted in HFSE relative to other incompatible elements, differ from the MREE-depleted series which is rich in both HFSE and incompatible elements. One may advocate the role of OIB-type mantle

  20. Hydrocarbon- Generating Model of the Area Covered With Volcanic Rock

    Guo Zhanqian; Zhang Yuwei


    The distribution of Oil & gas fields shows their close relationship with the most active tectonic regions. This is not a coincidence but having a scientific reasons. The crustal active regions, refer to the places where the active natural earthquake, volcanic activities, underground water happened, and the areas of the leaking off of natural gas to the surface of the crust. The magma of volcanic activities brings the organic "kitchen range body" hydrocarbon- generating model and inorganic genetic hydrocarbon to the regions covered by volcanic rock. Underground water brings a catalytic hydrocarbongenerating model for organic matter, and the leaking- off of H2 and CO2 contributes a synthetic hydrocarbon - generating model. Volcanic activities bring the assemblage of Source, Reservoir and Seal formed by the sediments and magma the sedimentary basins, and the hydrocarbon - generating system with a "water - volcano" binary structure is formed. All these conditions are favorable and excellent for the formation of oil & gas fields. The distribution of American oil & gas fields have very close relationship with the mines of Fe, Mn, Ct, Mo, W and V, deposits of Zn, Cu, V, Pb, Al and Hg, and the deposits of fluorite, sulfur, potassium salt, phosphate and halite, and the distribution of sulfate- chloride of river water. The reason why few oil & gas fields discovered in the regions covered by volcanic rock in western America maybe because of the view of "inconsistency between petroleum and volcano". Further more, It's very difficult to carry out a geophysical exploration in such kinds of regions.This paper examined a few hydrocarbon-generating models (systems) mentioned above and came up with some fresh ideas on the exploration in the areas covered with volcanic rocks.

  1. The strength of heterogeneous volcanic rocks: A 2D approximation

    Heap, Michael J.; Wadsworth, Fabian B.; Xu, Tao; Chen, Chong-feng; Tang, Chun'an


    Volcanic rocks typically contain heterogeneities in the form of crystals and pores. We investigate here the influence of such heterogeneity on the strength of volcanic rocks using an elastic damage mechanics model in which we numerically deform two-dimensional samples comprising low-strength elements representing crystals and zero-strength elements representing pores. These circular elements are stochastically generated so that there is no overlap in a medium representing the groundmass. Our modelling indicates that increasing the fraction of pores and/or crystals reduces the strength of volcanic rocks, and that increasing the pore fraction results in larger strength reductions than increasing the crystal fraction. The model also highlights an important weakening role for pore diameter, but finds that crystal diameter has a less significant influence for strength. To account for heterogeneity (pores and crystals), we propose an effective medium approach where we define an effective pore fraction ϕp‧ = Vp/(Vp + Vg) where Vp and Vg are the pore and groundmass fractions, respectively. Highly heterogeneous samples (containing high pore and/or crystal fractions) will therefore have high values of ϕp‧, and vice-versa. When we express our numerical samples (more than 200 simulations spanning a wide range of crystal and pore fractions) in terms of ϕp‧, we find that their strengths can be described by a single curve for a given pore diameter. To provide a predictive tool for the strength of heterogeneous volcanic rocks, we propose a modified version of 2D solution for the Sammis and Ashby (1986) pore-emanating crack model, a micromechanical model designed to estimate strength using microstructural attributes such as porosity, pore radius, and fracture toughness. The model, reformulated to include ϕp‧ (and therefore crystal fraction), captures the strength curves for our numerical simulations over a sample heterogeneity range relevant to volcanic systems. We find

  2. Remagnetization of lava flows spanning the last geomagnetic reversal

    Vella, Jérôme; Carlut, Julie; Valet, Jean-Pierre; Goff, Maxime Le; Soler, Vicente; Lopes, Fernando


    Large directional changes of remanent magnetization within lava flows that cooled during geomagnetic reversals have been reported in several studies. A geomagnetic scenario implies extremely rapid geomagnetic changes of several degrees per day, thus difficult to reconcile with the rate of the earth's core liquid motions. So far, no complete rock magnetic model provides a clear explanation. We revisited lava flows sandwiched between an underlying reverse and an overlying normal polarity flow marking the last reversal in three distinct volcanic sequences of the La Palma Island (Canary archipelago, Spain) that are characterized by a gradual evolution of the direction of their remanent magnetization from bottom to top. Cleaning efficiency of thermal demagnetization was not improved by very rapid heating and cooling rates as well as by continuous demagnetization using a Triaxe magnetometer. We did not observe partial self-reversals and minor changes in magnetic grain sizes are not related to the within-flow directional changes. Microscopic observations indicate poor exsolution, which suggests post-cooling thermochemical remagnetization processes. This scenario is strongly reinforced by laboratory experiments that show large resistance to thermal demagnetization when thermoremanence was acquired over a long time period. We speculate that in the present situation exsolution was reactivated during in field reheating and yielded formation of new magnetite, yet magnetic domain state rearrangements could also play a role. Initial reheating when the overlying flow took place, albeit moderate (less than 200-300 °C), was enough to produce overlying components with significantly higher unblocking temperatures.

  3. Trace Element Geochemistry of Cenozoic Volcanic Rocks in Shandong Province

    陈道公; 李彬贤; 等


    The Cenozoic volcanic rock of Shandong Province are mainly alkalic and strongly alkalic basaltic rocks.The Contents of major and trace elements including transitional,incompatible and rare-earth elements were determined.The chemical characterisitics of major and trace elements indicate that these basaltic rocks were derived from a mantle source and probably represent a primary magma,I,e.,unmodifiecd partical melts of mantle peridotite in terms of Mg values,correlatione between P2O5 and Ce,Sr,Ni and Rb concentrations,mantle xenoliths,etc.The abundances of trace elements vary systematically from west to east.The compatible transition elements such as Co,Ni,and Cr show a remarkable depletion,whereas the incompatible and rare-earth elements are abundant as viewed from the chondrite-nor-malized patterns.The chemical composition and correlation are consistent with the tectonic setting.According to the batch and fractional partial melting theory,the trace element contents of Shandong volcanic rocks can be calculated from the two-component mixing model.

  4. Petrography, Geochemistry and Petrogenesis of Volcanic Rocks, NW Ghonabad, Iran

    Sedigheh Zirjanizadeh


    Full Text Available Introduction The study area is located in NW Gonabad, Razavi Khorasan Province, northern Lut block and eastern Iran north of the Lut Block. Magmatism in NW Gonabad produced plutonic and volcanic rock associations with varying geochemical compositions. These rocks are related to the Cenozoic magmatic rocks in Iran and belong to the Lut Block volcanic–plutonic belt. In this study, petrogenesis of volcanic units in northwest Gonabad was investigated. The volcanic rocks are andesites/trachyandesites, rhyolites, dacites/ rhyodacites and pyroclastics.These rocks show porphyritic, trachytic and embayed textures in phenocrysts with plagioclase, sanidine and quartz (most notably in dacite and rhyolite, hornblende and rare biotite. The most important alteration zones are propylitic, silicification and argillic.Four kaolinite- bearing clay deposits have been located in areas affectedby hydrothermal alteration of Eocene rhyolite, dacite and rhyodacite. Analytical techniques Five samples were analyzed for major elements by wavelength dispersive X-ray fluorescence (XRF and six samples were analyzed for trace elements using inductively coupled plasma-mass spectrometry (ICP-MS in the Acme Laboratories, Vancouver (Canada.Sr and Nd isotopic compositions were determined for four whole-rock samples at the Laboratório de GeologiaIsotópica da Universidade de Aveiro, Portugal. Results Petrography. The rocks in this area are consist of trachyte, andesite/ trachyandesite, dacite/ rhyodacite, principally as ignimbrites and soft tuff. The textures of phenocrysts are mainly porphyritic, glomerophyric, trachytic and embayed textures in plagioclase, hornblende and biotite. The groundmasses consist of plagioclase and fine-grainedcrystals of hornblende. Plagioclase phenocrysts and microlitesare by far the most abundant textures in andesite - trachyandesites (>25% and in size from 0.01 to 0.1mm. Euhedral to subhedral hornblende phenocrysts areabundant (3-5%and 0.1 to 0

  5. Study on Mechanism of Formation of Volcanic Rock in North Altay by Using Rare Earths

    袁峰; 周涛发; 岳书仓


    The characteristics of rare earth elements in the Devonian and Carboniferous volcanic rocks were studied in the north Altay. And the mechanism of formation of volcanic rocks were discussed by using the rare earth elements. The correlativity of rare earth elements and major elements shows that the fractional crystallization is undistinguishable during the formation of Devonian and Carboniferous volcanic rocks, and the mechanism of formation of volcanic rocks may be the partial melting. The further study of the relationship of manifold rare earth elements shows that the mechanism of formation of Devonian and Carboniferous volcanic rocks in the north Altay is the partial melting. And the result also shows that the rare earth elements in the Devonian and Carboniferous volcanic rocks inherited the characteristics of those in its source materiels.

  6. Case Study of Volcanic Rock Reservoir--Beibao Area of Nanbao Sag

    Guo Qijun; Wan Zhimin; Jiao Shouquan; Liu Laixiang


    @@ Preface Crude has been produced from volcanic rock reservoirs inAmerica, Libya, Indonesia and Japan, according to the available data. Oil and gas Production from volcanic rock reservoirs in Japan accounts for 30% of the total, with the recoverable reserves estimated at 48% of the total. Oil and gas traps of volcanic rock reservoir with better reservoir properties and flow potential have been discovered in Liaohe, Erlian, Jizhong, Huanghua, Jiyang, Linqing,Jiangsu and Xinjiang of China since the 1970's. However,the systematic study of volcanic rock reservoir is just in the beginning.

  7. Fragmentation Speed and Permeability of hot Volcanic Rocks

    Scheu, B.; Mueller, S.; Spieler, O.; Dingwell, D. B.


    The speed of fragmentation may control the explosive behaviour of silicic volcanoes. It is directly affected by the gas pressure within the volcano, which else influences the eruptive behaviour of the volcano. We used two techniques to analyse the speed of fragmentation. At both the fragmentation is triggered by the rapid decompression inside a high-pressure autoclave. Firstly at room temperature a set of two pressure transducers record the differential pressure loss above and below the cylindrical sample (d = 25mm, l = 60 mm). Secondly at temperatures of 850°C two platinum wires are inserted at known distance into the sample cylinder and used as electrical conductors to record the rupture time. The recorded time difference and the distance between the conductors are used to recalculate the speed of the fragmentation wave. This speed depends on porosity, texture and initial pressure difference. Further, the results show a decrease of fragmentation speed while propagating through the rock sample. We propose this effect to be linked to the density changes of the gas and therefore the reducing flow rates through the rock sample (gas permeability). Up to today permeability measurements have only been performed on cold porous rocks (e.g. Eichelberger et al. 1986, Klug & Cashman 1996), because measurements with higher temperatures are not possible with common gas permeameters. Investigating the permeability of volcanic rocks in a hot state (up to 850°C) provides a better insight into the degassing processes under natural conditions. Therefore, any new experimental set-up is expected to yield information about the temperature dependency of permeability in volcanic rocks. The present experiments have been performed on samples with a wide range of porosities, collected from block-and-ash flows on Merapi (Indonesia), Unzen (Japan) and from pumices on Lipari Island (Italy). Permeability was measured using a modified set-up of the fragmentation apparatus. Below the sample a

  8. Reorganization Of Magnetic Carriers Due To Pressure-Solution, A New Remagnetization Mechanism From The Southwestern Pyrenees

    Oliva-Urcia, B.; Pueyo, E. L.; Larrasoa¤a, J. C.


    The ultimate cause of widespread remagnetizations remains elusive. Mechanisms such as orogenic fluids, burial diagenesis dolomitization, dedolomitization, maturation of organic matter, migration of hydrocarbons and thermal metamorphism can also account for widespread remagnetizations, but only under specific geochemical scenarios that do not apply to many widespread remagnetizations. Paleomagnetic analyses in the South Pyrenean Zone in Spain, show the presence of a widespread remagnetization in the Internal Sierras, the southern border of the Axial Zone and the northern part of the Jaca-Pamplona basin. This remagnetization component has always a reverse polarity, was acquired after folding and affects Triassic red beds, Upper Cretaceous carbonatic rocks and Tertiary turbidites. The areal distribution of the remagnetization coincides with a domain of pressure-solution cleavage, outside of the cleavage domain there is not such a secondary component. This circumstance together with the apparent correlation between the intensity of the remagnetization and the density of cleavage planes lead us to propose a new, and alternative remagnetization mechanism that can explain widespread remagnetizations in orogens. The mechanism that produces the new orientation of magnetic minerals is related to the evolution of the orogenic wedge when the highest tectonic load produces an extensive cleavage domain in the core of the Western Pyrenees. Formation of pressure-solution cleavage planes drives the partial dissolution of the matrix allowing the ferromagnetic minerals (less soluble and heavier) to reorient freely in the presence of the ambient magnetic field. We are aware of other studies reporting remagnetizations in relation of cleavage formation. The appeal of our new mechanism is that it does not necessarily involve the dissolution or neoformation of magnetic minerals, as typically assumed regardless of clear evidence, but rather reorganization of preexisting magnetic grains

  9. Late Cretaceous intraplate silicic volcanic rocks from the Lake Chad region: An extension of the Cameroon volcanic line?

    Shellnutt, J. G.; Lee, T.-Y.; Torng, P.-K.; Yang, C.-C.; Lee, Y.-H.


    Silicic volcanic rocks at Hadjer el Khamis, near Lake Chad, are considered to be an extension of the Cameroon volcanic line (CVL) but their petrogenetic association is uncertain. The silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within-plate granitoids. In situ U/Pb zircon dating yielded a mean 206Pb/238U age of 74.4 ± 1.3 Ma indicating the magmas erupted ˜10 million years before the next oldest CVL rocks (i.e., ˜66 Ma). The Sr isotopes (i.e., ISr = 0.7021-0.7037) show a relatively wide range but the Nd isotopes (i.e., 143Nd/144Ndi = 0.51268-0.51271) are uniform and indicate that the rocks were derived from a moderately depleted mantle source. Thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes. The silicic rocks are more isotopically similar to Late Cretaceous basalts identified within the Late Cretaceous basins (i.e., 143Nd/144Ndi = 0.51245-0.51285) of Chad than the uncontaminated CVL rocks (i.e., 143Nd/144Ndi = 0.51270-0.51300). The age and isotopic compositions suggest the silicic volcanic rocks of the Lake Chad region are related to Late Cretaceous extensional volcanism in the Termit basin. It is unlikely that the silicic volcanic rocks are petrogenetically related to the CVL but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the Central Atlantic Ocean and/or the Pan-African Orogeny.

  10. Cooling-dominated cracking in thermally stressed volcanic rocks

    Browning, John; Meredith, Philip; Gudmundsson, Agust


    Most studies of thermally induced cracking in rocks have focused on the generation of cracks formed during heating and thermal expansion. Both the nature and the mechanism of crack formation during cooling are hypothesized to be different from those formed during heating. We present in situ acoustic emission data recorded as a proxy for crack damage evolution in a series of heating and cooling experiments on samples of basalt and dacite. Results show that both the rate and the energy of acoustic emission are consistently much higher during cooling than during heating. Seismic velocity comparisons and crack morphology analysis of our heated and cooled samples support the contemporaneous acoustic emission data and also indicate that thermal cracking is largely isotropic. These new data are important for assessing the contribution of cooling-induced damage within volcanic structures and layers such as dikes, sills, and lava flows.

  11. Paleointensity determinations from Middle Miocene volcanic rocks, Far East, Russia

    Shcherbakov, Valeriy; Shcherbakova, Valentina; Zhidkov, Grigoriy; Bretstein, Yury


    Paleomagnetic and paleointensity studies of a representative collection of Middle Miocene volcanic rocks of (12.4-10.0) Ma age from Far East, Russia were carried out. A comprehensive rock magnetic investigation has been performed in order to determine the mineralogy of magnetic fraction and to identify the origin of NRM. Successful Thellier palaeointensity determinations with pTRM checks were obtained for 4 sites (33 samples). The Thellier experiments were complimented by the Wilson's express method. Values of the VDMs range from 3.1 to 7.5 (10^22 Am^2) with the mean VDM = 5.5×10^22 Am^2. A total 88 Thellier-type palaeointensity determinations were found in the World Paleointensity database for the 5-23 Ma period with the overall mean VDM = 5×10^22 Am^2. The VDM distribution is characterized by high variance from 1.5 to 12 (10^22 Am^2). No significant difference between the VDM distributions for the normal and reverse polarity intervals was revealed.

  12. Magnetic reorientation induced by pressure solution: A potential mechanism for orogenic-scale remagnetizations

    Oliva-Urcia, Belén; Pueyo, Emilio L.; Larrasoaña, Juan C.


    New paleomagnetic analyses of Triassic, Cretaceous and Eocene strata in the south-central Pyrenees show evidence for a widespread remagnetization, located along the southern border of the Axial Zone, the Internal Sierras, and the northern part of the Jaca-Pamplona basin. This remagnetization, always reversed in polarity, was acquired after an extensive period of Late Eocene-Early Oligocene folding and tilting in the area, and affects limestones, sandstones, marls and red beds. In addition, a characteristic prefolding component was identified in 30% of Upper Cretaceous and Triassic red beds. These results, together with a revaluation of previously published paleomagnetic data from the central Pyrenees, indicate that the spatial distribution of the postfolding remagnetization coincides with that of a domain of pressure solution cleavage. A relationship between the intensity of the remagnetization and the characteristic (prefolding component) with respect to the density of cleavage surfaces, leads us to propose a mechanism for the remagnetization related to the development of pressure solution cleavage that is framed within the tectonic evolution of the central Pyrenees. Partial dissolution of rock under tectonic compression leads to the liberation and subsequent accumulation of insoluble minerals in cleavage planes. Magnetic grains are part of the relatively insoluble residue, and they reorient in the presence of the ambient field after they are freed during dissolution of the rock matrix. Chemical reequilibrium (dissolution and/or neoformation of magnetic carriers) during this process cannot be excluded. The remagnetization mechanism we propose can help to explain widespread remagnetizations in low to moderately deformed rocks without the need of large-scale migration of orogenic fluids.

  13. Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado

    Wobus, Reinhard A.; Mochel, David W.; Mertzman, Stanley A.; Eide, Elizabeth A.; Rothwarf, Miriam T.; Loeffler, Bruce M.; Johnson, David A.; Keating, Gordon N.; Sultze, Kimberly; Benjamin, Anne E.; Venzke, Edward A.; Filson, Tammy


    The Guffey volcanic center is the largest within the 2000 km2 mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which represent the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the Guffey center was followed by extrusion of a thick series of basalt, trachybasalt, and shoshonite flows and lahars. Plugs, dikes, and vents ranging from basalt to rhyolite cut the thick mafic deposits, and felsic tuffs and tuff breccias chemically identical to the small rhyolitic plutons are locally preserved. Whole-rock major and trace element analyses of 80 samples, ranging almost continuously from 47% to 78%SiO2, indicate that the rocks of the Guffey center are among the most highly enriched in K2O (up to 6%) and rare earth elements (typically 200-300 ppm) of any volcanic rocks in Colorado. These observations, along with the relatively high concentrations of Ba and Rb and the depletion of Cr and Ni, suggest an appreciable contribution of lower crustal material to the magmas that produced the Thirtynine Mile volcanic rocks.

  14. Magmatic Source Composition and Magmatism of the Volcanic Rocks in the Area of Kuruktag, Southern Xinjiang

    JIANG Changyi; BAI Kaiyin; HI Aizhi; ZHAO Xiaoning; ZHANG Hongbo


    In the Sinian-Cambrian strata in the area of Kuruktag, southern Xingjiang, four layers of volcanic rocks occurred in the Early Sinian Beiyixi Formation, Late Sinian Zhamoketi Formation and Shuiquan Formation, and Early Cambrian Xishanbulake Formation, respectively. Volcanics of the Shuiquan Formation and Xishanbulake Formation are of alkali basalt series, those of the Zhamoketi Formation are of alkali basalt series and tholeiite series, and those of the Beiyixi Formation are obviously characterized by bimodal assemblage and mostly belong to alkali volcanics. Multi-element distribution patterns of the rocks show continental tumescence characters of interplate basalt.Fractional crystallization of plagioclase led to negative Eu-anomalies of some volcanics and the cumulation of olivine resulted in high MgO and low SiO2 content of some volcanic rocks. The SiO2 saturability of volcanic rocks of the Xishanbulake Formation and Shuiquan Formation is lower than that of tholeiite of the Zhamoketi Formation. Correspondingly, the abundance of incompatible elements in the first two formations is higher than those in the last formation, and the differences can be attributed to the different degrees of partial melting. The intense fractionation of REE and the obvious depletion of HREE suggest that these volcanic rocks were derived from garnet Iherzolite of the mantle in the continental lithosphere. The Ba/Nb, La/Nb, Ba/La, Ba/Th and Rb/Nb rations demonstrate that these volcanic rocks were exclusively derived from the enriched mantle, mainly the EMI type mantle.

  15. A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

    徐达伟; 萧炎宏


    The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosedand contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz,and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assem-blages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. Thisindicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result ofCaledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks shouldbe re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age:prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpellyite to lowergreenschist facies for the lower part of the Middle Ordovician volcanic rocks, and lower greenschist facies for the LowerOrdovician volcanic rocks. The P-T conditions are estimated as T = 240 - 290C and P = 1.5 - 4.5 kbar for the lower partof the Middle Ordovician rocks, and T = ~ 300~C for the Lower Ordovician rocks. The variations of mineral assemblagesoccurring at different domains of the volcanic rocks were controlled by the variations of the effective bulk composition inthose domains during metamorphism. The geochemical characteristics of Mg-Al chromite in the Mayaxueshan volcanicrocks are consistent with an origin of island arc environment.

  16. Evolution and genesis of volcanic rocks from Mutnovsky Volcano, Kamchatka

    Simon, A.; Yogodzinski, G. M.; Robertson, K.; Smith, E.; Selyangin, O.; Kiryukhin, A.; Mulcahy, S. R.; Walker, J. D.


    This study presents new geochemical data for Mutnovsky Volcano, located on the volcanic front of the southern portion of the Kamchatka arc. Field relationships show that Mutnovsky Volcano is comprised of four distinct stratocones, which have grown over that past 80 ka. The youngest center, Mutnovsky IV, has produced basalts and basaltic andesites only. The three older centers (Mutnovsky I, II, III) are dominated by basalt and basaltic andesite (60-80% by volume), but each has also produced small volumes of andesite and dacite. Across centers of all ages, Mutnovsky lavas define a tholeiitic igneous series, from 48-70% SiO2. Basalts and basaltic andesites have relatively low K2O and Na2O, and high FeO* and Al2O3 compared to volcanic rocks throughout Kamchatka. The mafic lavas are also depleted in the light rare earth elements (REEs), with chondrite-normalized La/Sm arc volcanic rocks worldwide. Radiogenic isotope ratios (Sr, Nd, Pb, Hf) are similar for samples from all four eruptive centers, and indicate that all samples were produced by melting of a similar source mixture. No clear age-progressive changes are evident in the compositions of Mutnovsky lavas. Mass balance and assimilation-fractional crystallization (AFC) modeling of major and rare earth elements (REEs) indicate that basaltic andesites were produced by FC of plagioclase, clinopyroxene and olivine from a parental basalt, combined with assimilation of a melt composition similar to dacite lavas present at Mutnovsky. This modeling also indicates that andesites were produced by FC of plagioclase from basaltic andesite, combined with assimilation of dacite. Dacites erupted from Mutnovsky I and II have low abundances of REEs, and do not appear to be related to mafic magmas by FC or AFC processes. These dacites are modeled as the products of dehydration partial melting at mid-crustal levels of a garnet-free, amphibole-bearing basaltic rock, which itself formed in the mid-crust by emplacement of magma that

  17. Fracture Detection in Geothermal Wells Drilled in Volcanic Rocks

    Gonfalini, Mauro; Chelini, Walter; Cheruvier, Etienne; Suau, Jean; Klopf, Werner


    The Phlegrean Fields, close to Naples, are the site of important geothermal activity. The formations are volcanic and mostly tuffites. They are originally very tight but the geothermal alteration locally produces fractures with large increase in permeability. The lack of geological markers makes well-to-well correlation quite difficult. Thus the local detection of fractured zones in each well is very important for the evaluation of its potential. The Mofete 8 D well is a typical example. A rather complete logging program was run for fracture detection. Standard methods turned out to be disappointing. However several non-standard detectors were found to be very consistent and, later on, in excellent agreement with the analysis of cuttings. They are derived from the Dual Laterolog, the SP, the Temperature log and, most particularly, the Acoustic Waveforms from the Long Spacing Sonic. The Dual Laterolog and the Temperature Log indicate invasion by fresh and cold mud filtrate; the SP behaves as in a typical Sand-Shale sequence. Sonic Waveforms were first analyzed by a purely empirical method derived from consistent log patterns. A practical algorithm compares the total energy measured in each of the two fixed time windows located the one before, the other after the fluid arrivals. The altered zones (i.e. fractured and permeable) are clearly shown by a complete reversal of the relative energy of these two windows. A more scientific method was then applied to the Waveforms; it is based on both logging experiments and physical considerations. The energy carried by the tube wave is separated by a frequency discrimination: it correlates very well with formation alteration, thus also with the other indicators including the empirical Waveform method. It should have two advantages: – It should permit at least a semi quantitative permeability evaluation – It seems to be promising in other formations: non-volcanic geothermal wells and even hydrocarbon-bearing rocks. 10 refs

  18. Discovery of Enclaves from Cenozoic Pulu Volcanic Rocks in West Kunlun Mountains and Its Geological Implications


    In this paper, we present the occurrence and mineral components of the enclaves firstly discovered in the Cenozoic Pulu volcanic rocks in west Kunlun Mountains, and propose that the enclave is accumulated by fractional crystallization within high-level magma chamber. In addition, the chemical compositions of its primary magma are calculated. The calculated compositions are similar to those of the Kangxiwa volcanic rocks that belong to the same volcanic belt in the Pulu volcanic region, suggesting their origin from the same source region. However, the temperatures and oxygen fugacity of magmas at high-level magma chamber decreased along with fractional crystallization.

  19. Characteristics and geological significance of olivine xenocrysts in Cenozoic volcanic rocks from western Qinling

    SU Benxun; ZHANG Hongfu; XIAO Yan; ZHAO Xinmiao


    Cenozoic volcanic rocks from the Haoti, Dangchang County of the western Qinling Mountains, contain a few clearlyzoned olivines. These olivines are relatively big in grain sizes and usually have cracks or broken features. Their cores have similar compositions (Mg# = 90.4- 91.0) to those for the peridotitic xenoliths entrained in host volcanic rocks and their rims are close to the compositions of olivine phenocrysts (Mg# = 85.5 81.9). The CaO contents in these zoned olivines are lower than 0.1%. These features demonstrate that the clearly zoned olivines are xenocrysts and disaggregated from mantle peridotites. The zoned texture was the result of the interaction between the olivine and host magma. Available data show that the volcanic rocks would have been derived from the mantle source metasomatized by subducted hydrathermally-altered oceanic crust. The formation of these Cenozoic volcanic rocks was perhaps related to the rapid uplift of the Tibetan Plateau.

  20. Influence of mesostasis in volcanic rocks on the alkali-aggregate reaction

    Tiecher, Francieli


    Mesostasis material present in the interstices of volcanic rocks is the main cause of the alkali-aggregate reaction (AAR) in concretes made with these rock aggregates. Mesostasis often is referred to as volcanic glass, because it has amorphous features when analyzed by optical microscopy. However, this study demonstrates that mesostasis in the interstitials of volcanic rocks most often consists of micro to cryptocrystalline mineral phases of quartz, feldspars, and clays. Mesostasis has been identified as having different characteristics, and, thus, this new characterization calls for a re-evaluation of their influence on the reactivity of the volcanic rocks. The main purpose of this study is to correlate the characteristics of mesostasis with the AAR in mortar bars containing basalts and rhyolites. © 2012 Elsevier Ltd. All rights reserved.

  1. Basic and ultrabasic volcanic rocks from the Argyll Group (Dalradian) of NE Scotland


    The Dalradian Supergroup of the Scottish Highlands is a largely metasedimentary succession of Neoproterozoic to Early Cambrian age, metamorphosed during the Caledonian Orogeny. The rocks were deposited on the Laurentian margin during and following the break-up of Rodinia. This rift setting is evidenced, in the upper half of the succession, by the presence of several volcanic sequences. A significant development of these volcanic rocks occurs in the NE Grampian Highlands. There, the Blackwater...

  2. K—Ar Geochronology and Evolution of Cenozoic Volcanic Rocks in Eastrn China

    王慧芬; 杨学昌; 等


    Cenozoic volcanic rocks widespread in eastern China constitute an important part of the circum-Pacific volcanic belt.This paper presents more than 150K-Ar dates and a great deal of petrochemical analysis data from the Cenozoic volcanic rocks distributed in Tengchong,China's southeast coast,Shandong,Hebei,Nei Monggol and Northeast China.An integrated study shows that ubiquitous but uneven volcanic activities prevailed from the Eogene to the Holocene,characterized as being multi-eqisodic and multicycled.For example,in the Paleocene(67-58Ma),Eocene(57-37.5Ma),Miocene(22-18,16-19Ma),Pliocene(8-3Ma),and Early Pleistocene-Middle Pleistocene(1.2-0.5Ma) there were upsurges of volcanism,while in the Oligocene there was a repose period.In space,the older Eogene volcanic rocks are distributed within the region or in the central part of the NE-NNE-striking fault depression,while the younger Neogene and Quaternary volcanic rocks are distributed in the eastern and western parts.Petrologically,they belong essentially to tholeiite-series and alkali-series basalts,with alkalinity in the rocks increasing from old to youg.The above regularities are controlled by both global plate movement and regional inherent tectonic pattern.

  3. Sr, Nd, Pb Isotope geochemistry and magma evolution of the potassic volcanic rocks, Wudalianchi, Northeast China

    Junwen, W.; Guanghong, X.; Tatsumoto, M.; Basu, A.R.


    Wudalianchi volcanic rocks are the most typical Cenozoic potassic volcanic rocks in eastern China. Compositional comparisons between whole rocks and glasses of various occurrences indicate that the magma tends to become rich in silica and alkalis as a result of crystal differentiation in the course of evolution. They are unique in isotopic composition with more radiogenic Sr but less radiogenic Pb.87Sr /86 Sr is higher and143Nd/144Nd is lower than the undifferentiated global values. In comparison to continental potash volcanic rocks, Pb isotopes are apparently lower. These various threads of evidence indicate that the rocks were derived from a primary enriched mantle which had not been subjected to reworking and shows no sign of incorporation of crustal material. The correlation between Pb and Sr suggests the regional heterogeneity in the upper mantle in terms of chemical composition. ?? 1989 Institute of Geochemistry, Chinese Academy of Sciences.

  4. Uranium mineralization in fluorine-enriched volcanic rocks

    Burt, D.M.; Sheridan, M.F.; Bikun, J.; Christiansen, E.; Correa, B.; Murphy, B.; Self, S.


    Several uranium and other lithophile element deposits are located within or adjacent to small middle to late Cenozoic, fluorine-rich rhyolitic dome complexes. Examples studied include Spor Mountain, Utah (Be-U-F), the Honeycomb Hills, Utah (Be-U), the Wah Wah Mountains, Utah (U-F), and the Black Range-Sierra Cuchillo, New Mexico (Sn-Be-W-F). The formation of these and similar deposits begins with the emplacement of a rhyolitic magma, enriched in lithophile metals and complexing fluorine, that rises to a shallow crustal level, where its roof zone may become further enriched in volatiles and the ore elements. During initial explosive volcanic activity, aprons of lithicrich tuffs are erupted around the vents. These early pyroclastic deposits commonly host the mineralization, due to their initial enrichment in the lithophile elements, their permeability, and the reactivity of their foreign lithic inclusions (particularly carbonate rocks). The pyroclastics are capped and preserved by thick topaz rhyolite domes and flows that can serve as a source of heat and of additional quantities of ore elements. Devitrification, vapor-phase crystallization, or fumarolic alteration may free the ore elements from the glassy matrix and place them in a form readily leached by percolating meteoric waters. Heat from the rhyolitic sheets drives such waters through the system, generally into and up the vents and out through the early tuffs. Secondary alteration zones (K-feldspar, sericite, silica, clays, fluorite, carbonate, and zeolites) and economic mineral concentrations may form in response to this low temperature (less than 200 C) circulation. After cooling, meteoric water continues to migrate through the system, modifying the distribution and concentration of the ore elements (especially uranium).

  5. Induced polarization of volcanic rocks. 1Surface versus quadrature conductivity

    Revil, A.; Breton, M. Le; Niu, Q.; Wallin, E.; Haskins, E.; Thomas, D. M.


    We performed complex conductivity measurements on 28 core samples from the hole drilled for the Humu´ula Groundwater Research Project (Hawai´i Island, HI, USA). The complex conductivity measurements were performed at 4 different pore water conductivities (0.07, 0.5, 1.0 or 2.0, and 10 S m-1 prepared with NaCl) over the frequency range 1 mHz to 45 kHz at 22 ± 1°C. The in-phase conductivity data are plotted against the pore water conductivity to determine, sample by sample, the intrinsic formation factor and the surface conductivity. The intrinsic formation factor is related to porosity by Archie's law with an average value of the cementation exponent m of 2.45, indicating that only a small fraction of the connected pore space controls the transport properties. Both the surface and quadrature conductivities are found to be linearly related to the cation exchange capacity of the material, which was measured with the cobalt hexamine chloride method. Surface and quadrature conductivities are found to be proportional to each other like for sedimentary siliclastic rocks. A Stern layer polarization model is used to explain these experimental results. Despite the fact that the samples contain some magnetite (up to 5% wt.), we were not able to identify the effect of this mineral on the complex conductivity spectra. These results are very encouraging in showing that galvanometric induced polarization measurements can be used in volcanic areas to separate the bulk from the surface conductivity and therefore to define some alteration attributes. Such a goal cannot be achieved with resistivity alone.

  6. Induced polarization of volcanic rocks - 1. Surface versus quadrature conductivity

    Revil, A.; Le Breton, M.; Niu, Q.; Wallin, E.; Haskins, E.; Thomas, D. M.


    We performed complex conductivity measurements on 28 core samples from the hole drilled for the Humu'ula Groundwater Research Project (Hawai'i Island, HI, USA). The complex conductivity measurements were performed at 4 different pore water conductivities (0.07, 0.5, 1.0 or 2.0, and 10 S m-1 prepared with NaCl) over the frequency range 1 mHz to 45 kHz at 22 ± 1 °C. The in-phase conductivity data are plotted against the pore water conductivity to determine, sample by sample, the intrinsic formation factor and the surface conductivity. The intrinsic formation factor is related to porosity by Archie's law with an average value of the cementation exponent m of 2.45, indicating that only a small fraction of the connected pore space controls the transport properties. Both the surface and quadrature conductivities are found to be linearly related to the cation exchange capacity of the material, which was measured with the cobalt hexamine chloride method. Surface and quadrature conductivities are found to be proportional to each other like for sedimentary siliclastic rocks. A Stern layer polarization model is used to explain these experimental results. Despite the fact that the samples contain some magnetite (up to 5 per cent wt.), we were not able to identify the effect of this mineral on the complex conductivity spectra. These results are very encouraging in showing that galvanometric induced polarization measurements can be used in volcanic areas to separate the bulk from the surface conductivity and therefore to define some alteration attributes. Such a goal cannot be achieved with resistivity alone.

  7. REE Geochemical Characteristics of Volcanic Rocks in Zhejiang and Jiangxi Provinces and Their Gological Significance



    Based on the data of 64 samples ,the REE geochemical characteristics of volcanic rocks in northern Zhejiang and eastern Jiangxi provinces are discussed in this paper.The REE distribution patterns in acid and intermediate-acid volcanic rocks in these areas display some similarities,as indicated by rightward-inclined V-shaped curves with negative Eu anomalies,which are parallel to earch other.In addi-tion,their REE parameters(ΣREE,ΣLREE/ΣHREE,δEu,Ce/Yb,La/Sm,La/Yb,etc)also va-ry over a narrow range with small deviations.HREE are particularly concentrated in the volcanic rocks as-sociated with uranium mineralization.The initial 87Sr/86Sr ratio in the volcanic rocks is about 0.7056-0.7139.All these features in conjunction with strontium isotopic data indicate that the rock-forming materials come from the sialic crust.The REE distribution patterns and REE geochemical parameters of the volcanic rocks ,as well as La/Sm-La and Ce/Yb-Eu/Yb diagrams may be applied to the sources of rock-forming and ore-forming materials.

  8. Petrographic analysis and correlation of volcanic rocks in Bostic 1-A well near Mountain Home, Idaho

    Arney, B.H.; Gardner, J.N.; Belluomini, S.G.


    Detailed examination of volcanic rock cuttings from the Bostic 1-A well near Mountain Home, Idaho, provides data that correlate the stratigraphy of the well with the regional stratigraphy of the western Snake River Plain. The Bostic 1-A well penetrates basalt of the Middle Pleistocene Bruneau Formation and underlying sedimentary rocks of the Upper Pliocene Glenns Ferry Formation. Basalt underlying the Glenns Ferry Formation is most likely Banbury Basalt of Middle Pliocene age or Banbury equivalent. A 350-ft interval of felsic volcanics is then intersected above another 600 ft of basalt. The well bottoms in altered felsic volcanics. The lowest 600 ft of basalt flows has not been correlated with any basalt observed on the surface. From the established stratigraphy of the region, and from petrographic evidence, the silicic volcanic rocks occurring both above and below the lowermost basalts in the well are probably lower Pliocene Idavada Volcanics. North of the well, in the Mt. Bennett Hills, Idavada Volcanics overlie crystalline rocks of the Idaho batholith. No estimate of depth to plutonic bedrock can be made from the well data alone. Stratigraphic comparisons suggest as little as 0.2 to 0.3 km more of Idavada lie beneath the Bostic 1-A well. Results of geophysical studies suggest additional basalt lies beneath the Bostic 1-A rather than granitic rocks of the batholith.

  9. 231Pa systematics in postglacial volcanic rocks from Iceland

    Turner, Simon; Kokfelt, Thomas; Hoernle, Kaj; Lundstrom, Craig; Hauff, Folkmar


    Several recent studies have highlighted the potential of combined 238U-230Th and 235U-231Pa systematics to constrain upwelling rates and the role of recycled mafic lithologies in mantle plume-derived basalts. Accordingly, we present measurements of the 231Pa concentrations from 26 mafic volcanic rocks from Iceland, including off-axis basalts from the Snaefellsnes Peninsula, to complement previously published 238U-230Th-226Ra data. 231Pa concentrations vary from 27 to 624 fg/g and (231Pa/235U) ratios from 1.12 to 2.11 with the exception of one anomalous sample from the Southeast Rift which has a 231Pa deficit with (231Pa/235U) = 0.86. An important new result is that basalts from the Southeast Rift and the Snaefellsnes Peninsula define a trend at relatively low (231Pa/235U) for a given (230Th/238U) ratio. Many of the remaining samples fall in or around the global field for ocean island basalts but those from the Mid-Iceland Belt and the Southwest Rift/Reykjanes Peninsula extend to higher (231Pa/235U) ratios at a given (230Th/238U), similar to mid-ocean ridge basalts. In principle, these lavas could result from melting of peridotite at lower pressures. However, there is no reason to suspect that the Mid-Iceland Belt and the Southwest Rift lavas reflect shallower melting than elsewhere in Iceland. In our preferred model, these lavas reflect melting of garnet peridotite whereas those from the Southeast Rift and the Snaefellsnes Peninsula contain a significant contribution (up to 20%) of melt from garnet pyroxenite. This is consistent with incompatible trace element and radiogenic isotope evidence for recycled oceanic crust in these lavas. There is increasing agreement that the displacement of ocean island basalts to lower (231Pa/235U) ratios at a given (230Th/238U), compared to mid-ocean ridge basalts, reflects the role of recycled mafic lithologies such as garnet pyroxenite as well as higher average pressures of melting. It now seems likely that this interpretation may

  10. Recognition and Significance of Volcanic Rocks of the Anda Depression of the Songliao Basin in Jurassic-Cretaceous

    Fuhong Gao; Dongpo Wang; Xinrong Zhang; Guixia Ji; Jian Zhao


    A series of volcanic rocks were developed in the deep part of Anda faulting depression. The reflection of therocks are mainly stratiform and hummocky. Three kinds of volcanic facies, e.g. explosion facies, effusion facies andsub-volcanic facies,were recognized by different reflecting characteristics in the seismic profile. The volcanic rocks areformed during three episodes of volcanic activities from Shahezi Formation to Yingcheng Formation. The volcanismshave relation to the formation of Songliao Basin and regional tectonics of northeastern China. Some of the volcanic rocksare good oil and gas reservoirs.

  11. Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India-Asia collision?

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C; van Hinsbergen, Douwe J J; Dekkers, Mark J; Guo, Zhaojie; Walip, Ross; Li, Xiaochun; Zhang, Xiaoran; Liu, Dongdong; Kapp, Paul


    ...). New zircon U-Pb dates indicate an age of ~52Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary...

  12. Geochemical characteristics of the oceanic island- type volcanic rocks in the Chiang Mai zone, northern Thailand

    SHEN Shangyue; FENG Qinglai; ZHANG Zhibin; CHONGPAN Chonglakmani


    The oceanic island volcanic rocks in the Chiang Mai zone, northern Thailand, are usually covered by Lower Carboniferous and Upper Permian shallow-water carbonate rocks, with the Hawaii rocks and potash trachybasalt being the main rock types. The alkaline series is dominant with sub-alkaline series occurring in few cases. The geochemical characteristics are described as follows: the major chemical compositions are characterized by high TiO2, high P2O5 and medium K2O; the rare-earth elements are characterized by right-inclined strong LREE-enrichment patterns; the trace element patterns are of the upward-bulging K-Ti enrichment type; multi-component plots falling within the fields of oceanic island basalts and alkali basalts, belonging to the oceanic island-type volcanic rocks, which are similar to the equivalents in Deqin and Gengma (the Changning-Menglian zone) of Yunnan Province, China.

  13. Spatial variation of volcanic rock geochemistry in the Virunga Volcanic Province: Statistical analysis of an integrated database

    Barette, Florian; Poppe, Sam; Smets, Benoît; Benbakkar, Mhammed; Kervyn, Matthieu


    We present an integrated, spatially-explicit database of existing geochemical major-element analyses available from (post-) colonial scientific reports, PhD Theses and international publications for the Virunga Volcanic Province, located in the western branch of the East African Rift System. This volcanic province is characterised by alkaline volcanism, including silica-undersaturated, alkaline and potassic lavas. The database contains a total of 908 geochemical analyses of eruptive rocks for the entire volcanic province with a localisation for most samples. A preliminary analysis of the overall consistency of the database, using statistical techniques on sets of geochemical analyses with contrasted analytical methods or dates, demonstrates that the database is consistent. We applied a principal component analysis and cluster analysis on whole-rock major element compositions included in the database to study the spatial variation of the chemical composition of eruptive products in the Virunga Volcanic Province. These statistical analyses identify spatially distributed clusters of eruptive products. The known geochemical contrasts are highlighted by the spatial analysis, such as the unique geochemical signature of Nyiragongo lavas compared to other Virunga lavas, the geochemical heterogeneity of the Bulengo area, and the trachyte flows of Karisimbi volcano. Most importantly, we identified separate clusters of eruptive products which originate from primitive magmatic sources. These lavas of primitive composition are preferentially located along NE-SW inherited rift structures, often at distance from the central Virunga volcanoes. Our results illustrate the relevance of a spatial analysis on integrated geochemical data for a volcanic province, as a complement to classical petrological investigations. This approach indeed helps to characterise geochemical variations within a complex of magmatic systems and to identify specific petrologic and geochemical investigations

  14. Geochemical characteristics of island-arc volcanic rocks in the Nan-Nam Pat-Phetchabun zone, northern Thailand

    SHEN Shangyue; FENG Qinglai; YANG Wenqiang; ZHANG Zhibin; Chongpom Chonglakmani


    Late Permian-Early Triassic (P2-T1) volcanic rocks distributed on the eastern side of ocean-ridge and oceanic-island basalts in the Nan-Uttaradit zone were analyzed from aspects of petrographic characteristics, rock assemblage, REE, trace elements, geotectonic setting, etc., indicating that those volcanic rocks possess the characteristic features of island-arc volcanic rocks. The volcanic rock assemblage is basalt-basaltic andesite-andesite. The volcanic rocks are sub-alkaline, dominated by calc-alkaline series, with tholeiite series coming next. The chemical composition of the volcanic rocks is characterized by low TiO2 and K2O and high Al2O3 and Na2O. Their REE patterns are of the flat, weak LREE-enrichment right-inclined type. The trace elements are characterized by the enrichment of large cation elements such as K, Rb and Ba, common enrichment of U and Th, and depletion of Nb, Ta, Zr and Hf. The petrochemical plot falls within the field of volcanic rocks, in consistency with the plot of island-arc volcanic rocks in the Jinsha River zone of China. This island-arc volcanic zone, together with the ocean-ridge/oceanic island type volcanic rocks in the Nan-Uttaradit zone, constitutes the ocean-ridge volcanic rock-island-arc magmatic rock zones which are distributed in pairs, indicating that the oceanic crust of the Nan-Uttaradit zone once was of eastward subduction. This work is of great significance in exploring the evolution of paleo-Tethys in the Nan-Uttaradit zone.

  15. Petrogenesis of Cenozoic Potassic Volcanic Rocks in the Nangqên Basin


    The Nangqên basin is one of the Tertiary pull-apart basins situated in the east of the Qiangtang block. Similar to the adjacent Dengqên basin and Baxoi basin, there occurred a series of potassic volcanic and sub-volcanic rocks, ranging from basic, intermediate to intermediate-acid in lithology. Based on the study of petrology, mineralogy and geochemistry, including REEs, trace elements, isotopic elements and chronology, the authors concluded that the Cenozoic potassic volcanic rocks in the Nangqên basin were formed in the post-collisional intraplate tectonic settings. The relations between the basic, intermediate and intermediate-acid rocks are neither differentiation nor evolution, but instead the geochemical variability is mainly attributable to the different partial melting degrees of the mantle sources formed at depths of 50(80 km. The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixing of components mainly derived from the crust. The recycling model can be described as follows: after they had subducted to the mantle wedge, the crust-derived rocks were metasomatized with the mantle materials. In view of the fact that the ratio of crust-derived rocks increases by the age of volcanism, it can be concluded that the sources of the potassic rocks moved upwards progressively with time. The underplating of small scattered magmas upwelling from the asthenosphere may have induced partial melting of the sources of the volcanic rocks in some pull-apart basins in the Hengduanshan area and the intense tectonic movements of large-scale strike-slip belts provided conduits for the ascending melts.

  16. Petrogenesis of Cenozoic Volcanic Rocks in Tengchong Region of Western Yunnan Province,China

    从柏林; 陈秋媛; 张儒瑷; 吴根耀; 徐平


    The Tengchong Cenozoic volcanic rocks belong to the high-K calc-alkaline rock series.They are strongly depleted in high field strength (HFS) elements and enriched in large-ion lithophile(LIL) elements and LREE.The generation of Tengchong volcanic rocks has been considered to be relatedto the evolution of the Neo-Tethys.The Indian Plate was subducted beneath the southeastern Asia conti-nent,which resulted in the formation of Indo-Burman Arc in the Late Cretaeeous-Palaeocene time.Thecollision between the Indian continent and Indo-Burman Arc started in Eocene and lasted to the present.The Andaman Sea and the Inner Burman Tertiary Basin are a back-arc basin that has been extended sincethe Late Miocene.A distinct characteristic of Tengchong volcanics is that they show a chemical affinityrelated to island arc but their generation postdated the subduetion of the ocean plate.

  17. Geologic Features of Wangjiatun Deep Gas Reservoirs of Volcanic Rock in Songliao Basin

    SHAN Xuanlong; CHEN Shumin; WU Dawei; Zang Yudong


    Wangjiatun gas pool is located at the north part of Xujiaweizi in Songliao basin. Commercial gas flow has been found in the intermediate and acid volcanic rock of upper Jurassic - lower Cretaceous, which makes a breakthrough in deep nature gas prospecting in Songliao basin. The deep natural gas entrapment regularity is discussed in the paper by the study of deep strata, structure and reservoir. Andesite, rhyolite and little pyroclastic rock are the main reservoirs. There are two types of volcanic reservoir space assemblage in this area: the pore and fissure and the pure fissure. Changes had taken place for volcanic reservoir space during long geologic time, which was controlled by tectonic movement and geologic environment.The developed degree of reservoir space was controlled by tectonic movement, weathering and filtering, corrosion and Filling. There are three types of source- reservoir-caprock assemblage in this area: lower source- upper reservoir model,upper source - lower reservoir model and lateral change model. Mudstone in Dengluoku formation and the compacted volcanic rock of upper Jurassic - lower Cretaceous are the caprock for deep gas reservoirs. Dark mudstone of deep lacustrine facies in Shahezi formation and lower part of Dengluoku formation are the source rock of deep gas. It can be concluded that deep gas pools are mainly volcanic lithologic reservoirs.

  18. Geochemical Characteristics of Danfeng Meta-Volcanic Rocks in Shangzhou Area,Shaanxi Province


    The Danfeng meta-volcanics in the Shangzhou area, Shaanxi Province are characterized by oceanic island arc volcanic geochemistry. They are a suite of low-K tholeiitic series and calc-alkaline series meta-volcanic rocks derived from different sources respectively.These meta-volcanics have high Th/Ta ratios and low contents of Ni,Ta,Ti,Y and Yb, suggesting that they were influenced by the subduction zone components.Many lines of evidence show that the Danfeng meta-volcanics were produced in an oceanic island are setting of the supra-subduction zone at the southern margin of the North China Block during the Early Paleozoic.

  19. Paleomagnetic Results of the Red Soil-Volcanic Rock Series of Yingfengling Section, Southern Leizhou Peninsula

    王俊达; 梁池生


    The Yingfengling section composed of red soil and volcanic rocks can be distinguished into 8 stratigraphic units and 4 red soil-volcanic rock cycles. 64 paleomagnetic-orientated sam ples were collected from the bottom to the top of the section. Natural remanent magnetization and magnetic susceptibility were firstly measured. All the samples were stepwisely treated with thermal or/and alternating fields. Four clear polarity segments were recorded in the section.Compared with the geo-magnetic polarity scale, the section was formed since the late Olduvai subchron, about 1.37 Ma. B. P.

  20. Zircon U-Pb geochronology of the volcanic rocks from Fanchang-Ningwu volcanic basins in the Lower Yangtze region and its geological implications

    YAN Jun; LIU HaiQuan; SONG ChuanZhong; XU XiSheng; AN YaJun; LIU Jia; DAI LiQun


    The latest eruptions in two important Mesozoic volcanic basins of Fanchang and Ningwu located in the middle-lower reaches of the Yangtze River formed the bimodal volcanic rocks of the Kedoushan Formation and ultrapotassic volcanic rocks of the Niangniangshan Formation,respectively.The representative volcanic rocks of the two Formations were selected for LA-ICPMS zircon U-Pb dating.The results indicate that there exist a large amount of magmatic zircons as indicated by high Th/U ratios in these volcanic rocks.The weighted mean age of 21 analyses is 130.7±1.1 Ma for the Kedoushan Formation,and that of 20 analyses is 130.6±1.1 Ma for the Niangniangshan Formation.These U-Pb ages are interpreted to represent the formation times of the volcanic rocks.In combination with other known geochronological data for Mesozoic volcanic rocks from the Lower Yangtze region,it is proposed that the latest volcanic activations in the Jinniu,Luzong,Fanchang and Ningwu volcanic basins probably came to end prior to ca.128 Me.There is no significant time interval between the early and later volcanic activities in the Luzong and Ningwu basins,suggesting e short duration of volcanic activities and thus implying the onset of an extensional tectonic setting at about 130 Ma in the Lower Yangtze region.Integrated studies reveal that the Early Cretaceous magmatic activities and their geochronological framework in the Lower Yangtze region are a response to progressively dynamic deep processes that started with the transformation of tectonic setting from compression to extension,followed by delaminating of the lower part of the thickened lithosphere,lithospheric thinning,asthenosphere upwelling,and crust-mantle interaction.

  1. Location, age, and rock type of volcanic rocks younger than 5 million years in Arizona and New Mexico

    Aldrich, M.J. Jr.; Laughlin, A.W.


    As part of the assessment of the Hot Dry Rock geothermal energy potential of Arizona and New Mexico, a compilation of the locations and ages of volcanic rocks less than 5 Myr was made. The locations of those rocks less than 3 Myr are shown on a map of the region. Because the compiled information has many uses in addition to geothermal exploration, the entire compilation is presented as a tabulation. The table is organized first by state and secondly by latitude and longitude within each state. Rock type, age and error, method of dating, and original reference are also given. The K-Ar dates have not been recalculated using the most recent decay constants for /sup 40/K. A few references gave only verbal descriptions of sample location; these locations were converted to approximate latitude and longitude.

  2. A preliminary evaluation of volcanic rock powder for application in agriculture as soil a remineralizer

    Ramos, Claudete G., E-mail: [Laboratory of Environmental Researches and Nanotechnology Development, Centro Universitário La Salle, Mestrado em Avaliação de Impactos Ambientais, Victor Barreto, 2288 Centro, 92010-000 Canoas, RS (Brazil); Querol, Xavier [Institute of Environmental Assessment and Water Research (IDÆA-CSIC), C/Luis Solé y Sabarís s/n, 08028 Barcelona (Spain); Oliveira, Marcos L.S. [Laboratory of Environmental Researches and Nanotechnology Development, Centro Universitário La Salle, Mestrado em Avaliação de Impactos Ambientais, Victor Barreto, 2288 Centro, 92010-000 Canoas, RS (Brazil); Pires, Karen [Departamento Nacional de Produção Mineral (DNPM), Washington Luiz, 815, Centro, 90010-460 Porto Alegre, RS (Brazil); Kautzmann, Rubens M. [Laboratory of Environmental Researches and Nanotechnology Development, Centro Universitário La Salle, Mestrado em Avaliação de Impactos Ambientais, Victor Barreto, 2288 Centro, 92010-000 Canoas, RS (Brazil); Oliveira, Luis F.S., E-mail: [Laboratory of Environmental Researches and Nanotechnology Development, Centro Universitário La Salle, Mestrado em Avaliação de Impactos Ambientais, Victor Barreto, 2288 Centro, 92010-000 Canoas, RS (Brazil)


    Mineralogical and geochemical characteristics of volcanic rock residue, from a crushing plant in the Nova Prata Mining District, State of Rio Grande do Sul (RS), Brazil, in this work named rock powder, were investigated in view of its potential application as soil ammendment in agriculture. Abaut 52,400 m{sup 3} of mining waste is generated annually in the city of Nova Prata without a proper disposal. The nutrients potentially available to plants were evaluated through leaching laboratory tests. Nutrient leaching tests were performed in Milli-Q water; citric acid solution 1% and 2% (AC); and oxalic acid solution 1% and 5% (AO). The bulk and leachable contents of 57 elements were determined by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Mining waste were made up by CaO, K{sub 2}O, SiO{sub 2}, Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3}, and P{sub 2}O{sub 5}. The analysis by X-ray diffraction (XRD) showed the major occurence of quartz, anorthite, cristobalite, sanidine, and augite. The water leachable concentrations of all elements studied were lower than 1.0 mg/kg, indicating their low solubility. Leaching tests in acidic media yield larger leachable fractions for all elements being studied are in the leachate of the AO 1%. These date usefulness of volcanic rock powder as potential natural fertilizer in agriculture in the mining district in Nova Prata, Rio Grande do Sul, Brazil to reduce the use of chemical fertilizers. - Highlights: • Volcanic rock powder as fertilizer in agriculture • Volcanic rock powder as a source of nutrients to plants • This technology may favor the use of volcanic rock in agriculture.

  3. The geochemistry of volcanic, plutonic and turbiditic rocks from Sumba, Indonesia

    Lytwyn, J.; Rutherford, E.; Burke, K.; Xia, C.


    Rocks that reveal the geology of Sumba for times before the Later Miocene (˜16 Ma) are relatively few and are not particularly well exposed. This has led to uncertainty about the nature of the basement rocks of the island and especially about whether Sumba originated as a fragment of Australia, or of that part of southeastern Eurasia which many authors have called Sundaland. A third possibility is that Sumba is underlain by arc material generated on the ocean floor and is not a fragment of either continent. We have studied the geochemistry of volcanic, plutonic and turbiditic rocks collected from Sumba in an attempt to provide additional insight into the island's origin and history between Late Cretaceous (˜86 Ma) and Early Miocene (˜16 Ma) times. Late Cretaceous to Early Oligocene (˜31 Ma) volcanic rocks on Sumba range compositionally from basalts to andesites, and are of typical oceanic island-arc affinity, exhibiting geochemical characteristics similar to those of high-Al basalts and their derivatives. Compositions indicate evolution along both calc-alkaline and tholeiitic trends. Some samples show indications of possible modifications by slab-derived melts and/or related fluids and also of contamination by turbiditic sediments. Gabbros and diorites collected from the Paleocene Tanadaro intrusion are compositionally similar to the associated volcanic rocks and, we consider, represent the plutonic equivalents of high-Al basalt. The geochemistry of Cretaceous turbiditic sedimentary rocks on Sumba indicates close proximity to an intra-oceanic island-arc environment. These results are consistent with the geochemical, sedimentological, stratigraphic, paleontological and paleomagnetic results of other investigators which together indicate that: (1) Late Cretaceous to Early Oligocene volcanic, plutonic and volcaniclastic rocks of Sumba are island-arc- and forearc-related; (2) the arc involved appears to have been what we refer to as the Great Indonesian Volcanic

  4. Search for magnetic monopoles in polar volcanic rocks

    Bendtz, K; Hächler, H -P; Hirt, A M; Mermod, P; Michael, P; Sloan, T; Tegner, C; Thorarinsson, S B


    For a broad range of values of magnetic monopole mass and charge, the abundance of monopoles trapped inside the Earth would be expected to be enhanced in the mantle beneath the geomagnetic poles. A search for magnetic monopoles was conducted using the signature of an induced persistent current following the passage of igneous rock samples through a SQUID-based magnetometer. A total of 24.6 kg of rocks from various selected sites, among which 23.4 kg are mantle-derived rocks from the Arctic and Antarctic areas, was analysed. No monopoles were found and a 90% confidence level upper limit of $1.6\\cdot 10^{-28}$ is set on the monopole to nucleon ratio in the search samples.

  5. Search for Magnetic Monopoles in Polar Volcanic Rocks

    Bendtz, K.; Milstead, D.; Hächler, H. -P.


    For a broad range of values of magnetic monopole mass and charge, the abundance of monopoles trapped inside Earth would be expected to be enhanced in the mantle beneath the geomagnetic poles. A search for magnetic monopoles was conducted using the signature of an induced persistent current...... following the passage of igneous rock samples through a SQUID-based magnetometer. A total of 24.6 kg of rocks from various selected sites, among which 23.4 kg are mantle-derived rocks from the Arctic and Antarctic areas, was analyzed. No monopoles were found, and a 90% confidence level upper limit of 9.8 x...... 10(-5)/g is set on the monopole density in the search samples. DOI:10.1103/PhysRevLett.110.121803...

  6. Nature and origin of secondary mineral coatings on volcanic rocks of the Black Mountain, Stonewall Mountain, and Kane Springs Wash volcanic centers, southern, Nevada

    Taranik, James V.; Hsu, Liang C.; Spatz, David M.; Chenevey, Michael J.


    The following subject areas are covered: (1) genetic, spectral, and LANDSAT Thematic Mapper imagery relationship between desert varnish and tertiary volcanic host rocks, southern Nevada; (2) reconnaissance geologic mapping of the Kane Springs Wash Volcanic Center, Lincoln County, Nevada, using multispectral thermal infrared imagery; (3) interregional comparisons of desert varnish; and (4) airborne scanner (GERIS) imagery of the Kane Springs Wash Volcanic Center, Lincoln County, Nevada.

  7. Chronology and geochemistry of the volcanic rocks in Woruo Mountain region,Northern Qiangtang depression:Implications to the Late Triassic volcanic-sedimentary events


    A suite of sedimentary-volcaniclastic rocks intercalated with the volcanic rocks unconformably overlies the Triassic Xiaochaka Formation in the Woruo Mountain region, Qiangtang Basin, northern Tibet. The vitric tuff from the base of these strata gives a SHRIMP zircon U-Pb age of 216 ± 4.5 Ma, which represents the age of the Late Triassic volcanic-sedimentary events in the Woruo Mountain region, and is consistent with that of the formation of the volcanic rocks from the Nadi Kangri Formation in the Nadigangri-Shishui River zone. There is a striking similarity in geochemical signatures of the volcanic rocks from the Woruo Mountain region and its adjacent Nadigangri-Shishui River zone, indicating that all the volcanic rocks from the Qiangtang region might have the same magmatic source and similar tectonic setting during the Late Triassic. The proper recognition of the Late Triassic large-scale volcanic eruption and volcanic-sedimentary events has important implications for the interpretation of the Late Triassic biotic extinction, climatic changes and regressive events in the eastern Tethyan domain, as well as the understanding of the initiation and nature, and sedimentary features of the Qiangtang Basin during the Late Triassic-Jurassic.

  8. Chronology and geochemistry of the volcanic rocks in Woruo Mountain region, Northern Qiangtang depression: Implications to the Late Triassic volcanic-sedimentary events

    WANG Jian; FU XiuGen; CHEN WenXi; WANG ZhengJiang; TAN FuWen; CHEN Ming; ZHUO JieWen


    A suite of sedimentary-volcaniclastic rocks intercalated with the volcanic rocks unconformably overlies the Triassic Xiaochaka Formation in the Woruo Mountain region, Qiangtang Basin, northern Tibet. The sents the age of the Late Triassic volcanic-sedimentary events in the Woruo Mountain region, and is consistent with that of the formation of the volcanic rocks from the Nadi Kangri Formation in the Nadigangri-Shishui River zone. There is a striking similarity in geochemical signatures of the volcanic rocks from the Woruo Mountain region and its adjacent Nadigangri-Shishui River zone, indicating that all the volcanic rocks from the Qiangtang region might have the same magmatic source and similar tectonic setting during the Late Triassic. The proper recognition of the Late Triassic large-scale volcanic eruption and volcanic-sedimentary events has important implications for the interpretation of the Late Triassic biotic extinction, climatic changes and regressive events in the eastern Tethyan domain,as well as the understanding of the initiation and nature, and sedimentary features of the Qiangtang Basin during the Late Triassic-Jurassic.

  9. Mineral Chemistry and Geochemistry of Volcanic Rocks in The North of Pasinler (Erzurum

    Oktay KILIÇ


    Full Text Available In the north of Pasinler (Erzurum, Upper Miocene-Pliocene volcanic rocks crop out. These volcanites are composed of basaltic andesite, andesite, dacite, rhyolite lavas and rhyolitic pyroclastics. The rocks show porphyritic, microlitic porphyritic, hyalo-microlitic porphyritic, vitrophyric, glomeroporphyritic, pilotaxitic and hyalopilitic textures. The investigated volcanites contain plagioclase (An29-80, olivine (Fo65-82, clinopyroxene (augite, orthopyroxene (enstatite, amphibole (Mg#: 0.57-0.71, biotite (phlogopite: 0.44-0.47, annite: 0.33-0.37, sanidine, quartz and opaque mineral (titano-magnetite and ilmenite. The volcanic rocks are calc-alkaline in character and have medium to high-K contents. Major oxide and trace element variations point out open-system magmatic differentiation in the evolution of rocks. Geochemical data indicate an important role of fractionation of phenocryst phases in the rocks during differentiation process. However, it is considered that assimilation±magma mixing might have accompanied to the process. High LILE (K, Rb, Ba, Th and relatively low HFSE (Nb, Ta, Hf, Zr contents of the rocks indicate that these rocks derived from parental magmas carrying subduction signature.

  10. Thermal Waters in Maguarichi, Chihuahua, Mexico: Influence on Volcanic Rocks Alteration

    Mascote, C. R.; Espejel-Garcia, V. V.; Villalobos-Aragon, A.


    Piedras de Lumbre, Maguarichi, is located 294 km. to the SW of Chihuahua city, in northern Mexico, in the Sierra Madre Occidental (SMO). The study area is composed of a set of igneous volcanic rocks affected by hydrothermal flows, which apparently run along a fault. Outcrops of hot springs, going out with high pressure, are active all over the year and have no seasonal flow changes. The hydrothermal flows, approximately 20, that reach the surface area at Piedras de Lumbre, are altering the volcanic rocks that surround the hot springs. The study area is highly altered, and evidenced by a variety range of colors in the rock surfaces. The rock samples collected at the region show a crystal growth due to the influence of the salts from the thermal water. The rocks closest to the water openings have a change in its mineralogy, with the mafic minerals, present in andesites, been replaced by carbonates and sulfates, leaving only the clear mineral pseudomorphs. On the crust of the rocks a white layer of material (salts), product of the thermal waters has precipitated. The alteration is perceived only about 5 m. or less around the hot springs. The water, which has high contents of arsenic and sulfates has exerted a strong alteration in rhyolitic and andesitic rocks.

  11. Engineering geological characterization of volcanic rocks of ethiopian and sardinian highlands to be used as construction materials

    Engidasew, Tesfaye Asresahagne


    This thesis presents the results of the study conducted on the “Geoengineering characterization of volcanic rocks from Ethiopian and Sardinian highlands to be used as construction materials”. Though, the two project areas are geographically far apart, both are partly covered with volcanic rocks mainly consisting of basic and subordinate felsic rocks. The research was conducted in two countries; part I, the Ethiopian Project area located on the northwestern central Highlands of ...

  12. Supracrustal rocks in the Kuovila area, Southern Finland: structural evolution, geochemical characteristics and the age of volcanism

    Pietari Skyttä; Asko Käpyaho; Irmeli Mänttäri


    The supracrustal rocks of the Kuovila area in the Palaeoproterozoic Svecofennian Uusimaa Belt, southern Finland, consist mainly of volcaniclastic rocks associated with banded iron formations (BIFs) and marbles. Small ZnS and PbS mineralizations are occasionally located within the marbles. Some primary features are well preserved in the sedimentary and volcanic rocks, including lamination in tuffites and banded iron formations. Geochemical results show that the volcanism was bimodal and it mai...

  13. Is there a geochemical link between volcanic and plutonic rocks in the Organ Mountains caldera?

    Memeti, V.; Davidson, J.


    Results from separate volcanic and plutonic studies have led to inconsistent conclusions regarding the origins and thus links between volcanic and plutonic systems in continental arcs and the magmatic processes and time scales responsible for their compositional variations. Some have suggested that there is a geochemical and geochronological disconnect between volcanic and plutonic rocks and hence have questioned the existence of magma mush columns beneath active volcanoes. Investigating contemporary volcanic and plutonic rocks that are spatially connected is thus critical in exploring these issues. The ca. 36 Ma Organ Mountains caldera in New Mexico, USA, represents such a system exposing contemporaneous volcanic and plutonic rocks juxtaposed at the surface due to tilting during extensional tectonics along the Rio Grande Rift. Detailed geologic and structural mapping [1] and 40Ar/39Ar ages of both volcanics and plutons [2] demonstrate the spatial and temporal connection of both rock types with active magmatism over >2.5 myr. Three caldera-forming ignimbrites erupted within 600 kyr [2] from this system with a total erupted volume of 500-1,000 km3 as well as less voluminous pre- and post-caldera trachyte and andesite lavas. The ignimbrite sequence ranges from a crystal-poor, high-SiO2 rhyolite at the base to a more crystal-rich, low-SiO2 rhyolite at the top. Compositional zoning with quartz-monzonite at the base grading to syenite and alaskite at the top is also found in the Organ Needle pluton, the main intrusion, which is interpreted to be the source for the ignimbrites [1]. Other contemporaneous and slightly younger plutons have dioritic to leucogranitic compositions. We examined both volcanic and plutonic rocks with petrography and their textural variations with color cathodoluminescence, and used whole rock element and Sr, Nd and Pb isotope geochemistry to constrain magma compositions and origins. Electron microprobe analyses on feldspars have been completed to

  14. Fractal approach in petrology: Small-angle neutron scattering experiments with volcanic rocks

    Lucido, G.; Triolo, R.; Caponetti, E.


    Following Mandelbrot's pioneering work in 1977, we attempt to use the concept of fractal dimension in petrology. Fractal dimension is an intensive property of matter which offers a quantitative measure of the degree of surface roughness. Neutron scattering experiments have been performed on 18 volcanic rocks from different localities. The scattered intensity as a function of the momentum transfer obeys a power law whose exponent varies, for the rock samples presented, between -3 and -4. We conclude that, at the molecular level, our volcanic rocks are not fractal volumes. With regard to the particle-matrix interface, it is not possible to provide a determination at the present stage of research. Our findings suggest it is feasible to verify the degree of surface irregularity at a resolution which is relevant to many aspects of petrology.

  15. Holocene volcanic rocks in Jingbo Lake region--Diversity of magmatism

    FAN Qicheng; SUN Qian; LI Ni; WANG Tuanhua


    During the time from 5500 a to 5200 a BP more than 10 Holocene volcanoes in Jingbo Lake region erupted and the volcanic rocks covered an area of about 500 km2. Holocene volcanic rocksin Jingbo Lake region belong to the potassium-rich rocks and contain three rock types: trachybasalts, basanites and phonotephrites. Various types of magmatism formed in a small area and in a short period of time came from partial melting of potassically-metasomatised lithospheric mantle. The diversity of magmatism can be explained by that Jingbo Lake is situated in the back-arc extensional region of East Asian continent subducted by the Pacific Ocean, and potassic fluid derived from mantle wedge or dehydration of subducted slab can result in a high heterogeneity of the mantle beneath this region. Based on the pressure estimation of clinopyroxene megacrysts, we estimate that phonotephrite magma fractionally crystallize at ca. 52-54 km down the earth.

  16. Petrography and petrology of Quaternary volcanic rocks from Ghezel Ghaleh, northwest Qorveh

    Alireza Bajelan


    Full Text Available Introduction In the east and northeast of Sanandaj in the Qorveh-Bijar-Takab axis, there are series of basaltic composition volcanoes with Quaternary age. The study area is part of the Sanandaj-Sirjan zone and is located between 47°52' and 47°57' E longitudes and 35°26 and '35°30' N latitudes. Due to the location of the volcanic cone on Pliocene clastic sediments and Quaternary travertine, the age of these volcanoes is considered to be Quaternary. The cones mostly consist of low scoria, ash, volcanic bombs, lapilli deposits and basaltic lava (Moein Vaziri and Aminsobhani, 1985. Petrological and geochemical studies have been carried out to evaluate Quaternary magmatism in the area and to determine the nature of the lithological characteristics, such as the evaluation of source rocks and magma type, degree of partial melting and the tectonic setting of Ghezel Ghaleh rocks (Moein Vaziri, 1997. Simplified geological map of the study area is characterized by ER-Mapper software. Materials and methods In the course of field studies in the region, 40 samples were taken, 30 thin sections were prepared and polished. XRD analyses were performed on some whole rock samples. All major, minor and trace elements were assessed by ICP-MS at Lab Weft Laboratory in Australia. Results Based on the classification of structural zones, the area is located in the Sanandaj-Sirjan zone, hundred kilometers away from the main Zagros thrust along the NW-SE direction. After early Cimmerian orogeny, andesitic volcanic activity took place (Moein Vaziri and Aminsobhani, 1985. A major secondary mineral in these rocks is iddingsite, formed by hydration and oxidation of the olivine (Shelley, 1993. According to SiO2 against Na2O + K2O (TAS diagram (Irvine and Baragar , 1971 and cationic R1 and R2 diagram (De La Roche et el., 1980, volcanic rocks of the area indicate alkaline series. Discussion To obtain more information on the tectonic setting of these rocks, the Zr/Y-Zr diagram

  17. Geochemical Characteristics of the Cenozoic Volcanic Rocks in Central Qiangtang, Tibet: Relation with the Uplift of the Qinghai Tibet Plateau

    TAN Fuwen; PAN Guitang; XU Qiang


    The Cenozoic volcanic rocks in central Qiangtang are tectonically outcropped in the transitional area where crust of the Qinghai-Tibet Plateau thins northwards and the Passion's ratios of the crust increases abnormally northwards. Of all Cenozoic volcanic rocks of northern Tibet, the volcanic rocks in Qiangtang area is the oldest one with ages from 44.1±1.0 Ma to 32.6±0.8 Ma. Petrological and geochemical studies of the volcanic rocks in central Qiangtang suggest they formed in the extension environment of post collision-orogeny and were the product of mixture of magmas from crust and mantle. The uplift of the northern plateau is closely related to decoupling of mantle lithosphere,crustal extension and thinning as well as volcanism. Therefore, it is inferred that the main uplift of the northern plateau began from about 40 Ma ago.

  18. Petrogenesis and tectonic implication of the Late Triassic post-collisional volcanic rocks in Chiang Khong, NW Thailand

    Qian, Xin; Wang, Yuejun; Feng, Qinglai; Zi, Jian-Wei; Zhang, Yuzhi; Chonglakmani, Chongpan


    The volcanic rocks exposed within the Chiang Khong-Lampang-Tak igneous zone in NW Thailand provide important constraints on the tectonic evolution of the eastern Paleotethys ocean. An andesite sample from the Chiang Khong area yields a zircon U-Pb age of 229 ± 4 Ma, significantly younger than the continental-arc and syn-collisional volcanic rocks (ca. 238-241 Ma). The Chiang Khong volcanic rocks are characterized by low MgO (1.71-6.72 wt.%) and high Al2O3 (15.03-17.76 wt.%). They are enriched in LILEs and LREEs and depleted in HFSEs, and have 87Sr/86Sr (i) ratios of 0.7050-0.7065, εNd (t) of - 0.32 to - 1.92, zircon εHf (t) and δ18O values of 3.5 to - 11.7 and 4.30-9.80 ‰, respectively. The geochemical data for the volcanic rocks are consistent with an origin from the enriched lithospheric mantle that had been modified by slab-derived fluid and recycled sediments. Based on available geochronological and geochemical evidences, we propose that the Late Triassic Chiang Khong volcanic rocks are equivalent to the contemporaneous volcanic rocks in the Lancangjiang igneous zone in SW China. The formation of these volcanic rocks was possibly related to the upwelling of the asthenospheric mantle during the Late Triassic, shortly after slab detachment, which induced the melting of the metasomatized mantle wedge.

  19. Anisotropy of Anhysteretic Remanent Magnetization: A Tool To Estimate Trm Deviations In Volcanic Rocks

    Gattacceca, J.; Rochette, P.

    In order to assess the paleomagnetic direction deviations due to anisotropy in volcanic rocks, we studied the anisotropies of magnetic susceptibility (AMS), of anhysteretic remanent magnetization (AARM) and of thermoremanent magnetization (ATRM) of a set of Miocene pyroclastic rocks from Sardinia (Italy). The main magnetic carrier is pseudo-single domain titanomagnetite. AARM and ATRM were determined with a 3-position measurement scheme. The measurements show that there is no general relation between the degrees of AMS and ATRM (as this relation depends on the ti- tanomagnetite grain size spectrum), while the degree of AARM and ATRM are almost identical. Measuring the AMS is thus nearly irrelevant to quantitatively estimate TRM deviations due to anisotropy in volcanic rocks. Instead, measuring the AARM provides a reliable and relatively fast method to correct paleomagnetic direction deviations in volcanic rocks (inclination shallowing due to horizontal planar fabric in most cases). This is confirmed by a case study on a succession of four welded pyroclastic flows : an apparent paleosecular variation pattern is almost entirely explained by the effect of ATRM.

  20. New high-precision 40Ar/39Ar ages on Oligocene volcanic rocks of northwestern Kenya

    Brown, Francis H.; Jicha, Brian R.


    New, high-precision 40Ar/39Ar ages from volcanic rocks in northwestern Kenya are provided for some areas of exposure in this remote area. We report seven 40Ar/39Ar ages generated from single crystal total fusion experiments on alkali feldspar separated from volcanic rocks in the Mogila, Songot, and Lokwanamur Ranges and the Gatome valley. A rhyolite from the lower part of the sequence in the Mogila Range yielded ages of 32.31 ± 0.06 Ma and 32.33 ± 0.07 Ma, and a rhyolite near the top of that sequence yielded 31.67 ± 0.04 Ma. A single sample from the Songot Range yielded an age of 32.49 ± 0.07 Ma, slightly older than the rocks collected from Mogila. In both ranges the early Oligocene rhyolites are underlain by basalts, as is also the case in the Labur Range. Ages of 25.95 ± 0.03 Ma, 25.91 ± 0.04 Ma, and 27.15 ± 0.03 Ma were measured on alkali feldspar from rhyolites from the Lokwanamur Range, and the nearby Gatome valley. All of these rocks are part of an episode of widespread volcanism in northwestern Kenya in the mid-to late Oligocene that is not currently known from the Ethiopian Rift Valley.

  1. Upper Cretaceous to Pleistocene melilitic volcanic rocks of the Bohemian Massif: petrology and mineral chemistry

    Skála Roman


    Full Text Available Upper Cretaceous to Pleistocene volcanic rocks of the Bohemian Massif represent the easternmost part of the Central European Volcanic Province. These alkaline volcanic series include rare melilitic rocks occurring as dykes, sills, scoria cones and flows. They occur in three volcanic periods: (i the Late Cretaceous to Paleocene period (80–59 Ma in northern Bohemia including adjacent territories of Saxony and Lusatia, (ii the Mid Eocene to Late Miocene (32.3–5.9 Ma period disseminated in the Ohře Rift, the Cheb–Domažlice Graben, Vogtland, and Silesia and (iii the Early to Late Pleistocene period (1.0–0.26 Ma in western Bohemia. Melilitic magmas of the Eocene to Miocene and Pleistocene periods show a primitive mantle source [(143Nd/144Ndt=0.51280–0.51287; (87Sr/86Srt=0.7034–0.7038] while those of the Upper Cretaceous to Paleocene period display a broad scatter of Sr–Nd ratios. The (143Nd/144Ndt ratios (0.51272–0.51282 of the Upper Cretaceous to Paleocene rocks suggest a partly heterogeneous mantle source, and their (87Sr/86Srt ratios (0.7033–0.7049 point to an additional late- to post-magmatic hydrothermal contribution. Major rock-forming minerals include forsterite, diopside, melilite, nepheline, sodalite group minerals, phlogopite, Cr- and Ti-bearing spinels. Crystallization pressures and temperatures of clinopyroxene vary widely between ~1 to 2 GPa and between 1000 to 1200 °C, respectively. Nepheline crystallized at about 500 to 770 °C. Geochemical and isotopic similarities of these rocks occurring from the Upper Cretaceous to Pleistocene suggest that they had similar mantle sources and similar processes of magma development by partial melting of a heterogeneous carbonatized mantle source.

  2. Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

    Charles W. Messo


    Full Text Available The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

  3. Petrogenesis of volcanic rocks that host the world-class Agsbnd Pb Navidad District, North Patagonian Massif: Comparison with the Jurassic Chon Aike Volcanic Province of Patagonia, Argentina

    Bouhier, Verónica E.; Franchini, Marta B.; Caffe, Pablo J.; Maydagán, Laura; Rapela, Carlos W.; Paolini, Marcelo


    We present the first study of the volcanic rocks of the Cañadón Asfalto Formation that host the Navidad world-class Ag + Pb epithermal district located in the North Patagonian Massif, Patagonia, Argentina. These volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of Gondwana and represent the mafic to intermediate counterparts of the mainly silicic Jurassic Chon Aike Volcanic Province. Lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions, whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation. LA-ICPMS Usbnd Pb ages of zircon crystals from the volcanic units yielded Middle Jurassic ages of 173.9 ± 1.9 Ma and 170.8 ± 3 Ma. In the Navidad district, volcanic rocks of the Cañadón Asfalto Formation show arc-like signatures including high-K basaltic-andesite to high-K dacite compositions, Rb, Ba and Th enrichment relative to the less mobile HFS elements (Nb, Ta), enrichment in light rare earth elements (LREE), Ysbnd Ti depletion, and high Zr contents. These characteristics could be explained by assimilation of crustal rocks in the Jurassic magmas, which is also supported by the presence of zircon xenocrysts with Permian and Middle-Upper Triassic ages (281.3 Ma, 246.5, 218.1, and 201.3 Ma) and quartz xenocrysts recognized in these volcanic units. Furthermore, Sr and Nd isotope compositions suggest a contribution of crustal components in these Middle Jurassic magmas. High-K basaltic andesite has initial 87Sr/86Sr ratios of 0.70416-0.70658 and ξNd(t) values of -5.3 and -4. High-K dacite and andesite have initial 87Sr/86Sr compositions of 0.70584-0.70601 and ξNd(t) values of -4,1 and -3,2. The range of Pb isotope values (206Pb/204Pb = 18.28-18.37, 207Pb/204Pb = 15.61-15.62, and 208Pb/204Pb = 38.26-38.43) of Navidad volcanic rocks and ore minerals suggest mixing Pb sources with contributions of

  4. Paralavas in the Cretaceous Paraná volcanic province, Brazil - A genetic interpretation of the volcanic rocks containing phenocrysts and glass


    Full Text Available ABSTRACT The occurrences of glassy rocks containing long and curved phenocrysts in the Paraná volcanic province, South America, are here interpreted as paralavas. The large number of thin (0.1-0.5 m dikes and sills of glassy volcanic rocks with hopper, hollow or curved, large crystals of clinopyroxene (up to 10 cm, plagioclase (up to 1 cm, magnetite and apatite are contained in the core of thick (>70 m pahoehoe flows. They are strongly concentrated in the state of Paraná, coincident with the presence of the large number of dikes in the Ponta Grossa arch. These rocks were previously defined as pegmatites, although other names have also been used. A paralava is here interpreted as the product of melting of basaltic rocks following varied, successive processes of sill emplacement in high-kerogen bituminous shale and ascent of the resultant methane. As the gas reached the lower portion of the most recent lava flow of the volcanic pile, the methane reacted with the silicate and oxide minerals of the host volcanic rock (1,000 ºC and thus elevated the local temperature to 1,600 ºC. The affected area of host rock remelted (possibly 75 wt.% and injected buoyantly the central and upper portion of the core. This methane-related mechanism explains the evidence found in the paralavas from this volcanic province, one of the largest in the continents.

  5. Saturated Zone Plumes in Volcanic Rock: Implications for Yucca Mountain

    S. Kelkar; R. Roback; B. Robinson; G. Srinivasan; C. Jones; P. Reimus


    This paper presents a literature survey of the occurrences of radionuclide plumes in saturated, fractured rocks. Three sites, Idaho National laboratory, Hanford, and Oak Ridge are discussed in detail. Results of a modeling study are also presented showing that the length to width ratio of a plume starting within the repository footprint at the Yucca Mountain Project site, decreases from about 20:1 for the base case to about 4:1 for a higher value of transverse dispersivity, indicating enhanced lateral spreading of the plume. Due to the definition of regulatory requirements, this lateral spreading does not directly impact breakthrough curves at the 18 km compliance boundary, however it increases the potential that a plume will encounter reducing conditions, thus significantly retarding the transport of sorbing radionuclides.

  6. Petrographic and geochemical characterization of the Triassic and Jurassic magmatic and volcanic rocks of southeastern Ecuador

    Villares, Fabián; Eguez, Arturo; Yanez, Ernesto


    Formely, the subandean zone in the southeastern Ecuador involved large volcanic and magmatic rocks included in the Misahualli Formation and Zamora batholith, both as expression of the Jurassic cal-alcaline volcanic arc. The aim of the project carried out by the INIGEMM (Instituto Nacional de Investigación Geológico Minero Metalúrgico) was discriminate the volcanic products including a continuous set going from basalts to ryolithes and volcanoclastic rocks. Geochemical characterization was done using representative 16 whole - rock chemical analysis. The oldest rocks of the investigated area called Pachicutza Unit, include greenish to black, massive basalts and basaltic andesites, locally showing pillows structures. The texture is aphanitic to microporphyritic with slight crystal growth of plagioclase and pyroxenes. The Unit include also local pyroclastic breccias and tuffs showing variable skarnification related to the intrusion of the jurassic Zamora Batholith. Two samples of basalts show tholeiitic affinity, corresponding to an N- MORB, probably representing an early stage in opening of a regional Triassic rift reported since Colombia to Peru in the Andes. These geochemical characteristics are similar to the amphibolites of Monte Olivo Unit in the Real Cordillera. The Jurassic large volcanic assembly of the Misahualli Formation was also differenciated. Basal volcanics include green, subporphyritic andesites and volcanic breccias possibly generated at an early stage of the volcanic arc, caused by a change of extensive to compressive regime. Continental volcano sedimentary and sedimentary rock were discriminate as Nueva Esperanza and Suarez Units, respectively. The volcanosedimentary sequence include massive to laminate tuffs and tuffites of intermediate composition. The sediments of the Suarez Unit include dominant conglomerats and sandstones of fluvial domain. The regional volcanic sequence is completed by the Las Peñas Unit that includes aphanitic to

  7. Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet

    Geng Quanru; Pan Guitang; Jin Zhenmin; Wang Liquan; Liao Zhongli


    The Early Jurassic bimodal volcanic rocks in the Yeba Formation, situated between Lhasa, Dagzê and Maizhokunggar, composed of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia, are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys. Based on systematic field investigations, we carried out geochemical studies on representative rock samples. Major and trace element compositions were analyzed for these rock samples by XRF and ICP-MS respectively, and an isotope analysis of Rb-Sr and Sm-Nd was carried out by a MAT 262 mass spectrograph. The results show that the SiO2 contents in lava rocks are 41 %-50.4 % and 64 %-69 %, belonging to calc-alkaline basalt and dacite. One notable feature of the basalt is its low TiO2 content, 0.66 %-1.01 %, much lower than those of continental tholeiite. The ΣREE contents of basalt and dacite are 60.3-135 μg/g and 126.4-167.9 μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb*=0.54-1.17 averaging 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*=0.74-1.06 averaging 0.86. Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone. The spinal lherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages. The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.

  8. Methane-rich fluid inclusions and their hosting volcanic reservoir rocks of the Songliao Basin, NE China

    WANG Pu-Jun; HOU Qi-jun; CHENG Ri-hui; LI Quan-lin; GUO Zhen-hua; HUANG Yu-long


    Methane-rich fluids were recognized to be hosted in the reservoir volcanic rocks as primary inclusions.Samples were collected from core-drillings of volcanic gas reservoirs with reversed δ12C of alkane in the Xujiaweizi depression of the Songliao Basin. The volcanic rocks are rhyolite dominant being enriched in the more incompatible elements like Cs, Rb, Ba, Th, U and Th with relative high LREE, depleted HREE and negative anomalies of Ti and Nb,suggesting a melt involving both in mantle source and crustal assimilation. Primary fluids hosted in the volcanic rocks should have the same provenance with the magma. The authors concluded that the enclosed CH4 in the volcanics are mantle/magma-derived alkane and the reversed δ13C of alkane in the corresponding gas reservoirs is partly resulted from mixture between biogenic and abiogenic gases.

  9. Geochemical study of volcanic and associated granitic rocks from Endau Rompin, Johor, Peninsular Malaysia

    Azman A Ghani; Ismail Yusoff; Meor Hakif Amir Hassan; Rosli Ramli


    Geochemical studies and modelling show that both volcanic and granitic magmas from the western part of the Johor National Park, Endau Rompin are different and probably have different sources. The geochemical plot suggests that both dacite/rhyolite and andesite probably have a common origin as in many of the geochemical plots, these two groups form a similar trend. Volcanic rocks have a transitional geochemical character between tholeiite and calc alkaline on a Y versus Zr plot. (La/Yb)N versus La and TiO2 versus Zr modelling show that the crystallization of both granitic and volcanic magmas are controlled by a different set of minerals. The rare earth elements (REE) patterns of some of the granite and volcanic samples have pronounced negative Eu anomaly indicating plagioclase fractionation. The difference between both profiles is that the granite samples show a concave shape profile which is consistent with liquids produced by partial melting of quartz feldspathic rocks containing amphibole among the residual phase. Both magmas were generated at a different time during the subduction of Sibumasu beneath the Indochina blocks.

  10. K-Ar dating, whole-rock and Sr-Nd isotope geochemistry of calc-alkaline volcanic rocks around the Gümüşhane area: implications for post-collisional volcanism in the Eastern Pontides, Northeast Turkey

    Aslan, Zafer; Arslan, Mehmet; Temizel, İrfan; Kaygusuz, Abdullah


    Volcanic rocks from the Gümüşhane area in the southern part of the Eastern Pontides (NE Turkey) consist mainly of andesitic lava flows associated with tuffs, and rare basaltic dykes. The K-Ar whole-rock dating of these rocks range from 37.62 ± 3.33 Ma (Middle Eocene) to 30.02 ± 2.84 Ma (Early Oligocene) for the andesitic lava flows, but are 15.80 ± 1.71 Ma (Middle Miocene) for the basaltic dykes. Petrochemically, the volcanic rocks are dominantly medium-K calc-alkaline in composition and show enrichment of large ion lithophile elements, as well as depletion of high field strength elements, thus revealing that volcanic rocks evolved from a parental magmas derived from an enriched mantle source. Chondrite-normalized rare-earth element patterns of the volcanic rocks are concave upwards with low- to-medium enrichment (LaCN/LuCN = 3.39 to 12.56), thereby revealing clinopyroxene- and hornblende-dominated fractionations for andesitic-basaltic rocks and tuffs, respectively. The volcanic rocks have low initial 87Sr/86Sr ratios (0.70464 to 0.70494) and ɛNd(i) values (+1.11 to +3.08), with Nd-model ages (TDM) of 0.68 to 1.02 Ga, suggesting an enriched lithospheric mantle source of Proterozoic age. Trace element and isotopic data, as well as the modelling results, show that fractional crystallization and minor assimilation played an important role in the evolution of the volcanic rocks studied. The Eocene to Miocene volcanism in the region has resulted from lithospheric delamination and the associated convective thinning of the mantle, which led to the partial melting of the subduction-metasomatized lithospheric mantle.

  11. Scaling Up the Pulse-Remagnetization Experiment for Large Animals

    Kirschvink, J. L.; Hilburn, I. A.; Golash, H. N.; Wang, C. X.; Wu, D. A.; Mizuhara, Y.; Shimojo, S.; Matani, A.


    Pulse-remagnetization has been a commonly-used technique in rock magnetic studies for producing IRM curves for the past 33 years. Typical circuits involve charging a bank of capacitors to a desired voltage, and then discharging it on command through a silicon-controlled rectifier into a solenoid magnet coil. Back-oscillations are prevented by dissipating the energy with a suitably-configured diode, and the intensity of the single magnetic peak resulting from this procedure is proportional to the charging voltage. However, this technique also has been applied many times in biology to measure the coercivity spectrum of various populations of magnetoctactic bacteria, as well as in successful tests of the hypothesis that biological magnetite is the biophysical transducer for magnetic field sensitivity (magnetoreception) in animals. We have recently discovered earth-strength magnetic effects on the brainwave patterns of a large mammal. Experimental results indicate that the effects are not a result of either electrical induction or an axially-symmetric biophysical compass; therefore, the most likely explanation is a polar compass provided by specialized sensory cells containing chains of single-domain biological magnetite. If so, we should be able to invert the magnetic polarity of this response via a properly-configured pulse-remagnetization experiment, and thereby place constraints on the microscopic coercivity of the magnetite crystals in the hypothesized receptor cells. To achieve this end, we have constructed a large-volume IRM coil capable of producing a uniform magnetic field pulse of up to 70 mT over the volume of the target animal's head, while also applying a co-axial 1 mT static DC biasing field. Applying a DC-biased IRM pulse to our animal subjects has proven to be surprisingly difficult, both due to the complexity of engineering a coaxial pulse and biasing coil system of this volume, and also due to the care and safety protocols necessary to ensure the well

  12. Basic-ultrabasic and volcanic rocks in Chagbu-Shuanghu area of northern Xizang (Tibet),China

    邓万明; 尹集祥; 呙中平


    The widespread Early Permian and Triassic sequences outcropping in the Chagbu-Shuanghu area of northern Xizang, China, are mainly characterized by volcanic rocks belonging to tholeiite with subordinate veins of diallagite. wehrlite and limburgites schlierens. These ultrabasic rocks do not carry plastic deformational fabrics from upper mantle and may result from the crystallization of fused mass derived from mantle under condition of deeper crust or earlier segregation of tholeiitic magma. These volcanic rocks, as interlayers or lens, are generally involved in slates, limestones and pebbly slates or breccia and geochemically different from MORB. It is reasonable to conclude from research results that the volcanic activities during the Early Permian and Late Triassic would be able to occur in an intraplale environment suffering initial extension of continental crust or an aulacogen. Therefore, these basic-ultrabasic and volcanic rocks did not constitute an ophiolitic association with an occurrence 6f the

  13. Volcanic stratigraphy of intermediate to acidic rocks in southern Paraná Magmatic Province, Brazil

    Liza Angélica Polo


    Full Text Available This article presents the first map in detail scale for an area covered by Palmas type volcanic rocks in the south border of the eocretaceous Paraná Magmatic Province, south Brazil. The study of the structural features coupled with petrography and geochemistry made it possible to separate these rocks into three main volcanic sequences and recognize their stratigraphy. The older Caxias do Sul sequence rests directly over the first low-Ti basalt flows (Gramado type, and corresponds to the stacking of lobated lava flows, laminar flows and lava domes, mostly emitted as continuous eruptions; only the latest eruptions are intercalated with thin sandstone deposits. These rocks have dacitic composition (~ 68 wt% SiO2 with microphenocrysts of plagioclase and subordinate pyroxenes and Ti-magnetite immersed in glassy or devitrified matrix. A second volcanic sequence, named Barros Cassal, is composed of several lava flows of basaltic andesite, andesitic and dacitic composition (~ 54; ~ 57 and ~ 63 wt% SiO2 , respectively, with microphenocrysts of plagioclase, pyroxenes and Ti-magnetite. The frequent intercalation of sandstone between the flows attests to the intermittent behaviour of this event. The upper sequence, Santa Maria, is made up of more silica-rich (~ 70 wt% SiO2 rocks occurring as laminar flows, lobated flows and lava-domes. These rocks have rhyolitic composition with microphenocrysts of plagioclase and Ti-magnetite set in a glassy or devitrified matrix with microlites. The structures and textures of all three silicic sequences favor the interpretation that they had a predominantly effusive character, which is thought to be a reflection of the remarkably high temperatures of the lavas (~ 1,000 ºC.

  14. Geochronology of the volcanic rocks in the Lu-Zong basin and its significance

    David; COOKE; Sebastien; MEFFRE


    The Lu-Zong (Lujiang-Zongyang) basin is one of the most important volcanic basins in the middle and lower reaches of the Yangtze River area, China. It comprises four shoshonitic volcanic units, which are, in an ascending order, the Longmenyuan, Zhuanqiao, Shuangmiao and Fushan Groups. The LA-ICP MS U-Pb zircon ages of the four units are: 134.8±1.8 Ma for the Longmenyuan Group, 134.1±1.6 Ma for the Zhuanqiao Group, 130.5±0.8 Ma for the Shuangmiao Group, and 127.1±1.2 Ma for the Fushan Group. The results indicate that all volcanic rocks in the Lu-Zong basin were formed in the Early Cretaceous from about 135 Ma to 127 Ma, lasting 8-10 Ma. There were no Jurassic volcanic activities in all the volcanic basins including the Lu-Zong basin in the middle and lower reaches of the Yangtze River area. This work has provided new chronological results for the further study and understanding of the tec- tonic, magmatic and metallogenic processes of eastern China in the Mesozoic.

  15. Geochronology of the volcanic rocks in the Lu-Zong basin and its significance

    ZHOU TaoFa; FAN Yu; YUAN Feng; LU SanMing; SHANG ShiGui; David COOKE; Sebastien MEFFRE; ZHAO GuoChun


    The Lu-Zong (Lujiang-Zongyang) basin is one of the most important volcanic basins in the middle and lower reaches of the Yangtze River area, China. It comprises four shoshonitic volcanic units, which are,in an ascending order, the Longmenyuan, Zhuanqiao, Shuangmiao and Fushan Groups. The LA-ICP MS U-Pb zircon ages of the four units are: 134.8±1.8 Ma for the Longmenyuan Group, 134.1±1.6 Ma for the Zhuanqiao Group, 130.5±0.8 Ma for the Shuangmiao Group, and 127.1+1.2 Ma for the Fushan Group. The results indicate that all volcanic rocks in the Lu-Zong basin were formed in the Early Cretaceous from about 135 Ma to 127 Ma, lasting 8-10 Ma. There were no Jurassic volcanic activities in all the volcanic basins including the Lu-Zong basin in the middle and lower reaches of the Yangtze River area.This work has provided new chronological results for the further study and understanding of the tectonic, magmatic and metallogenic processes of eastern China in the Mesozoic.

  16. Mineral chemistry and petrogenesis of the Gurgur Mount volcanic rocks (Northeast Takab

    Dariush Esmaeily


    Full Text Available Andesitic and andesitic-basaltic lavas are widespread over most of the ground surface of the Gurgur area altered mostly by the hydrothermal solutions. The main rock forming minerals in these rocks are plagioclase, pyroxene and olivine affected by the hydrothermal solutions. The altered rocks do contain minerals including calcite, sericite and chlorite. Given the results obtained and the mineral chemistry studies, the clinopyroxenes formed in the area are, chemically, calkalkaline and of diopside-augite type formed in subvolcanic to near surface levels contemporaneous with magma ascending. Plagioclase minerals show zoning textures and lie within the two andesine and albite-oligoclase fields. These units, in terms of total rock chemistry, are classified as the calk-alkaline volcanic rocks formed in the continental arcs. On the other hand, on the trace elements chondrite-normalized diagrams and enriched mantle-normalized multi- element diagrams, the LREE enrichment relative to the HREE is observed. The LILE (i.e. Rb, K and Th and the LREE (e.g. La, Ce and Nd show an enrichment in comparison to the HFSE (Zr, Hf, Nb, Yb, Y and Sm. Given the Nd/Th (1.42-1.15, Zr/Nb (12.27-21.22, Ba/La (18.64-29.77 as well as LILE enrichment associated with depletion in Nb, Ta and Ti, an environment related to the subduction zones can be proposed for the area under study. Moreover, the similarity between the REE distribution pattern and the incompatible elements point to the genetic relationship between these rocks. Finally, on the base of the obtained data, it can be concluded that the volcanic rocks in the Gurgur Mountain were likely formed during the extended magmatism of the Urumieh-Dokhtar in the Cenozoic.

  17. Thermo-physical rock properties of greywacke basement rock and intrusive lavas from the Taupo Volcanic Zone, New Zealand

    Mielke, P.; Weinert, S.; Bignall, G.; Sass, I.


    Greywacke of the Waipapa and Torlesse (Composite) Terrane form the basement of the Taupo Volcanic Zone (TVZ), New Zealand. Together with inferred buried lavas, domes and igneous complexes they are likely to be the dominant rock type prevailing at depths > 4 km beneath the TVZ. A fundamental understanding of the rock properties of the deep formations is of utmost importance for the exploration of deep unconventional geothermal resources. An outcrop analogue study was conducted to improve the understanding of the thermo-physical rock properties of likely deep buried rock formations beneath the TVZ. A total of 145 core samples were taken at 10 locations inside and outside the TVZ and their grain and bulk density, porosity, matrix permeability, bulk thermal conductivity and specific heat capacity, and the compressional and shear wave velocities measured on oven-dry samples. Additional tests of the unconfined compressive strength were conducted for selected greywacke samples to quantify their mechanical rock strength. The obtained data indicates that the thermo-physical rock properties are mainly controlled by porosity, and minor by mineralogy, texture and grain size. Samples from Waipapa-type and Torlesse-type greywacke exhibit minor rheological differences, with Waipapa-type greywacke having lowest porosity (about 1% vs. 3%) and highest bulk thermal conductivity (2.5 W m- 1 K- 1 vs. 1.7 W m- 1 K- 1) and specific heat capacity (0.8 kJ kg- 1 K- 1 vs. 0.7 kJ kg- 1 K- 1). Matrix permeability is rock properties due to their wide range of porosity (rock properties were tested at laboratory conditions (ambient temperature and pressure), which do not reflect the in situ conditions at greater depth. With depth, thermal conductivity and acoustic wave velocity are likely to decrease caused by micro fractures resulting from thermal cracking of the rock, while specific heat capacity increases. The data presented in this paper are expected to improve the statistical confidence on

  18. Zircon U/Pb Dating of Cretaceous Adakitic Volcanic Rocks in the Eastern Part of North Dabie Mountains

    薛怀民; 董树文; 刘晓春


    Mesozoic volcanic rocks in the eastern part of the North Dabie Mountains are rich inNa ( Na2O=4.03%, Na2O/K2O = 1.31 ), Sr (865μg/g) and Ba ( 1361μg/g) , and high inSr/Y ratio (66.1) but low in Nb, Y and HREE. They have experienced strong fractionation ofREE [ (La/Yb)N = 26.6 ], and are similar to adakite in geochemical characteristics. The U-Pbdating of zircon from the volcanic rocks is ( 129.2 + 2.6) Ma, belonging to Early Cretaceous.These rocks are similar to the volcanic rocks of North Huaiyang not only in age and rare-earth el-ement and trace element geochemistry, but also in the formation temperature and pressure of theminerals. The results indicated that the delamination of mountain root and underplating of mafic-ultramafic magma had happened in the Dabie orogen before Early Cretaceous. Mesozoic mag-matism was intense in the North Dabie Mountains, including the intrusion of mafic-ultramaficmagma, uplifting of gneiss dome, explosion of volcanic rocks and intrusion of granitic magma.The Mesozoic volcanic rocks in the eastern part of the North Dabie Mountains may be one part ofthe Mesozoic volcano-intrusive complex belt of North Huaiyang. The existence of Mesozoic vol-canic remnant cap means the denudation of the Dabie orogenic belt was not very strong sinceEarly Cretaceous.

  19. Petrogenesis of the Cenozoic Volcanic Rocks from the Northern Par of Qinghai-Xizang(Tibet) Plateau


    Based on electron probe analyses of the minerals and bulk composition of the Cenozoic volcanic rocks from Yumen and Hoh Xil lithodistricts,Qinghai-Xizang plateau,the forming conditions including the temperature and pressure of those rocks are studied in this paperAccording to the thermodynamic calculation results of mineral-melt equilibrium,the depth of the asthenosphere superface(about 75-130km)for the northern part of the Qinghai-Xizang plateau during the Cenozoic is suggested.Finally,this paper indicates that the Cenozoic volcanic rocks in the northern part of the Qinghai-Xizang plateau mainly consist of shoshonite series.Their forming temperature is 630-1039℃ and forming pressure is between 2.3-4.0GPa .The rocks were formed in the intracontinental orogenic belt,of which the primary magma was originated from a particular enrichment upper mantle and accreted crust-mantle belt of directly from asthenospheric superface as a result of partial of pyrolite.

  20. The nature and origin of mineral coatings on volcanic rocks of the Black Mountain, Stonewall Mountain and Kane Springs Wash volcanic centers in southern Nevada

    Taranik, J. V.; Noble, D. D.; Hsu, L. C.; Hutsinpiller, A.


    Four LANDSAT thematic mapping scenes in southern Nevada were requested at two different acquisition times in order to assess the effect of vegetation on the signature of the volcanic units. The remote sensing data acquisition and analysis portion are nearly completed. The LANDSAT thematic mapping data is of good quality, and image analysis techniques are so far successful in delineating areas with distinct spectral characteristics. Spectrally distinct areas were correlated with variations in surface coating and lithologies of the volcanic rocks.

  1. Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)

    Neres, Marta; Silva, Pedro F.; Ikene, Moha; Martins, Sofia; Hafid, Ahmid; Mata, João; Almeida, Francisco; Youbi, Nasrrddine; Boumehdi, Ahmed


    Paleomagnetic data able to constrain the paleoposition of the West African Craton (WAC) during Paleo-Mesoproterozoic are absent, mainly due to gaps on the sedimentary record and intense remagnetizations. Dykes that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) have recently been subjected to geochronological studies, which revealed ages between Paleoproterozoic and early Neoproterozoic. Therefore, these dykes represent a window of opportunity for paleomagnetic studies aiming to infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. We determined the paleomagnetic directions and evaluated their meaning by rock magnetic and mineral analyses, complemented by petrographic observations. Results revealed that these rocks record the presence of a complex history of remagnetization events, mostly assigned to several Phanerozoic thermal/chemical events. In particular, we found components assigned to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases (magnetite and hematite). The primary (magmatic) thermo-remanent magnetization of the dykes was obliterated during these events through multiple thermal and chemical remagnetizations. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma. The authors wish to acknowledge FCT (Portugal) - CNRST (Morocco) bilateral agreement for its major contribution without which this work wouldn't be possible. Publication supported by project FCT UID/GEO/50019/2013 - Instituto Dom Luiz.

  2. Simulated Lunar Environment Spectra of Silicic Volcanic Rocks: Application to Lunar Domes

    Glotch, T. D.; Shirley, K.; Greenhagen, B. T.


    Lunar volcanism was dominated by flood-style basaltic volcanism associated with the lunar mare. However, since the Apollo era it has been suggested that some regions, termed "red spots," are the result of non-basaltic volcanic activity. These early suggestions of non-mare volcanism were based on interpretations of rugged geomorphology resulting from viscous lava flows and relatively featureless, red-sloped VNIR spectra. Mid-infrared data from the Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter have confirmed that many of the red spot features, including Hansteen Alpha, the Gruithuisen Domes, the Mairan Domes, Lassell Massif, and Compton Belkovich are silicic volcanic domes. Additional detections of silicic material in the Aristarchus central peak and ejecta suggest excavation of a subsurface silicic pluton. Other red spots, including the Helmet and Copernicus have relatively low Diviner Christiansen feature positions, but they are not as felsic as the features listed above. To date, the SiO2 content of the silicic dome features has been difficult to quantitatively determine due to the limited spectral resolution of Diviner and lack of terrestrial analog spectra acquired in an appropriate environment. Based on spectra of pure mineral and glass separates, preliminary estimates suggest that the rocks comprising the lunar silicic domes are > 65 wt.% SiO2. In an effort to better constrain this value, we have acquired spectra of andesite, dacite, rhyolite, pumice, and obsidian rock samples under a simulated lunar environment in the Planetary and Asteroid Regolith Spectroscopy Environmental Chamber (PARSEC) at the Center for Planetary Exploration at Stony Brook University. This presentation will discuss the spectra of these materials and how they relate to the Diviner measurements of the lunar silicic dome features.

  3. Petrogenesis and tectonic implications of Early Jurassic volcanic rocks of the Raohe accretionary complex, NE China

    Wang, Zhi-Hui; Ge, Wen-Chun; Yang, Hao; Bi, Jun-Hui; Ji, Zheng; Dong, Yu; Xu, Wen-Liang


    The Raohe accretionary complex, located at the border between the Russian Far East and Northeastern China, is a significant part of the western Pacific Oceanic tectonic regime. Due to lack of precise age and geochemical constraints, the tectonic setting and petrogenesis of the magmatic rocks in this area remain undefined, resulting in debate about crustal growth mechanisms and subduction-related accretionary processes in Northeastern China. Here, we report whole-rock major and trace element and Sr-Nd isotope data, together with zircon U-Pb ages and in situ zircon Hf isotope data for calc-alkaline andesites, dacites, rhyolites, rhyolitic crystal tuffs, Nb-enriched andesites and basaltic andesites, and high-Mg andesites of the Raohe accretionary complex in NE China. Samples were collected from Late Triassic to Early Jurassic strata. However, geochronological results in this study indicated that the studied magmatism occurred in the Early Jurassic (187-174 Ma). The calc-alkaline volcanic rocks possess geochemical characteristics typical of arc magmas that form at active continental margins, such as moderate enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletions in high field strength elements (HFSEs). They have positive εHf(t) values of +3.4 to +10.6 and relatively high (87Sr/86Sr)i values of 0.7047-0.7102. While the Nb-enriched andesites and basaltic andesites have higher TiO2, Hf, Nb, and Zr contents and higher Nb/Ta (24.0-87.6), Nb/U (11.9-75.9), (Nb/Th)PM (0.67-2.70), and (Nb/La)PM (1.95-5.00) ratios than typical arc basalts. They have negative εNd(t) values (-5.5 to -6.0) and relatively variable (87Sr/86Sr)i values of 0.7047-0.7114, suggesting an origin via the partial melting of mantle wedge peridotite that had been metasomatized by slab-derived melt. The high-Mg volcanic rocks, characterized by high MgO and Mg#, TiO2, Al2O3, Cr, Ni, (La/Yb)N and (La/Sm)N, but low Ba/Th ratios, are geochemically similar to

  4. Strontium isotopic ratios of the volcanic rocks from Dogo of the Oki Islands, Japan

    Kurasawa, Hajime


    The isotopic composition of strontium and the abundances of rubidium and strontium in volcanic rocks from Dogo of the Oki Islands and Takashima in the northwest Kyushu, West Japan, and Ulrungdo of Korea, have been determined. The rubidium and strontium contents for alkakine basalts range from 27.6 to 51.2 ppm and 444 to 723 ppm, and 148 to 208 ppm and 3.7 to 205 ppm for intermediate to felsic suites, respectively. The alkaline basalts are divided into two groups with /sup 87/Sr//sup 86/Sr ratios of the restricted ranges of 0.70481 - 0.70496 and 0.70540 - 0.70575, respectively. However, the /sup 87/Sr//sup 86/Sr ratios of intermediate to felsic rocks of Dogo are relatively high and variable ranging from 0.70706 to 0.71019, which probably reflect the contamination and/or produced by body or partial melting of the basement rocks in this area without crustal assimilation of basaltic magma. In addition, the lead isotopic results indicate that the melting of Precambrian basement rocks possibly yields less radiogenic lead. In the southwest Japan, the /sup 87/Sr//sup 86/Sr ratios of Cenozoic basaltic rocks are clearly different between the San-in and the northwest Kyushu regions, which includes Jeju island. The higher /sup 87/Sr//sup 86/Sr ratios of basalts from the San-in region than that of basalts from the northwest Kyushu region also reflect the different properties of the upper mantle, which means there is regional heterogeneity of Sr isotopic ratios under the southwest Japan arcs. Furthermore, the relatively high and variable /sup 87/Sr//sup 86/Sr ratios of volcanic rocks are particularly concentrated in the southwestern Japan arcs which has probably more continental properties than northeastern Japan arcs.

  5. Quaternary Basanitic Rocks within the Eastern Anatolian Volcanism (Turkey): Petrological and Geochemical Constrains

    Özdemir, Yavuz; Mercan, Çaǧrı; Oyan, Vural; Atakul-Özdemir, Ayşe


    The Eastern Anatolian Cenozoic continental intraplate volcanism was initiated in Middle Miocene as a result of the convergence between the Arabian and Anatolian plates. The origin of Eastern Anatolian volcanism has been the focus of many petrological studies that have aimed to resolve the relative contributions of asthenospheric mantle and/or lithospheric mantle with/without subduction component in the genesis of magmas that compositionally have many affinities to ocean island basalts (OIB) and volcanic arcs. Volcanism in the region characterized by mainly stratovolcanoes, basaltic lava plateaus and are dominantly spread at the northern parts of Bitlis Pötürge Massif (BPM). Our study focuses on a small scale Quaternary basaltic system that firstly observed within the BPM. The volcanic rocks of our study located 50 km to the south of Lake Van and are basanitic in composition. They exposed along K-G striking tensional fissures and crosscut the Upper unit of the Bitlis Massif. Initial products of the volcanism are scoria fall deposits. Thick basanitic lava flows overly the pyroclastics and formed columnar structures. The basanites are generally fine-grained with phenocrysts of olivine+clinopyroxene. The groundmass is typically of clinopyroxene, olivine and Ti magnetite and Cr spinel with interstitial nepheline. The olivine phenocrysts are typically euhedral to subhedral with Forsterite contents of Fo73-83. Clinopyroxenes are highly calcic and show modest variations in Wo47-52-En34-42-Fs10-15 and are weakly zoned with mg# 89-87 at cores to 86-84 at rims. Nephelines occur as minor minerals within the networks of other groundmass minerals. Ti rich and Fe-Cr spinels occur as inclusions in olivine and clinopyroxenes as well as within the groundmass. LILE and LREE enrichments over HFSE and HREE suggest similarities with magmas generated from enriched mantle sources. EC-AFC modeling of trace element and isotope compositions indicates that assimilation of crustal

  6. Petrography, geochemistry and tectonic setting of Salmabad Tertiary volcanic rocks, southeast of Sarbisheh, eastern Iran

    Masoumeh Goodarzi


    Full Text Available Introduction The area reviewed and studied in this paper is located 5 km southeast of Sarbisheh city at eastern border of the Lut block (Jung et al., 1983; Karimpour et al., 2011; Richards et al., 2012 in eastern Iran between 59° 47′ and 59° 53′ E longitude and 32°30′ and 32°34′ N latitude. The magmatic activity in the Lut block began in middle Jurassic (165-162 Ma and reached its peak in Tertiary (Jung et al., 1983. Volcanic and subvolcanic rocks of Tertiary age cover over half of Lut block with up to 2000 m thickness and formed due to subduction prior to the collision of the Arabian and Asian plates (Camp and Griffis, 1982; Tirrul et al., 1983; Berberianet et al., 1982. Most of magmatic activity in the Lut block formed in middle Eocene (Karimpour et al., 2011 The andesitic volcanics were erupted together with the dacites and rhyodacites during a time interval of some 50 Ma from early Cretaceous to early Neogene. It can be assumed that the intensity of the volcanic activity was varying significantly during this time span (Jung et al., 1983.Tertiary volcanic rocks (Eocene-Oligocene to Pliocene with intermediate composition associated with pyroclastic rocks cropped out in eastern parts of Salmabad village, southeast of Sarbisheh. The main purpose of this paper is better understand the tectono-magmatic setting of the Tertiary volcanic rocks in southeast of Sarbisheh, eastern Iran based on geochemical characteristics. Materials and methods Eleven samples were analyzed for major elements by inductively coupled plasma (ICP technologies and trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS, following a lithium metaborate/tetraborate fusion and nitric acid total digestion, at the SGS Laboratories, Toronto, Canada. Results In the Salmabad area, Tertiary volcanic rocks with mainly intermediate (andesitic composition are exposed associated with pyroclastic deposits such as tuff, breccia and agglomerate

  7. Remagnetization and Clay diagenesis in Jurassic Sediments of Skye, Scotland

    Basu, A.; Elliott, W.; Wampler, J.; Elmore, R.; Engel, M. H.


    The thrust of this study is to test the hypothesized connection between magnetite authigenesis and diagenetic reactions forming illite, an idea based on the results of paleomagnetic, rock magnetic, geochemical, and petrographic/SEM studies on Jurassic sedimentary rocks of Skye, Scotland. The Jurassic rocks in southern Skye contain a dual polarity magnetization residing in magnetite that is interpreted as a chemical remanent magnetization (CRM) and has directions similar to those of early Tertiary igneous rocks on Skye. The presence-absence test and the timing of acquisition for this CRM suggest that magnetite authigenesis is related to the smectite-to-illite conversion, and that clay diagenesis is a viable remagnetization mechanism. The clay fractions consist of illite-smectite (I-S), kaolinite, and trace amounts of chlorite. The I-S exhibits Kalkberg order (IISI) and typically contains ˜ 85% to 90% illite layers based on the positions of the 001 reflection. The 2M1, 1M and 1Md illite polytypes are present in variable proportions depending on size fraction. The K-Ar dates of I-S in the finest fraction range from 100 to 131 Ma while the K-Ar dates of I-S of the coarse fractions range from 160-210 Ma. In all cases, the dates of the finest I-S are much greater than the Tertiary CRM ages recorded in these rocks (65 Ma). The K-Ar dates decrease with decrease in particle size. At first approximation, the decrease is thought to reflect smaller amounts of detrital illite in the finer fractions. The youngest K-Ar dates of I-S are significantly greater than the CRM age recorded in these sediments. The most likely explanation of the discordance between K-Ar dates and CRM is that the finest fractions contain small amounts of detrital illite.

  8. Geology and geochemistry of palaeoproterozoic low-grade metabasic volcanic rocks from Salumber area, Aravalli Supergroup, NW India

    L S Shekhawat; M K Pandit; D W Joshi


    The Palaeoproterozoic Aravalli Supergroup in Salumber region includes a basal unit of metabasic volcanic rocks (Salumber volcanic rocks) overlain by a volcaniclastic/conglomerate one. Although these volcanic rocks have been metamorphosed to green-schist facies, some primary volcanic features are still preserved. This metabasic volcanic sequence can be further differentiated on the basis of textural variations, and the mineral assemblages are: (a) oligoclase + actinolite + chlorite + epidote; and (b) oligoclase + hornblende+ chlorite + biotite + Fe-Ti oxides. The SiO2 content ranges from ∼47.7 to 55.8% and MgO from ∼4.2 to 12.8%. Geochemical characteristics allow their subdivision into high Mg and Fe tholeiites. Inverse relationship of MgO with silica, alkalis and Zr is generally consistent with fractionation mechanism, also suggested by a change in colour of the rocks from dark greenish to light greenish towards the upper parts of the sequence. These metabasic volcanic rocks are enriched in incompatible trace elements and LREE (La = 30 − 40 × chondrite, Lu = 2 − 5 × chondrite), and demonstrate affinity mainly with MORB and within plate settings in geochemical tectonic discrimination schemes. The geochemical characteristics suggest a complex evolutionary history envisaging derivation of the melt from an enriched heterogeneous lithospheric source.

  9. Research on isotope geology: Isotopes ages of volcanic rocks from Ryeongnam Massif, Korea

    Shin, Seong Cheon; Chi, Se Jung; Kim, Yoo Sook [Korea Institute of Geology Mining and Materials, Taejon (Korea, Republic of)


    Chronostratigraphy of most volcanic rocks in the Ryeongnam Massif have been undefined or mis-classified in different geological maps due to total absence of reported isotope ages. Twenty-four new isotope ages are given for age-undefined volcanic units and some related igneous bodies. Most of volcanic rocks show high [La/Yb]n ratios and LREE enrichments which are characteristics of subduction-related high-K calc-alkali volcanic rocks occurred in the active continental margin. Preliminary results on carbon and oxygen stable isotope ratios({delta}{sup 13}C=-1.7{approx}-6.2 per mil; {delta}{sup 18} O=-21.6{approx}-24.7 per mil) of druse- or phenocryst-calcite from andesitic and basaltic rocks in the southern coastal region indicate a magmatic origin. Based on new K-Ar whole-rock ages, chronological guidelines are established as follows: 1) Gayasan andesite (78{+-}4Ma) - Gurye andesitic tuff (81{+-}4Ma); 2) Gurye andesite (68{+-}4Ma) - Suncheon andesitic tuff (67{+-}3Ma) - Yeosu basaltic andesite (67{+-}3Ma) - Narodo andesite (70{+-}3Ma); 3) Taebaeg Baegbyeongsan basaltic andesite (62{+-}3Ma) - Gurye Obongsan andesite (64{+-}3Ma) - Yeosu dacite (63{+-}3Ma) - Dolsando andesite (62{+-}3Ma) - Jangheung Buyongsan andesite (65{+-}3Ma); 4) Suncheon Joryedong andesite (55{+-}2Ma) - Goheung andesite (56{+-}3Ma); 5) Taebaeg Baegbyeonsan basaltic andesite (48{+-}2Ma) - Yeosu basalt (51{+-}3Ma). Resetted age (49{+-}2Ma) of an intrusive rhyolite implies the timing of thermal alteration in the Wondong Fe-Mine of the Taebaegsan Mineralized Belt. K-Ar hornblende ages of two hornblendite stocks in the southern Jangsu suggests apparent emplacement-ages of late Triassic (210{+-}9Ma) and early Permian (274{+-}10Ma), independently. K-Ar hornblende age (1023{+-}37Ma) of the Ogbang amphibolite implies a reduction of original age due to later thermal effect probably attributed to either later intrusion or regional metamorphism. (author). 56 refs., 19 tabs., 14 figs.

  10. Neogene seismites and seismic volcanic rocks in the Linqu area, Shandong Province, E China

    Tian H.S.


    Full Text Available The Yishu Fault Zone runs through the centre of Shandong Province (E China; it is a deep-seated large fault system that still is active. Two volcanic faulted basins (the Shanwang and Linqu Basins in the Linqu area, west of the fault zone, are exposed to rifting, which process is accompanied by a series of tectonic and volcanic earthquakes with a magnitude of 5-8. Lacustrine sediments in the basins were affected by these earthquakes so that seismites with a variety of soft-sediment deformation structures originated. The seismites form part of the Shanwang Formation of the Linqu Group. Semi-consolidated fluvial conglomerates became deformed in a brittle way; these seismites are present at the base of the Yaoshan Formation. Intense earthquakes triggered by volcanic activity left their traces in the form of seismic volcanic rocks associated with liquefied-sand veins in the basalt/sand intercalations at the base of the Yaoshan Formation. These palaeo-earthquake records are dated around 14-10 Ma; they are responses to the intense tectonic extension and the basin rifting in this area and even the activity of the Yishu Fault Zone in the Himalayan tectonic cycle.

  11. Geochemistry of Mesoproterozoic Volcanic Rocks in the Western Kunlun Mountains:Evidence for Plate Tectonic Evolution

    ZHANG Chuanlin; DONG Yongguan; ZHAO Yu; WANG Aiguo; GUO Kunyi


    Mesoproterozoic volcanic rocks occurring in the north of the western Kunlun Mountains can be divided into two groups. The first group (north belt) is an reversely-evolved bimodal series. Petrochemistry shows that the alkalinity of the rocks decreases from early to late: alkaline→calc-alkaline→tholeiite, and geochemistry proves that the volcanic rocks were formed in rifting tectonic systems. The sedimentary facies shows characteristics of back-arc basins. The second (south belt) group, which occurs to the south of Yutian-Minfeng-Cele, is composed of calc-alkaline island arc (basaltic) andesite and minor rhyolite. The space distribution, age and geochemistry of the two volcanite groups indicate that they were formed in a back-arc basin (the first group) and an island arc (the second group) respectively and indicate the plate evolution during the Mesoproterozoic. The orogeny took place at ~1.05 Ga, which was coeval with the Grenville orogeny. This study has provided important geological data for exploring the position of the Paleo-Tarim plate in the Rodinia super-continent.

  12. A Backarc Basin Origin for the Eocene Volcanic Rocks North of Abbas Abad, East of Shahrud, Northeast Iran

    Khalatbari Jafari, M.; Mobasher, K.; Davarpanah, A.; Babaie, H.; La Tour, T.


    The region in northeastern Iran, bordered by the Miami fault and the Doruneh fault, mainly exposes the Eocene volcanic and Tertiary sedimentary rocks and sporadic outcrops of pre- Jurassic metamorphic rocks such as gneiss and mica-schist. We have divided the volcanic and volcanic-sedimentary rocks into six main units: E1 through the youngest E6. North of Abbas Abad, the Lower Eocene is conglomerate, sandstone, and red shale with lenses of nummulite-bearing limestone at the base, and dacitic lava (E1) at the top. The nummulites give an Early Eocene age for the limestone lenses. The E2 unit includes vesicular basalt, intercalated, intraformational conglomerate, and lenses of nummulite-bearing limestone. E3 is volcanic- sedimentary, and is made of green tuff, tuffite, shale, and nummulite bearing limestone. E4 includes basalt and vesicular trachy-basalt, and E5 is mostly sedimentary, made of tan marl, sandstone, shale, and lenses of Middle Eocene nummulite-bearing limestone. The E6 unit is the most extensive, with at least three levels of nummulite-bearing limestone lenses which give a Middle to Early Eocene age. The volcanic rocks of the E6 unit include few hundred meters of epiclastic to hyaloclastic breccia, with intercalations of lava at the base. These are overlain by four horizons of aphyric olivine basalt and basalt, and phyric trachy-andesite and trachy-basalt. The volume of the aphyric lavas decreases, and that of the phyric lavas increases upsection. The Eocene volcanic sequence is covered by turbidite; the marl washings give an Eocene-Oligocene age range. Chondrite-normalized multi-element plots indicate enrichment of the Eocene Abbas Abad volcanic rocks in the LILE elements, with variable ratios of La/Yb (4.36-19.33) and La/Sm (3.10-7.91). These plots show a gentle slope, and the volcanic rocks in the E1 to E4 units are less enriched than those in the E6 unit, probably reflecting the difference in the original source for the melt. The multi-element plots

  13. The Effect of Degassing Efficiency on the Fragmentation Behavior of Volcanic Rocks

    Mueller, S.; Scheu, B.; Spieler, O.; Dingwell, D. B.


    The degassing efficiency of volcanic rocks is a decisive measure for the eruptive style and thus the explosivity of a volcano, since it directly affects magma fragmentation behaviour. Vesicles in ascending magma may bear overpressure if the relevant magma viscosity entails a relaxation time scale which is significantly larger than the time scale of ambient pressure reduction due to magma ascent. As long as this overpressure does not overcome the tensile strength of the magma, the system is in a structurally stable state, eventually degassing quiescently via an interconnected pore network. However, if a decompressive event (e.g. sector collapse) disturbs this stable pressure situation, two possible scenarios are conceivable: (1) An interconnected pore network has been established whose permeability is sufficiently high, so vesicle overpressure can be reduced efficiently by gas filtration. (2) The permeability of the network (or cluster of isolated pores, respectively) is low and gas overpressure can not be reduced within the required time scale. In this case the expansion of the pressurized gas may cause bubble wall failure and magma fragmentation into pyroclasts. This study compares experimentally derived fragmentation threshold values of volcanic rock samples, determined with a shock-tube based setup, to unsteady-state permeability values of the same sample sets. In order to cover a wide range in rock properties, we analysed samples from a broad variety of volcanic deposits. Among the treated volcanoes were Colima (Mexico), Bezymianny (Russia), Krakatoa and Merapi, (both Indonesia), Unzen (Japan), Lipari and Campi Flegrei (both Italy), Pinatubo (Philippines), and Santorini (Greece). The correlation of extensive databases of both investigated parameters revealed that permeabilities above a transition zone between 10-13 and 10-12 m2 shift the fragmentation threshold towards higher values. By means of this dataset the influence of the permeability on fragmentation

  14. Late Cretaceous lithospheric extension in SE China: Constraints from volcanic rocks in Hainan Island

    Zhou, Yun; Liang, Xinquan; Kröner, Alfred; Cai, Yongfeng; Shao, Tongbin; Wen, Shunv; Jiang, Ying; Fu, Jiangang; Wang, Ce; Dong, Chaoge


    Petrological, geochemical and in-situ zircon U-Pb dating and Hf-isotope analyses have been carried out on a suite of basalt-andesite-rhyolite volcanic rocks exposed in the Liuluocun area, Hainan Island, SE China. Zircon analyses show that these volcanic rocks crystallized in the Early Cretaceous (ca. 102 Ma). The basalts are characterized by low MgO contents and mg-numbers but high rare earth element, high field strength element and large ion lithophile element contents and Nb-Ta negative anomalies. They have relatively uniform Sr-Nd isotope compositions with εNd(t) values of - 4.09 to - 3.63. The andesites show enrichment of high field strength element and rare earth element with negligible Eu anomalies. They have εNd(t) values of - 2.35 to - 3.88 and εHf(t) values of - 9.73 to - 1.13. The rhyolites have high K2O and SiO2 contents. They are characterized by prominent Eu, P and Ti negative anomalies and enrichment in large ion lithophile element, and show εHf(t) values of - 7.51 to + 0.47 and εNd(t) values of - 2.49 to - 2.69. Petrogenetic analysis indicates that the Liuluocun volcanic rocks were produced by incomplete reaction of the mantle wedge peridotite with felsic melts derived from partial melting of subducted sediment. All these characteristics, combined with geological observations, suggest that their formation was related to regional lithospheric extension in the South China Craton during the Early Cretaceous, which may have been caused by subduction of the Paleo-Pacific plate beneath the continental plate of China.

  15. Chronology and Geochemistry of Volcanic Rocks in the Cretaceous Suifenhe Formation in Eastern Heilongjiang, China

    JI Weiqiang; XU Wenliang; YANG Debin; PEI Fuping; JIN Ke; LIU Xiaoming


    Zircon U-Pb ages and geochemical data of volcanic rocks in the Suifenhe Formation in eastern Heilongjiang Province are reported, and their petrogenesis is discussed in this paper. The Suifenhe Formation mainly consists of basalt, andesite, and dacite. Zircon from andesite and dacite are euhedral in shape and show typical oscillatory zoning with high Th/U ratios (0.18-0.57), implying its magmatic origin. Zircon U-Pb dating results by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) indicate that the 206Pb/238U ages of zircons from andesite range within 105-106 Ma, yielding a weighted mean age of 105.5±0.8 Ma (n=14), and that 206Pb/238U ages of zircons from dacite are between 90-96 Ma, yielding a weighted mean age of 93.2±1.3 Ma (n =13). The volcanic rocks from the Suifenhe Formation are subalkaline series and show a calc-alkaline evolutionary trend with SiO2 content of 47.69%-65.47%, MgO contents of 1.42%-6.80% (Mg#= 45-53), and Na2O/K2O ratios of 1.83-3.63. They are characterized by enrichment in large ion lithophile elements (LILE) and lightrare-earth elements (LREE), depletion in heavy rare earth elements (HREE) and high field strength elements (HFSE) (e.g., Nb, Ta, Ti), and low initial 87Sr/86Sr ratios (0.7041-0.7057) and positive εNd(t)values (0.39-4.08), implying that they could be derived from a depleted magma source. Taken together,these results suggest that the primary magma of the volcanic rocks might originate from partial melting of the mantle wedge metasomatized by fluids derived from subducted slab under a tectonic setting of active continental margin.

  16. Geology and stratigraphy of the Challis Volcanic Group and related rocks, Little Wood River area, south-central Idaho

    Sandford, Richard F.; Snee, Lawrence W.


    The southwestern part of the Challis volcanic field occupies the valley of the Little Wood River and its tributaries in the Hailey and Idaho Falls 1??2? quadrangles of south-central Idaho. The Little Wood River area is a structurally controlled topographic basin that is partly filled by Eocene Challis Volcanic Group and younger rocks. Rock types in the Challis Volcanic Group of the Little Wood River area include, in order of decreasing abundance, andesite lava flows and tuff breccia, dacite lava flows and flow breccia, volcaniclastic sedimentary rocks, lithic tuff, nonvolcanic conglomerate, and rhyolite dikes. A basal nonvolcanic conglomerate, that locally rests on upper Paleozoic sedimentary rocks at a regional unconformity, was deposited prior to eruption of volcanic rocks. Andesite was the first volcanic rock erupted and is a voluminous sequence as thick as 3,000 ft (1,000 m). Locally thick volcaniclastic sedimentary rocks accumulated in topographic lows. A sharp transition marks the beginning of dacite eruption from fissures and flow-dome complexes. Dacite flows and breccias are as thick as 2,000 ft (600 m). An upper volcaniclastic unit was deposited in paleotopographic lows following emplacement of the main dacite unit. Next, a widespread, distinctive, lithic rich ash flow tuff, correlated with the tuff of Stoddard Gulch, was deposited over much of the area. Deposition of the tuff was followed by eruption of thin andesite and dacite lava flows and deposition of conglomeratic sedimentary rocks. The entire sequence was then intruded by a dacite flow-dome complex composed of at least three separate intrusions. The Challis Volcanic Group in the study area is calcalkaline. Andesitic rocks are typically high potassium basaltic andesite, high potassium andesite, shoshonite, and banakite (latite). Dacitic rocks are high potassium dacite and trachyte. Tuffs and vitrophyres range in composition from basaltic andesite to trachyte. The paleotopographic basin in which the

  17. Clinopyroxene application in petrogenesis identification of volcanic rocks associated with salt domes from Shurab (Southeast Qom

    Somayeh Falahaty


    Full Text Available Introduction The study area is located in the Shurab area that is about 50 Km Southeast of Qom. Volcanic rocks of the Shurab area have basaltic composition that is associated with salt and marl units. Igneous rocks of the Shurab area have not been comprehensively studied thus far. Clinopyroxene composition of volcanic rocks, and especially the phenocrysts show Magma chemistry and can help to identify magma series (Lebas, 1962; Verhooge, 1962; Kushiro, 1960, Leterrier et al., 1982, tectonic setting (Leterrier et al., 1982; Nisbet and Pearce, 1977 as well as temperature formation and pressure of rock formation. Some geologists have estimated temperature of clinopyroxene formation by clinopyroxene composition (Adams and Bishop, 1986 and clinopyroxene-olivine couple. So, clinopyroxene is used in this study in order to identify magma series, tectonic setting, plus the temperature and pressure of volcanic rocks of the Shurab. Material and method Clinopyroxene analyses were conducted by wavelength-dispersive EPMA (JEOL JXA-8800R at the Cooperative Centre of Kanazawa University (Japan. The analyses were performed under an accelerating voltage of 15 kV and a beam current of 20 nA. The ZAF program was used for data corrections. Natural and synthetic minerals of known composition were used as standards. The Fe3+ content in minerals was estimated by Droop method (Droop, 1987. Discussion In the Shurab area, the volcanic rocks area with basaltic composition are located 50 km Southeast of Qom. Their age is the early Oligocene and they are associated with the salty marl units of the Lower Red Formation (LRF. The hand specimens of the studied rocks look green. These rocks are intergranular, microlitic, porphyric, vitrophyric and amygdaloidal and they consist of olivine, pyroxene and plagioclase. Accessory minerals contain sphene, apatite and opaque. According to Wo-En-Fs diagram (Morimoto, 1988, clinopyroxenes indicate diopside composition. Clinopyroxenes are

  18. Petrology and geochemistry of mafic and ultramafic cumulates occurring as xenoliths in volcanic rocks from Polish part of Central European Volcanic Province.

    Dajek, Michał; Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros


    Mafic xenoliths coexisting with the peridotitic ones in rocks from Polish part of Cenozoic European Volcanic Province have been scarcely examined. (Bakun-Czubarow and Białowolska, 2003, Mineralogical Society of Poland- Spec. Pap. and references therein; Matusiak, 2006, Min. Polonica- Spec. Pap.; Puziewicz et al., 2011, JoP). In this study we present new results on mafic and ultramafic xenolithic rocks from the Wilcza Góra, Winna Góra, Góra Świątek, Mnisia Górka and Grodziec volcanic rocks in the Złotoryja-Jawor Volcanic Complex. The studied xenoliths are either plagioclase-free (clinopyroxenite, websterite) or plagioclase-bearing (anorthosite, gabbro, olivine-bearing gabbro and norite). Both the types may occur in the same volcanic rock. The cumulative xenoliths are smaller than peridotitic ones, blackish and show clear cumulative, coarse grained textures. Beside the rock-forming phases, the xenoliths occasionally contain spinel, sulfides and amphibole. Usually clinopyroxene grains occurring in gabbros are strongly corroded or disintegrated, while other phases are well-preserved. Contacts between xenolith and host volcanic rock are usually sharp with subhedral crystals of clinopyroxene growing at the xenolith surface. The mineral grains are usually zoned and chemical equilibrium between phases is scarce. Clinopyroxene in plagioclase-free rocks has composition of diopside with occasionally elevated Al, Ti and Cr contents. It's mg# varies from 0.89 to 0.79. It is slightly to moderately enriched in LREE; the REE patterns are concave, and the normalized values vary significantly between localities. It shows negative Sr anomaly, depth of Ti anomaly is variable. Orthopyroxene is Al-rich enstatite with mg# varying from 0.85 down to 0.75. Orthopyroxene in websterites is LREE depleted and show strong positive Ti and Zr-Hf anomalies. Opaques are ilmenite - Ti-magnetite solid solution and/or sulfides Clinopyroxene forming plagioclase-bearing rocks also has composition

  19. Suprasubduction volcanic rocks of the Char ophiolite belt, East Kazakhstan: new geochemical and first geochronological data

    Safonova, Inna; Simonov, Vladimir; Seltmann, Reimar; Yamamoto, Shinji; Xiao, Wenjiao


    The Char ophiolite belt is located in the western Central Asian Orogenic Belt, a world largest accretionary orogen, which has evolved during more than 800 Ma. The Char belt formed during Kazakhstan - Siberia collision. It has been known for hosting fragments of Late Devonian-Early Carboniferous oceanic crust, MORB, OPB and OIB, of the Paleo-Asian Ocean (Safonova et al., 2012). The Char is surrounded by two Paleozoic island-arc terranes: Zharma-Saur in the west and Rudny Altai in the east, however, until recent times, no island-arc units have been found within it. We were the first to find island-arc units as tectonic sheets occurring adjacent to those consisting of oceanic rocks. In places, island-arc andesites cut oceanic basalts. The Char volcanic and subvolcanic rocks of a probable suprasubduction origin are basalt, microgabbro, dolerite, andesite, tonalite and dacite. The mafic to andesitic volcanics possessing low TiO2 (0.85 wt.%av.) and show MgO vs. major elements crystallization trends suggesting two magma series: tholeiitic and calc-alkaline. The tholeiitic varieties are less enriched in incompatible elements then the calc-alkaline ones. Two samples are high-Mg and low-Ti andesibasalts similar to boninites. The rocks possess moderately LREE enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/Lapm = 0.14-0.47; Nb/Thpm = 0.7-1.6).The distribution of rare-earth elements (La/Smn = 0.8-2.3, Gd/Ybn = 0.7-1.9) and the results of geochemical modeling in the Nb-Yb system suggest high degrees of melting of a depleted harzburgite-bearing mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry indicates crystallization of melts at 1020-1180°C. Melt inclusion composition based numerical calculations show that primary melts were derived at 1350

  20. Petrology and petrogenesis of the Eocene Volcanic rocks in Yildizeli area (Sivas), Central Anatolia, Turkey

    Doğa Topbay, C.; Karacık, Zekiye; Genç, S. Can; Göçmengil, Gönenç


    -Na plagioclase + hornblend ± pyroxene ± biotite + opaques in a matrix comprised of mostly glass, microlites or crypto to micro crystalline feldspars. All the lavas show mainly pilotaxitic, intersertal, cumulophyric and poikilitic textures. Geochemically, Yıldızeli lavas ranging in composition from basalt to trachyandesite displaying the calc-alkaline affinity with medium-K and shoshonitic character. All intermediate and basic volcanic rocks show enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) such as Nb, Ta, Zr and Ti. Volcanic rocks of the Yıldızeli region display the following range in Sr and Nd initial isotope ratios: 87Sr/86Sr = 0.704389 to 0.706291 and 143Nd/144Nd = 0.512671. The major- trace element geochemistry and isotopic values suggest that Yıldızeli volcanics derived possibly from a mantle source which was modified by subduction related fluids or was contaminated by the continental crustal components.

  1. A Comprehensive Study on Dielectric Properties of Volcanic Rock/PANI Composites

    Kiliç, M.; Karabul, Y.; Okutan, M.; İçelli, O.


    Basalt is a very well-known volcanic rock that is dark colored and relatively rich in iron and magnesium, almost located each country in the world. These rocks have been used in the refused rock industry, to produce building tiles, construction industrial, highway engineering. Powders and fibers of basalt rocks are widely used of radiation shielding, thermal stability, heat and sound insulation. This study examined three different basalt samples (coded CM-1, KYZ-13 and KYZ-24) collected from different regions of Van province in Turkey. Polyaniline (PANI) is one of the representative conductive polymers due to its fine environmental stability, huge electrical conductivity, as well as a comparatively low cost. Also, the electrical and thermal properties of polymer composites containing PANI have been widely studied. The dielectric properties of Basalt/Polyaniline composites in different concentrations (10, 25, 50 wt.% PANI) have been investigated by dielectric spectroscopy method at the room temperature. The dielectric parameters (dielectric constants, loss and strength) were measured in the frequency range of 102 Hz-106 Hz at room temperature. The electrical mechanism change with PANI dopant. A detailed dielectrically analysis of these composites will be presented.

  2. Origin and accumulation mechanisms of petroleum in the Carboniferous volcanic rocks of the Kebai Fault zone, Western Junggar Basin, China

    Chen, Zhonghong; Zha, Ming; Liu, Keyu; Zhang, Yueqian; Yang, Disheng; Tang, Yong; Wu, Kongyou; Chen, Yong


    The Kebai Fault zone of the West Junggar Basin in northwestern China is a unique region to gain insights on the formation of large-scale petroleum reservoirs in volcanic rocks of the western Central Asian Orogenic Belt. Carboniferous volcanic rocks are widespread in the Kebai Fault zone and consist of basalt, basaltic andesite, andesite, tuff, volcanic breccia, sandy conglomerate and metamorphic rocks. The volcanic oil reservoirs are characterized by multiple sources and multi-stage charge and filling history, characteristic of a complex petroleum system. Geochemical analysis of the reservoir oil, hydrocarbon inclusions and source rocks associated with these volcanic rocks was conducted to better constrain the oil source, the petroleum filling history, and the dominant mechanisms controlling the petroleum accumulation. Reservoir oil geochemistry indicates that the oil contained in the Carboniferous volcanic rocks of the Kebai Fault zone is a mixture. The oil is primarily derived from the source rock of the Permian Fengcheng Formation (P1f), and secondarily from the Permian Lower Wuerhe Formation (P2w). Compared with the P2w source rock, P1f exhibits lower values of C19 TT/C23 TT, C19+20TT/ΣTT, Ts/(Ts + Tm) and ααα-20R sterane C27/C28 ratios but higher values of TT C23/C21, HHI, gammacerane/αβ C30 hopane, hopane (20S) C34/C33, C29ββ/(ββ + αα), and C29 20S/(20S + 20R) ratios. Three major stages of oil charge occurred in the Carboniferous, in the Middle Triassic, Late Triassic to Early Jurassic, and in the Middle Jurassic to Late Jurassic periods, respectively. Most of the oil charged during the first stage was lost, while moderately and highly mature oils were generated and accumulated during the second and third stages. Oil migration and accumulation in the large-scale stratigraphic reservoir was primarily controlled by the top Carboniferous unconformity with better porosity and high oil enrichment developed near the unconformity. Secondary dissolution

  3. Geochemical and isotopic composition of volcanic rocks of the heterogeneous Miocene (~ 23-19 Ma) Tepoztlán Formation, early Transmexican Volcanic Belt, Mexico

    Torres-Alvarado, Ignacio S.; Lenhardt, Nils; Arce, José Luis; Hinderer, Matthias


    We present the first geochemical data (major and trace elements, as well as Sr, Nd, and Pb isotopes) on volcanic rocks from the Tepoztlán Formation in the central Transmexican Volcanic Belt. The Tepoztlán Formation is up to 800 m thick and comprises a wide range of primary volcanic rocks (lavas, pyroclastic density current deposits, pyroclastic fall deposits), and their secondary reworked products due to mass flow (lahars) and fluvial processes. Magnetostratigraphy combined with K/Ar and Ar/Ar geochronology suggests an age of Early Miocene (23-19 Ma) for this Formation. Lava flows, pyroclastic rocks, dykes and volcanic clasts range from basaltic andesite to rhyolite, with a clear dominance of andesites and dacites. All samples are subalkaline and hy-normative. These rocks show homogeneous REE patterns with LREE enrichment and higher LILE concentrations with respect to HFSE, notably the typical negative anomaly of Nb, Ta, and Ti, suggesting a subduction-related magma genesis. Major and trace element concentrations show that either assimilation of heterogeneous continental crust or crustal recycling by subduction erosion and fractional crystallization are important processes in the evolution of the Tepoztlán Formation magmas. Isotopic compositions of the Tepoztlán Formation samples range from (87Sr/86Sr)t = 0.703693 to 0.704355 and (143Nd/144Nd)t = 0.512751 to 0.512882, falling within the mantle array. All geochemical characteristics indicate that these rocks originated from a heterogeneous mantle, modified and evolved through assimilation of country rock and fractional crystallization in the upper crust.

  4. Properties of middle-late Proterozoic volcanic rocks in South Qinling and the Precambrian continental break-up

    夏林圻; 夏祖春; 徐学义


    In South Qinling, the volcanic series of the middle-late Proterozoic Yunxi Group, Yaolinghe Group, Xi.xiang Group and Bikou Group have characteristics of the continental rift volcanic rocks or continental flood basalts and are formed in continental intraplate tensional setting. The enrichment of incompatible elements, high εNd values and low-medium 87Sr/86Sr initial ratios of these volcanic rocks indicate that they were derived from asthenospheric plume. Under the action of the intense pull-apart in lithosphere, the mantle plume upwelled, quickly decompressed and melted, and finally produced magma. This tensional process made the continental crust break and eventually led to an oceanic basin in late Proterozoic. The middle-late Proterozoic volcanism is a precursor of Precarabrian continental break-up in the South Qinling.

  5. Geochemical characteristics and petrogenesis of volcanic rocks from Baiyingaolao Formation in northeastern Hailar Basin

    LI Xu; SUN Deyou; GOU Jun


    The volcanic rocks from Baiyingaolao Formation in the northeastern Hailar Basin are mainly com-posed of rhyolite and trachydacite.U-Pb dating of zircon shows these volcanic rocks formed in Early Cretaceous (128-124 Ma).Geochemical data indicate that they are sub-alkaline series in composition and rich in alkali and potassium.All samples have similar rare earth element patterns characterized by high total rare earth ele-ments contents (∑REE=113.96 ×10 -6-204.33 ×10 -6),significant fractionation of heavy and light rare earth elements (∑LREE/∑HREE =3.10-4.52)with middle negative Eu anomalies (δEu=0.46-0.76). The trace elements are characterized by enrichment in large-ion lithophile elements such as K,Rb,LREE and depletion in high field strength elements e.g.Nb,Ta,HREE,P and Ti,while enriched in Th and U.Rhyolite and trachydacite contain low initial 87Sr/86Sr ratios (0.704 9-0.7 053)and positive εNd(t)values (ca.4. 15).These data suggest that the magma of rhyolite and trachydacite were derived from mafic lower crust newly accreted from mantle,with the evolutional trend of comagmatic fractional crystallization.

  6. Hot climate inhibits volcanism on Venus: Constraints from rock deformation experiments and argon isotope geochemistry

    Mikhail, Sami; Heap, Michael J.


    The disparate evolution of sibling planets Earth and Venus has left them markedly different. Venus' hot (460 °C) surface is dry and has a hypsometry with a very low standard deviation, whereas Earth's average temperature is 4 °C and the surface is wet and has a pronounced bimodal hypsometry. Counterintuitively, despite the hot Venusian climate, the rate of intraplate volcano formation is an order of magnitude lower than that of Earth. Here we compile and analyse rock deformation and atmospheric argon isotope data to offer an explanation for the relative contrast in volcanic flux between Earth and Venus. By collating high-temperature, high-pressure rock deformation data for basalt, we provide a failure mechanism map to assess the depth of the brittle-ductile transition (BDT). These data suggest that the Venusian BDT likely exists between 2 and 12 km depth (for a range of thermal gradients), in stark contrast to the BDT for Earth, which we find to be at a depth of ∼25-27 km using the same method. The implications for planetary evolution are twofold. First, downflexing and sagging will result in the sinking of high-relief structures, due to the low flexural rigidity of the predominantly ductile Venusian crust, offering an explanation for the curious coronae features on the Venusian surface. Second, magma delivery to the surface-the most efficient mechanism for which is flow along fractures (dykes; i.e., brittle deformation)-will be inhibited on Venus. Instead, we infer that magmas must stall and pond in the ductile Venusian crust. If true, a greater proportion of magmatism on Venus should result in intrusion rather than extrusion, relative to Earth. This predicted lower volcanic flux on Venus, relative to Earth, is supported by atmospheric argon isotope data: we argue here that the anomalously unradiogenic present-day atmospheric 40Ar/36Ar ratio for Venus (compared with Earth) must reflect major differences in 40Ar degassing, primarily driven by volcanism. Indeed

  7. Tectonic implications of paleomagnetic poles from Lower Tertiary Volcanic Rocks, south central Alaska

    Hillhouse, John W.; Grommé, C. Sherman; Csejtey, Bela, Jr.


    We have determined the paleolatitude of lower Tertiary volcanic rocks in southern Alaska to measure possible poleward translation of the Wrangellia and the Peninsular terranes after 50 m.y. ago. Previous paleomagnetic studies have shown that in Triassic and Jurassic time these terranes were located near the equator and have moved at least 3000 km poleward relative to the North American craton. Our sample localities are in the northern Talkeetna Mountains in mildly deformed andesite and dacite flows (50.4, 51.3, 53.9, and 56.3 m.y. by K-Ar) that overlap Lower Cretaceous flysch, Lower Permian volcanic rocks of Wrangellia, and Upper Triassic pillow basalt of the Susitna terrane. Results from 26 cooling units (23 of reversed polarity and 3 of normal polarity) give a mean paleomagnetic pole at 69.5°N, 179.6°E, α95 = 12.2°. Stratigraphic sections from opposite limbs of a syncline yield directional paths that pass the fold test, satisfying a necessary condition for primary origin of the magnetization. The corresponding mean paleolatitude (76°N) of the northern Talkeetna Mountains is 8°±10° higher than the latitude predicted from the Eocene reference pole for North America. Therefore, northward drift of the Talkeetna superterrane, which is the amalgamation of the Wrangellia and Peninsular terranes during and after Middle Jurassic time, was probably complete by 50 m.y. ago. Our results are consistent with paleomagnetic poles from uppermost Cretaceous and Paleocene volcanic sequences in Denali National Park, the Lake Clark region, northern Bristol Bay region, and near McGrath. These poles generally lie south of the cratonic poles, suggesting that the region between the Kaltag, Bruin Bay, and Castle Mountain faults has rotated counterclockwise relative to North America since the early Eocene.

  8. Petrology, Magnetic susceptibility, Tectonic setting and mineralization associated with Plutonic and Volcanic Rocks, Eastern Bajestan and Taherabad, Iran

    Malihe Ghoorchi


    Full Text Available Study area is located in district of Bajestan and Ferdows cities, NE of Iran. Structurally, this area is part of Lut block. The oldest exposed rocks, to the north of intrusive rocks and in Eastern Bajestan, are meta-chert, slate, quartzite, thin-bedded crystalline limestone and meta-argillite. The sedimentary units are: Sardar Formation (Carboniferous, Jamal Formation (Permian, Sorkh Shale and Shotori Formations (Triassic, carbonateous rocks (Cretaceous and lithostratigraphically equivalent to Kerman conglomerate (Cretaceous-Paleocene are exposed in this area. Based on relative age, magmatism in eastern Bajestan and Taherabad started after Late Cretaceous and it has been active and repeated during Tertiary time. At least, three episodes of volcanic activities are recognized in this area. The first stage was mainly volcanic flow with mafic composition and minor intermediate. The second episode was mainly intermediate in composition. The third stage was changed to acid-intermediate in composition. Since the plutonic rocks intruded the volcanic rocks, therefore they may be Oligo-Miocene age. Bajestan intrusive rocks are granite-granodiorite-quartz monzonite. Taherabad intrusive rocks are diorite-quartz diorite- monzonite-latite. Bajestan intrusive rocks are reduced type (ilmenite series and Taherabad intrusive rocks are oxidized type (magnetite series.Based on geochemical analysis including trace elements, REE and isotopic data, Bajestan intrusive rocks formed in continental collision zone and the magma has crustal origin. Taherabad intrusive rocks were formed in subduction zone and magma originated from oceanic crust. Taherabad intrusive rock has exploration potential for Cu-Au and pb.

  9. Discovery and Significance of High CH4 Primary Fluid Inclusions in Reservoir Volcanic Rocks of the Songliao Basin, NE China

    WANG Pujun; HOU Qijun; WANG Keyong; CHEN Shumin; CHENG Rihui; LIU Wanzhu; LI Quanlin


    Comparing compositions of the fluid inclusions in volcanic rocks to the contents and isotopes of the gases in corresponding volcanic reservoirs using microthermometry, Raman microspectroscopy and mass spectrum analysis, we found that: (1) up to 82 mole% methane exists in the primary inclusions hosted in the reservoir volcanic rocks; (2) high CH4 inclusions recognized in the volcanic rocks correspond to CH4-bearing CO2 reservoirs that are rich in helium and with a high 3He/4He ratio and which show reversed order of δ13C in alkane; (3) in gas reservoirs of such abiotic methane (>80%)and a mix of CH4 and CO2, the enclosed content of CH4 in the volcanic inclusions is usually below 42mole%, and the reversed order of δ13C in alkane is sometimes irregular in the corresponding gas pools;(4) a glassy inclusion with a homogeneous temperature over 900℃ also contains a small portion of CH4although predominantly CO2. This affinity between gas pool and content of inclusion in the same volcanic reservoirs demonstrates that magma-originated gases, both CH4 and CO2, have contributed significantly to the corresponding gas pools and that the assumed hydrocarbon budget of the bulk earth might be much larger than conventionally supposed.

  10. Petrology, Geochemistry and Nd-Sr-Pb Isotopic Properties of Volcanic Rocks in Daheishan Island, Penglai, Shandong Province

    Fu Yongtao; Li Anchun


    The major elements, trace elements, K-Ar age and Sr-Nd-Pb isotopic systems of the Cenozoic volcanic rocks in Daheishan Island and Cishan, Penglai, Shandong Province are measured. The volcanic rocks ( olivine-nephelinite and nepheline-basanite ) in Daheishan Island erupted periodically in an interval of 0.32 Ma, from 8.72 Ma, 8.39 Ma, 8.08 Ma to 7.73 Ma. The volcanic rocks are all rich in light REEs. They are similar to the OIB-type alkali basalt in the trace elements normalized model by primordial mantle: rich in high field elements such as Nb and Ta, and imcompatible elements such as Cs, Rb, Ba, Th, U. The volcanic rocks show a depletion of K and Rb elements. It is suggested by the trace elements that the olivine-nephelinite in Daheishan Island is originated from deep resources under the continental mantle. ε Nd (0) values of the volcanic rocks in Daheishan Island and Cisban are 5.31 ~ 8.51 and 7.33 respectively, suggesting that the volcanic rocks are from the depleted mantle resources, which have higher Sm/Nd ratios than the CHUR. 143Nd /144Nd ratios of Daheishan Island olivine-nephelinite and Cishan alkali basalts are 0.512 910 ~ 0.513 074 and 0.513 014 respectively. The 87Sr /86Sr of Daheishan Island volcanic rocks are lower than that of Cishan, 0.703 427 ~ 0.703 482 and 0.703 895 respectively. The Daheishan Island olivinenephelinite has the Pb isotopic values as follows: 206Pb /204pb = 18.028 9 ~ 17.972 8, 207Pb /204pb= 15.435 8 ~ 15.402 2 and 208Pb /204Pb = 38.087 6 ~ 37.997 5, lower than those of Cishan basanite. The Cishan basanite has 206Pb /204pb = 18.240 1, 207Pb /204Pb = 15.564 5 and 208Pb /204pb = 38.535. The authors suggest that the olivine-nephelinite in Daheishan Island is similar to the E-type MORB or Hawaii OIB, and the alkali basalts in Cishan similar to the Kerguelen OIB. The dominant mantle components of DM+PREMA and perhaps DM ( Dupal type ) are the dominant mantle components for volcanic rocks in Daheishan Island and Cishan. The

  11. Potash-rich volcanic rocks and lamprophyres in western Shandong Province: 40Ar39Ar dating and source tracing


    Highly precise 40Ar39Ar dating results demonstrate that the ages of potash-rich volcanic rocks in western Shandong Province are 114.7-124.3 Ma, and that of the lamprophyres is 119.6 Ma. The potash-rich volcanic rocks have relatively high (87Sr/86Sr)i ratios ( 0.708715-0.711418)and distinctly negative εNd values ( -11.47 - -17.54), and are enriched in radiogenic lead (206Pb/204pb=17.341-17.622,207pb/204Pb=15.525-15.538, 208Pb/204pb = 37.563-37.684).Similarly, the lamprophyres also have quite low εNd values ( -11.57 - -19.64). Based on the fact that the Sr, Nd and Pb isotopic compositions of potash-rich volcanic rocks are very consistent with that of the clinopyroxene separates, and by integrating comprehensive analyses of their tectonic settings,and extensive comparisons of the Sr, Nd isotopic compositions with that of the related simultaneous rocks, it is concluded that the potash-rich volcanic rocks and lamprophyres in western Shandong Province were most possibly derived from the partial melting of enriched mantle which was caused by source contamination and metasomatism of subducted continental crustal materials.``

  12. In situ Laser Induced Breakdown Spectroscopy as a tool to discriminate volcanic rocks and magmatic series, Iceland

    Roux, C.P.M., E-mail: [Laboratoire interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, BP 47 870, F-21078 Dijon Cedex (France); Rakovský, J.; Musset, O. [Laboratoire interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, BP 47 870, F-21078 Dijon Cedex (France); Monna, F. [Laboratoire ARTéHIS, UMR 6298 CNRS-Université de Bourgogne, 6 Boulevard Gabriel, F-21000 Dijon (France); Buoncristiani, J.-F.; Pellenard, P.; Thomazo, C. [Laboratoire Biogéosciences, UMR 6282 CNRS-Université de Bourgogne, 6 Boulevard Gabriel, F-21000 Dijon (France)


    This study evaluates the potentialities of a lab-made pLIBS (portable Laser-Induced Breakdown Spectroscopy) to sort volcanic rocks belonging to various magmatic series. An in-situ chemical analysis of 19 atomic lines, including Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, Na, Si, Sr and Ti, from 21 sampled rocks was performed during a field exploration in Iceland. Iceland was chosen both for the various typologies of volcanic rocks and the rugged conditions in the field in order to test the sturdiness of the pLIPS. Elemental compositions were also measured using laboratory ICP-AES measurements on the same samples. Based on these latter results, which can be used to identify three different groups of volcanic rocks, a classification model was built in order to sort pLIBS data and to categorize unknown samples. Using a reliable statistical scheme applied to LIBS compositional data, the classification capability of the pLIBS system is clearly demonstrated (90–100% success rate). Although this prototype does not provide quantitative measurements, its use should be of particular interest for future geological field investigations. - Highlights: • Portable LIBS applied to field geology • Fast semi-quantitative geochemical analysis of volcanic rocks and magmatic series • Discriminant analysis and statistical treatments for LIBS compositional data.

  13. Textural and Mineralogical Analysis of Volcanic Rocks by µ-XRF Mapping.

    Germinario, Luigi; Cossio, Roberto; Maritan, Lara; Borghi, Alessandro; Mazzoli, Claudio


    In this study, µ-XRF was applied as a novel surface technique for quick acquisition of elemental X-ray maps of rocks, image analysis of which provides quantitative information on texture and rock-forming minerals. Bench-top µ-XRF is cost-effective, fast, and non-destructive, can be applied to both large (up to a few tens of cm) and fragile samples, and yields major and trace element analysis with good sensitivity. Here, X-ray mapping was performed with a resolution of 103.5 µm and spot size of 30 µm over sample areas of about 5×4 cm of Euganean trachyte, a volcanic porphyritic rock from the Euganean Hills (NE Italy) traditionally used in cultural heritage. The relative abundance of phenocrysts and groundmass, as well as the size and shape of the various mineral phases, were obtained from image analysis of the elemental maps. The quantified petrographic features allowed identification of various extraction sites, revealing an objective method for archaeometric provenance studies exploiting µ-XRF imaging.

  14. A study on the geochemical characteristics of Upper Permian continental marginal arc volcanic rocks in the northern segment of South Lancangjiang Belt

    SHEN Shangyue; FENG Qinglai; WEI Qirong; ZHANG Zhibin; ZHANG Hu


    Geochemical characteristics of the Upper Permian ( P2 ) continental marginal arc volcanic rocks are described, which have been found recently around the areas of Xiaodingxi and Zangli on the eastern side of the Yunxian-Lincang granite, in terms of rock assemblage, petrochemistry, REE, trace elements, Pb isotopes, geotectonic environment and so on. The volcanic rock assemblage is dominated by basalt-andesite-dacite, with minor trachyte andecite-trachyte; the volcanic rock series is predominated by the calc-alkaline series, with minor tholleiite series and alkaline series rocks; the volcanic rocks are characterized by high Al2O3 and low TiO2 , with K2O contents showing extremely strong polarity; the REE distribution patterns are characterized by LREE enrichment and right-inclined type; trace elements and large cation elements are highly enriched, Ti and Cr are depleted, and P and Nb are partially depleted; the Pb composition is of the Gondwana type; the petrochemical points mostly fall within the field of island-arc volcanic rocks, in consistency with the projection of data points of continental marginal volcanic rocks in the southern segment of the South Lancangjiang Belt and the North Lancangjiang Belt. This continental marginal arc volcanic rock belt, together with the ocean-ridge and ocean-island volcanic rocks and ophiolites in the Changning-Menglian Belt, constitute the ocean-ridge volcanic rock, ophiolite-arc rock-magmatic rock belts which are distributed in pairs, indicating that the Lancangjiang oceanic crust subducted eastwards. This result is of great importance in constraining the evolution of the paleo-Tethys in the Lancangjiang Belt.

  15. Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications

    Wang, Chao; Ding, Lin; Liu, Zhi-Chao; Zhang, Li-Yun; Yue, Ya-Hui


    Limited geochronological and geochemical data from Early Cretaceous igneous rocks of the Gangdese Belt have resulted in a dispute regarding the subduction history of Neo-Tethyan Ocean. To approach this issue, we performed detailed in-situ zircon U-Pb and Hf isotopic, whole-rock elemental and Sr-Nd isotopic analyses on Late Mesozoic volcanic rocks exposed in the Liqiongda area, southern Lhasa terrane. These volcanic rocks are calc-alkaline series, dominated by basalts, basaltic andesites, and subordinate rhyolites, with a bimodal suite. The LA-ICPMS zircon U-Pb dating results of the basaltic andesites and rhyolites indicate that these volcanic rocks erupted during the Early Cretaceous (137-130 Ma). The basaltic rocks are high-alumina (average > 17 wt.%), enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), showing subduction-related characteristics. They display highly positive zircon εHf(t) values (+ 10.0 to + 16.3) and whole-rock εNd(t) values (+ 5.38 to + 7.47). The silicic suite is characterized by low Al2O3 (extracted from a source metasomatized by slab-derived components for the petrogenesis of mafic rocks, whereas the subsequent mafic magma underplating triggered partial melting of the juvenile crust to generate acidic magma. Our results confirm the presence of Early Cretaceous volcanism in the southern Lhasa terrane. Combined with the distribution of the contemporary magmatism, deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo-Tethyan oceanic lithosphere was flat-lying beneath the Lhasa terrane during the Early Cretaceous.

  16. Nature and origin of mineral coatings on volcanic rocks of the Black Mountain, Stonewall Mountain, and Kane Springs Wash volcanic centers, Southern Nevada

    Taranik, James V.; Hsu, Liang C.; Spatz, David


    Comparative lab spectra and Thematic Mapper imagery investigations at 3 Tertiary calderas in southern Nevada indicate that desert varnish is absorbant relative to underlying host rocks below about 0.7 to 1.3 microns, depending on mafic affinity of the sample, but less absorbant than mafic host rocks at higher wavelengths. Desert varnish occurs chiefly as thin impregnating films. Distribution of significant varnish accumulations is sparse and localized, occurring chiefly in surface recesses. These relationships result in the longer wavelength bands and high 5/2 values over felsic units with extensive desert varnish coatings. These lithologic, petrochemical, and desert varnish controlled spectral responses lead to characteristic TM band relationships which tend to correlate with conventionally mappable geologic formations. The concept of a Rock-Varnish Index (RVI) is introduced to help distinguish rocks with a potentially detectable varnish. Felsic rocks have a high RVI, and those with extensive desert varnish behave differently, spectrally, from those without extensive varnish. The spectrally distinctive volcanic formations at Stonewall Mountain provide excellent statistical class segregation on supervised classification images. A binary decision rule flow-diagram is presented to aid TM imagery analysis over volcanic terrane in semi-arid environments.

  17. Exotic lamproites or normal ultrapotassic rocks? The Late Miocene volcanic rocks from Kef Hahouner, NE Algeria, in the frame of the circum-Mediterranean lamproites

    Lustrino, Michele; Agostini, Samuele; Chalal, Youcef; Fedele, Lorenzo; Stagno, Vincenzo; Colombi, Francesco; Bouguerra, Abdellah


    The late Miocene (11-9 Ma) volcanic rocks of Kef Hahouner, 40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O > 3 wt.%, K2O/Na2O > 2.5, MgO > 3-4 wt.%, SiO2 < 55-57 wt.% and SiO2/K2O < 15. All the investigated samples show primitive mantle-normalized multi-element patterns typical of orogenic (arc-type) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb = 37-59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874-0.70961, 143Nd/144Nd = 0.51222-0.51223, 206Pb/204Pb = 18.54-18.60, 207Pb/204Pb = 15.62-15.70 and 208Pb/204Pb = 38.88-39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6-21.4 wt.%), and the absence of phlogopite are all at inconsistent with the definition of lamproite. We reviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as "normal" potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into "orogenic" and "anorogenic" types appears questionable.

  18. Newly developed evidence for the original Tethysan island-arc volcanic rocks in the southern segment of the South Lancangjiang Belt


    This paper re-describes the characteristics of pre-Ordovician (Pt3) metamorphic volcanic rocks in the Huimin-Manlai region of Yunnan Province from the aspects of petrographic characteristics, rock assemblage, petrochemistry, REE, trace elements, lead isotopes and geotectonic setting. The metamorphic volcanic rocks maintain blasto-intergranular and blasto-andesitic textures; the volcanic rocks are characterized by a basalt-andesite-dacite assemblage; the volcanic rocks are basic-intermediate-intermediate-acid in chemical composition, belonging to semi-alkaline rocks, with calc-alkaline series and tholeiite series coexisting, and they are characterized by low TiO2 contents; their REE distribution patterns are of the LREE-enrichment right-inclined type; the volcanic rocks are enriched in large cation elements and commonly enriched in Th and partly depleted in Ti, Cr and P, belonging to the Gondwana type as viewed from their Pb isotopic composition; petrochemically the data points fall mostly within the field of island-arc volcanic rocks. All these characteristics provided new evidence for the existence of original Tethysan island-arc volcanic rocks in the region studied.

  19. Carboniferous Bimodal Volcanic Rocks and Their Plate Tectonic Setting,Hainan Island

    夏邦栋; 施光宇; 等


    The Carboniferous volcanic rocks in western Hainan Island consist of a series of oceanic tholeite and rhyoporphyrite,showing bimodal nature.Similar geochemical characters,in terms of abun-daces and relative rations of incompatible elements and REE and the REE patterns,between the basalt and continental rift-associated tholeiite indicate the occurrence of Late Paleozoic rifting in the area.The basaltic magma,with a low degree of evolution,was originated from deep mantle,show-ing contamination by low crustal material.The rhyolite is thought to be formed from partial melting of the continental crust by higher thermal flow in a rift environment rather than from fractional crystallization of a basaltic magma.

  20. Discovery of the granulite xenoliths in Cenozoic volcanic rocks from Hoh Xil, Tibetan plateau

    LAI Shaocong; YI Haisheng; LIN Jinhui


    Two-pyroxene granulite and clinopyroxene granulite xenoliths have been recently discovered in the Late Paleogene to Neogene volcanic rocks (with ages in the range of 4.27~44.60 Ma) that outcropped in Hoh Xil, central Tibetan plateau. Based on the electron microprobe analysis data, the xenoliths provide constraints for the formation equilibrium temperatures of the two-pyroxene granulite being about 783 to 818 ℃ as determined by two-pyroxene thermometry and the forming pressure of the clinopyroxene granulite being about 0.845 to 0.858 GPa that is equivalent to 27.9~28.3 km depth respectively. It indicates that these granulite xenoliths represent the samples from the middle part of the thickened Tibetan crust. This discovery is important and significant to making further discussion on the component and thermal regime of the deep crust of the Tibetan plateau.

  1. Volcanological, petrographical and geochemical characteristics of Late Cretaceous volcanic rocks around Borçka-Artvin region (NE Turkey)

    Baser, Rasim; Aydin, Faruk; Oguz, Simge


    This study presents volcanological, petrographical and geochemical data for late Cretaceous volcanic rocks from the Borçka-Artvin region (NE Turkey) in order to investigate their origin and magmatic evolution. Based on the previous ages and recent field studies, the late Cretaceous time in the study area is characterized by two different bimodal volcanic periods. The first bimodal period of the late Cretaceous volcanism is mainly represented by mafic rock series (basaltic-basaltic andesitic pillow lavas and hyaloclastites) in the lower part, and felsic rock series (dacitic lavas, hyaloclastites, and pyrite-bearing tuffs) in the upper part. The second bimodal period of the late Cretaceous volcanism begins with mafic rock suites (basaltic-andesitic lavas and dikes-sills) and grades upward into felsic rock suites (biotite-bearing rhyolitic lavas and hyaloclastites), which are intercalated with hyaloclastites and red pelagic limestones. All volcano-sedimentary units are covered by Late Campanian-Paleocene clayey limestones and biomicrites with lesser calciturbidites. The mafic volcanic series of the study area, which comprise basaltic and andesitic rocks, generally show amygdaloidal and aphyric to porphyritic texture with phenocrysts of calcic to sodic plagioclase and augite in a hyalopilitic matrix of plag+cpx+mag. Zircon and magnetite are sometimes observed as accessory minerals, whereas chlorite, epidote and calcite are typical alteration products. On the other hand, the felsic volcanic series consisting of dacitic and rhyolitic rocks mostly display porphyritic and glomeroporphyritic textures with predominant feldspar, quartz and some biotite phenocrysts. The microgranular to felsophyric groundmass is mainly composed of aphanitic plagioclase, K-feldspar and quartz. Accessory minerals such as zircon, apatite and magnetite are common. Typical alteration products are sericite and clay minerals. Late Cretaceous Artvin-Borçka bimodal rock series generally display a

  2. K-Ar geochronology of the late cenozoic volcanic rocks of the Cordillera Occidental, southernmost Peru

    Tosdal, Richard M.; Farrar, Edward; Clark, Alan H.


    Twenty-four K-Ar radiometric ages are presented for late Cenozoic continental volcanic rocks of the Cordillera Occidental of southernmost Perú (lat. 16° 57'-17° 36'S). Rhyodacitic ignimbrite eruptions began in this transect during the Late Oligocene and continued episodically through the Miocene. The development of andesitic-dacitic strato volcanoes was initiated in the Pliocene and continues to the present. The earliest ignimbrite flows (25.3-22.7 Ma) are intercalated in the upper, coarsely-elastic member of the Moquegua Formation and demonstrate that this sedimentary unit accumulated in a trough, parallel to Andean tectonic trends, largely in the Oligocene. More voluminous ash-flow eruptions prevailed in the Early Miocene (22.8-17.6 Ma) and formed the extensively preserved Huaylillas Formation. This episode was coeval with a major phase of Andean uplift, and the pyroclastics overlie an erosional surface of regional extent incised into a Paleogene volcano-plutonic arc terrain. An age span of 14.2-8.9 Ma (mid-Late Miocene) is indicated for the younger Chuntacala Formation, which again comprises felsic ignimbrite flows, largely restricted to valleys incised into the pre-Huaylillas Formation lithologies, and, at lower altitudes, an extensive aggradational elastic facies. The youngest areally extensive ignimbrites, constituting the Sencca Formation, were extruded during the Late Miocene. In the earliest Pliocene, the ignimbrites were succeeded by more voluminous calcalkaline, intermediate flows which generated numerous large and small stratovolcanoes; these range in age from 5.3 to 1.6 Ma. Present-day, or Holocene, volcanism is restricted to several large stratovolcanoes which had begun their development during the Pleistocene (by 0.7 Ma). The late Oligocene/Early Miocene (ca. 22-23 Ma) reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the Cordilleras Occidental and Oriental of the Central Andes.

  3. Geochemistry of middle Tertiary volcanic rocks in the northern Aquarius Mountains, west-central Arizona

    Simmons, A.M.; Haxel, G.B.


    The northern Aquarius Mountains volcanic field ([approximately]50km east of Kingman) covers an area of 400 km[sup 2], bounded by upper Trout Creek (S), the Truxton Valley (N), the Big Sandy Valley (W), and Cross Mountain (E). The volcanic sequence rests upon a pre-middle Eocene erosional surface. The lowest units is a 250 m-thick unit of rhyolitic pyroclastic breccias and airfall tuffs. Successively younger units are: basanite flows and cinder cones; hornblende latite flows and domes; porphyritic dacite flows, domes, and breccias; alkali basalt intrusions; and low-silica rhyolite domes and small high=silica rhyolite flows. Dacite is volumetrically dominant, and erupted primarily from vents in and around Cedar Basin (Penitentiary Mtn 7.5[prime] quad.). Other geologists have obtained K-Ar dates [approximately]24--20 Ma for the basanites and latites. The alkali basalts, latites, dacites, and rhyolites evidently constitute a genetically-related high-K to shoshonitic calcalkaline suite with chemistry typical of subduction-related magmatism: enrichment in LILE and LREE, and depletion of Nb and Ta relative to K and La and of Ti relative to Hf and Yb. Each rock type is unique and distinguishable in K/Rb and Rb/Sr. The basanites are primitive (mg=0.75--0.78), have intraplate affinities (La/Nb[<=]1), and show consistent and distinctive depletion of K relative to the other LILE. The presence of these basanites in an early Miocene volcanic sequence is unusual or unexpected, as they predate (by [approximately]10 m.y.) the regional eruption of asthenosphere-derived basalts associated with Basin-and-Range extension.

  4. Physical Volcanology and Hazard Analysis of a Young Volcanic Field: Black Rock Desert, Utah, USA

    Hintz, A. R.


    The Black Rock Desert volcanic field, located in west-central Utah, consists of ~30 small-volume monogenetic volcanoes with compositions ranging from small rhyolite domes to large basaltic lava flow fields. The field has exhibited bimodal volcanism for > 9 Ma with the most recent eruption of Ice Springs volcano ˜ 600 yrs ago. Together this eruptive history along with ongoing geothermal activity attests to the usefulness of a hazard assessment. The likelihood of a future eruption in this area has been calculated to be ˜ 8% over the next 1 Ka (95% confidence). However, many aspects of this field such as the explosivity and nature of many of these eruptions are not well known. The physical volcanology of the Tabernacle Hill volcano, suggests a complicated episodic eruption that may have lasted up to 50 yrs. The initial phreatomagmatic eruptions at Tabernacle Hill are reported to have begun ~14 Ka. This initial eruptive phase produced a tuff cone approximately 150 m high and 1.5 km in diameter with distinct bedding layers. Recent mapping and sampling of Tabernacle Hill's lava field, tuff cone and intra-crater deposits were aimed at better constraining the eruptive history, physical volcanology, and explosive energy associated with this eruption. Blocks ejected during the eruption were mapped and analyzed to yield minimum muzzle velocities of 60 - 70 meters per second. These velocities were used in conjunction with an estimated shallow depth of explosion to calculate an energy yield of ˜ 0.5 kT.

  5. Paleomagnetic data from Early Cretaceous volcanic rocks of West Liaoning:Evidence for intracontinental rotation


    Detailed rock magnetic studies of 55 lavas from Yixian and Fuxin area, West Liaoning, show the primary carriers of remanence to be pseudo-single domain titanomagnetite. K/Ar dating indicates that the volcanic sequence spans 93 to 133 Ma. Stepwise thermal demagnetization successfully isolated well-defined characteristic magnetization (ChRM) in all lavas thermal-treated above 250℃. The mean paleodirections are D/I = 5.9°/58.8° (α95 = 2.9°) and D/I =179.2°/-59.9° (α95 = 5.2°) for 27 normally magnetized flows and 28 reversibly magnetized flows, respectively. It indicates that since the Early Cretaceous there is no significant horizontal movement and rotation between the Yixian-Fuxin area and Eurasia. However, Korea Peninsula may have undergone a clockwise rotation of 33.9° relative to the Yixian-Fuxin area during the Cretaceous. On the basis of characteristics of hotspot origins (core-mantle boundary or upper mantle), the clockwise rotation of Korea Peninsula relative to Eurasia is assumed to be mainly caused by an extensional force in the crust of eastern China, which was corresponding to intensive surface volcanic activities in this area.

  6. SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications

    XIE Guiqing; MAO Jingwen; LI Ruiling; ZHOU Shaodong; YE Huishou; YAN Quanren; ZHANG Zusong


    The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower valley of the Yangtze River, is one of the important volcanic basins in East China. Volcanic rocks in the Jinniu Basin are distributed mainly in the Majiashan Formation, the Lingxiang Formation and the Dasi Formation, consisting of rhyolite, basalt and basaltic andesite, (trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively, in which the Dasi volcanism is volumetrically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indicate that there exist a large amount of magmatic zircons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128±1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios, this age is considered to represent the crystallization time of the Dasi volcanism. The volcanic rocks in the Dasi, Majiashan and Lingxiang Formations share similar trace element and REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geology, it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Cretaceous, just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also suggested for tectonic regime in this region in the Cretaceous Period.

  7. SHRIMP dating of volcanic rock in the Zhangwu-Heishan area, West Liaoning province, China: Its relationship with coal-bearing strata

    Cai Houan; Xu Debin; Li Baofang; Shao Longyi


    The Zhangwu-Heishan area is located to the east of the Fuxin-Yixian Basin and is mostly covered with volcanic rock.At various periods,different geologists had varying opinions about their age and periods of volcanic eruptions,especially on sequences between volcanic rock and main coal-beating strata,which affect the direction of searching for coal,as well as prospecting the entire research area.During our study,we carried out detailed field investigations in this research area; observed and recorded the main representative outcrops of volcanic rock.We collected over 20 volcanic rock samples and tested the Sensitive High Resolution Ion Microprobe Ⅱ (SHRIMP Ⅱ) U-Pb isotope age of 11 samples.The age of our volcanic rock samples ranged between 56.0 ± 2.9 and 132.3 ± 2.3 Ma.After taking earlier investigations into consideration,we concluded that,except for a suite of paleogene olivine basalt,the volcanic rock in the Zhangwu-Heishan area is younger than the coal-beating Shahai Formation.It is therefore most unlikely to find coal seams equivalent to those of the early Cretaceous Shahai Formation in Fuxin Basin below volcanic rock.

  8. Discovery of Late Permian Jianshanzi volcanic rocks in Qinghe valley of northern Liaoning and its geologic significance

    Yuejun CHEN; Chunlin SUN; Yuewu SUN


    Jianshanzi volcanic rocks at Qinghe valley in the northern Liaoning were considered belonging to Early Proterozoic Gaojiayu Formation of Liaohe Group, or to Early Cambrian Beidagou Formation of Qinghezhen Group, or Middle-Late Proterozoic Shenjiapu Formation-Complex of Kaiyuan Group-complex. Dating the zircons from the dacite with schistosity is 2 506 Ma in method of U-Pb (SHRIMP). This evidence indicates the rocks may be referred to Late Permian in age. Discovery of the rocks is significant to re-recognize stratigraphic property of "Liaohe Group", regional geotectonic location and revolution of orogenic zone in Qinghe valley of the northern Liaoning.

  9. Geophysical, geochemical, mineralogical, and enivronmental data for rock samples collected in a mineralized volcanic environment, upper Animas River watershed, Colorado

    McCafferty, A.E.; Horton, R.J.; Stanton, M.R.; McDougal, R.R.; Fey, D.L.


    This report provides analyses of 90 rock samples collected in the upper Animas River watershed near Silverton, Colo., from 2001 to 2007. The samples are analyzed for geophysical, geochemical, mineralogical, and environmental rock properties of acid neutralizing capacity and net acid production. The database is derived from both published (n=68) and unpublished (n=32) data. New for all samples are geophysical measurements of electrical resistivity, density, and porosity. Rock samples were acquired from 12 geologic units that include key Tertiary volcanic and plutonic lithologies, all with varying degrees of alteration.

  10. Volcanostratigraphy, petrography and petrochemistry of Late Cretaceous volcanic rocks from the Görele area (Giresun, NE Turkey)

    Oguz, Simge; Aydin, Faruk; Baser, Rasim


    In this study, we have reported for lithological, petrographical and geochemical features of late Cretaceous volcanic rocks from the Çanakçı and the Karabörk areas in the south-eastern part of Görele (Giresun, NE Turkey) in order to investigate their origin and magmatic evolution. Based on the previous ages and recent volcano-stratigraphic studies, the late Cretaceous time in the study area is characterized by an intensive volcanic activity that occurred in two different periods. The first period of the late Cretaceous volcanism (Cenomanian-Santonian; 100-85 My), conformably overlain by Upper Jurassic-Lower Cretaceous massive carbonates (Berdiga Formation), is represented by bimodal units consisting of mainly mafic rock series (basaltic-andesitic lavas and hyaloclastites, dikes and sills) in the lower part (Çatak Formation), and felsic rock series (dacitic lavas and hyaloclastites, crystal- and pyrite-bearing tuffs) in the upper part (Kızılkaya Formation). The second period of the late Cretaceous volcanism (Santonian-Late Campanian; 85-75 Ma) is also represented by bimodal character and again begins with mafic rock suites (basaltic-basaltic andesitic lavas and hyaloclastites) in the lower part (Çağlayan Formation), and grades upward into felsic rock suites (biotite-bearing rhyolitic lavas, ignimbrites and hyaloclastites) through the upper part (Tirebolu Formation). These bimodal units are intercalated with volcanic conglomerates-sandstones, claystones, marl and red pelagic limestones throughout the volcanic sequence, and the felsic rock series have a special important due to hosting of volcanogenic massive sulfide deposits in the region. All volcano-sedimentary units are covered by Tonya Formation (Late Campanian-Paleocene) containing calciturbidites, biomicrites and clayey limestones. The mafic rocks in the two volcanic periods generally include basalt, basaltic andesite and minor andesite, whereas felsic volcanics of the first period mainly consists of

  11. Characteristics of Fault Zones in Volcanic Rocks Near Yucca Flat, Nevada Test Site, Nevada

    Donald Sweetkind; Ronald M. Drake II


    During 2005 and 2006, the USGS conducted geological studies of fault zones at surface outcrops at the Nevada Test Site. The objectives of these studies were to characterize fault geometry, identify the presence of fault splays, and understand the width and internal architecture of fault zones. Geologic investigations were conducted at surface exposures in upland areas adjacent to Yucca Flat, a basin in the northeastern part of the Nevada Test Site; these data serve as control points for the interpretation of the subsurface data collected at Yucca Flat by other USGS scientists. Fault zones in volcanic rocks near Yucca Flat differ in character and width as a result of differences in the degree of welding and alteration of the protolith, and amount of fault offset. Fault-related damage zones tend to scale with fault offset; damage zones associated with large-offset faults (>100 m) are many tens of meters wide, whereas damage zones associated with smaller-offset faults are generally a only a meter or two wide. Zeolitically-altered tuff develops moderate-sized damage zones whereas vitric nonwelded, bedded and airfall tuff have very minor damage zones, often consisting of the fault zone itself as a deformation band, with minor fault effect to the surrounding rock mass. These differences in fault geometry and fault zone architecture in surface analog sites can serve as a guide toward interpretation of high-resolution subsurface geophysical results from Yucca Flat.

  12. Mineralogy and geochemistry of xenoliths in the Eocene volcanic rocks from southwest of Jandaq

    Samineh Rajabi


    Full Text Available The Eocene volcanic rocks from the southwest of the Jandaq (Kuh-e-Godar-e-Siah, Central-East Iran micro-continent are andesitic basalt and andesite in composition. These rocks contain xenoliths with granulitic mineralogy. Mineral assemblage of these xenoliths is plagioclase + phlogopite + corundum + sillimanite + chlorite + phengite with granublastic, poiklioblastic and foliated textures in the pick metamorphic condition. Thermometry of phlogopite in these xenoliths suggests the average temperature 782oC. The characteristics of the xenoliths are consistent with the granolitic facies metamorphism of the Al-saturated Si-undersaturated crustal sediments at the lower crust condition. Melting of these granulites forms the magma which crystallized the S-type granitoids. Differentiation and crystallization of this magma causes the S-type granite formation. Therefore, the S-type granites in the study area are probably generated from melting of the granulites parts of which brought to the surface as xenoliths by Eocene magmatism in south of the Jandaq (Kuh-e-Godar-e-Siah. S-type granites in the study area are located along the Doruneh, Chupanan and Aeirakan faults in the Aeirakan area and Jandaq ophiolite. These granites are the source of uranium, thorium and uranium ore in southwest of the Aeirakan mountain.

  13. Crystallisation condition of the Quaternary basanites of volcanic centre Black Rock, monogenetic field Lunar Crater

    Turova, Mariia; Plechov, Pavel; Scherbakov, Vasily; Larin, Nikolay


    The Lunar Crater volcanic field is located in a tension zone Basin and Range Province (USA). This tension is connected with dives oceanic plate under the continental plate [1]. Lunar Crater consists of flows basalt, basanite, trachybasalt has a different age [2]. In this work we investigate the youngest rock - basanite. The basanite is highly crystalline consisting of about megacrysts (3-10 cm) 30-60 wt% phenocrysts ( 800-1500 µm) and microphenocrysts (100-800 µm) and 40-60% microlites (stress and style of tectonism of the Basin and Range province of the western United States //Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. - 1981. - T. 300. - №. 1454. - C. 407-434. 2. Wood, X., and Keinle, Y., 1990, Volcanoes of North America: Cambridge,United Kingdom, Cambridge University Press, 354 p. 3. Nimis P. Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems //Contributions to Mineralogy and Petrology. - 1999. - T. 135. - №. 1. - C. 62-74. 4. Ballhaus C., Berry R. F., Green D. H. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle //Contributions to Mineralogy and Petrology. - 1991. - T. 107. - №. 1. - C. 27-40.

  14. Characteristics of Fault Zones in Volcanic Rocks Near Yucca Flat, Nevada Test Site, Nevada

    Donald Sweetkind; Ronald M. Drake II


    During 2005 and 2006, the USGS conducted geological studies of fault zones at surface outcrops at the Nevada Test Site. The objectives of these studies were to characterize fault geometry, identify the presence of fault splays, and understand the width and internal architecture of fault zones. Geologic investigations were conducted at surface exposures in upland areas adjacent to Yucca Flat, a basin in the northeastern part of the Nevada Test Site; these data serve as control points for the interpretation of the subsurface data collected at Yucca Flat by other USGS scientists. Fault zones in volcanic rocks near Yucca Flat differ in character and width as a result of differences in the degree of welding and alteration of the protolith, and amount of fault offset. Fault-related damage zones tend to scale with fault offset; damage zones associated with large-offset faults (>100 m) are many tens of meters wide, whereas damage zones associated with smaller-offset faults are generally a only a meter or two wide. Zeolitically-altered tuff develops moderate-sized damage zones whereas vitric nonwelded, bedded and airfall tuff have very minor damage zones, often consisting of the fault zone itself as a deformation band, with minor fault effect to the surrounding rock mass. These differences in fault geometry and fault zone architecture in surface analog sites can serve as a guide toward interpretation of high-resolution subsurface geophysical results from Yucca Flat.

  15. Characteristics of Fault Zones in Volcanic Rocks Near Yucca Flat, Nevada Test Site, Nevada

    Sweetkind, Donald S.; Drake II, Ronald M.


    During 2005 and 2006, the USGS conducted geological studies of fault zones at surface outcrops at the Nevada Test Site. The objectives of these studies were to characterize fault geometry, identify the presence of fault splays, and understand the width and internal architecture of fault zones. Geologic investigations were conducted at surface exposures in upland areas adjacent to Yucca Flat, a basin in the northeastern part of the Nevada Test Site; these data serve as control points for the interpretation of the subsurface data collected at Yucca Flat by other USGS scientists. Fault zones in volcanic rocks near Yucca Flat differ in character and width as a result of differences in the degree of welding and alteration of the protolith, and amount of fault offset. Fault-related damage zones tend to scale with fault offset; damage zones associated with large-offset faults (>100 m) are many tens of meters wide, whereas damage zones associated with smaller-offset faults are generally a only a meter or two wide. Zeolitically-altered tuff develops moderate-sized damage zones whereas vitric nonwelded, bedded and airfall tuff have very minor damage zones, often consisting of the fault zone itself as a deformation band, with minor fault effect to the surrounding rock mass. These differences in fault geometry and fault zone architecture in surface analog sites can serve as a guide toward interpretation of high-resolution subsurface geophysical results from Yucca Flat.

  16. Cenozoic Adakite-type Volcanic Rocks in Qiangtang,Tibet and Its Significance

    LIU Shen; HU Ruizhong; FENG Caixia; CHI Xiaoguo; LI Cai; YANG Rihong; WANG Tianwu; JIN Wei


    Volcanic rocks in the study area, including dacite, trachyandesite and mugearite, belong to the intermediate-acid, high-K calc-alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that therocks are characterized by SiO2>59%, enrichment in Al2O3 (15.09-15.64%) and Na2O (>3.6%), high Sr (649-885 μg/g)and Sc, low Y contents (<17 μg/g), depletion in HREE (Yb<1.22 μg/g), (La/Yb)N>25, Sr/Y>40, MgO<3% (Mg#<0.35),weak Eu anomaly (Eu/Eu*=0.84-0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd andSr isotope data (87Sr/86Sr=0.7062-0.7079, 143Nd/144Nd=0.51166-0.51253, εNd= -18.61-0.02), show that the magmaresulted from partial melting (10%-40%) of newly underplated basaltic lower crust under high pressure (1-4 GPa), and thepetrogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will givean insight into the uplift mechanism of the Tibetan plateau.

  17. The role of pore fluids on deforming volcanic rocks: an experimental study

    Fazio, Marco; Benson, Philip; Vinciguerra, Sergio; Meredith, Philip


    Pore fluids play an important role on the process of the deformation of rocks. Not only it affects the mechanical properties and the elastic velocities of the material, but it is also responsible in the generation of a whole kind of seismicity, characterized by lower frequency and longer tail (i.e. Long Period, LP, and Hybrid events) than the Volcano-Tectonic (VT) signals, generated by simple shear. While great progress has been made in understanding VT events, LPs, Hybrid signals and the transition between these types of activity are not fully understood yet. This study, aiming in particular on the transition between VT and Hybrid events, shows the results of triaxial experiments on a volcanic rock, run both in dry and wet conditions, to better understand the role of the pore fluids on the final stage of the deformation tests, when the sample is approaching failure. This is achieved through a servo-controlled triaxial testing machine and a state-of the-art acoustic emissions (AEs) kit, composed by an array of 12 piezoelectric sensors surrounding the sample and by both a "triggered" unit, where the events are recorded only if a certain threshold is reached, and a "continuous" unit, where the data is recorded from the beginning to the end of the acquisition, fundamental when the AEs grow exponentially and the triggered unit cannot store at the same rate. The use of sensors of different dominant frequency allows us to better investigate the events occurring as the sample is approaching failure. In both conditions we observe a decrease of the dominant frequency of the seismic activity, due to two different processes: in dry conditions the coalescence of fractures, eventually leading to the major shear zone, creates relatively low-frequency VT events; the same occurs in wet conditions, but the movement of fluids, eased by the merging of the cracks, generates hybrid events. These two type of seismicity are then distinguished in terms of their source mechanism components

  18. Supracrustal rocks in the Kuovila area, Southern Finland: structural evolution, geochemical characteristics and the age of volcanism

    Pietari Skyttä


    Full Text Available The supracrustal rocks of the Kuovila area in the Palaeoproterozoic Svecofennian Uusimaa Belt, southern Finland, consist mainly of volcaniclastic rocks associated with banded iron formations (BIFs and marbles. Small ZnS and PbS mineralizations are occasionally located within the marbles. Some primary features are well preserved in the sedimentary and volcanic rocks, including lamination in tuffites and banded iron formations. Geochemical results show that the volcanism was bimodal and it mainly had volcanic arc affinity. Specific geochemical indicators suggesting a volcanic arc origin for the Kuovila volcanic rocks include: 1 Enrichment of LILE over the HFSE elements and 2 Distinctly low Nb and Ta contents in relation to Th, Ce and LREE. Geochemistry of the Kuovila area volcanic rocks is very similar to those of the Orijärvi and Kisko formations, located ~15 km NE of Kuovila. Felsic tuff in the Kuovila area was dated at 1891±4 Ma by the U-Pb system on zircons. Consequently volcanism was contemporaneous with magmatism in the adjacent Orijärvi area, thus representing the earliest identified volcanic stage in the southern Svecofennian Uusimaa Belt. Early deformation structures within the Kuovila area are suggested to relate to low-metamorphic or localized low-angle thrusting during D1. F1 folds were recumbent and the S1 cleavages are generally weak. Thrusting was followed by approximately N–S contraction with upright, peak-metamorphic F2 folding overprinting D1 structures and defining the Kuovila synform. Two separate intrusive phases include a synvolcanic granodiorite-diorite-gabbro association and a weakly S2-foliated syn-D2 granodiorite. Anatectic granites and associated migmatizing veins are absent, therefore suggesting that D2 pre-dates the ~1.84–1.82 Ga metamorphic event in the Southern Svecofennian Arc Complex (SSAC. D2 structuresin the Kuovila area are suggested to correlate with the early structures with associated axial planar

  19. Spectral characterization of volcanic rocks in the VIS-NIR for martian exploration

    De Angelis, Simone; Carli, Cristian; Manzari, Paola; De Sanctis, Maria Cristina; Capaccioni, Fabrizio


    Igneous effusive rocks cover much of the surface of Mars [1,2,3]. Initially only two types of lithologies were thought to constitute the Martian crust, i.e. a basaltic one and a more andesitic one [1,2], while more evolved lithologies were ruled out.Nevertheless a more complex situation is appearing in the last years. Recently several observations have highlighted the presence of evolved, acidic rocks. High-silica dacite units were identified in Syrtis Major caldera by thermal IR data [4]. Outcrops in Noachis Terra were interpreted as constituted of felsic (i.e. feldspar-rich) rocks essentially by the observation of a 1.3-µm spectral feature in CRISM data, attributed to Fe2+ in feldspars [5]. However different interpretations exist, invoking plagioclase-enriched basalts [6] rather than felsic products.The increasing of high-resolution and in-situ rover-based observations datasets and the changing of the initial paradigm justify a new systematic spectral study of igneous effusive rocks. In this work we focus on the spectral characterization of volcanic effusive rocks in the 0.35-2.5-µm range. We are carrying out measurements and spectral analyses on a wide ensemble of effusive samples, from mafic to sialic, with variable alkali contents, following the classification in the Total-Alkali-Silica diagram, and discussing the influence on spectral characteristics of different mineral assemblages and/or texture ([7], [8]). [1] Bandfield J.L., et al., Science, 287, 1626, 2000; [2] Christensen P.R., et al., J. Geophys. Res., 105, N.E4, 9609-9621, 2000; [3] Ehlmann B.L. & Edwards C.S., Annu. Rev. Earth Planet. Sci., 42, 291-315, 2014; [4] Christensen P.R., et al., Nature, 436, 504-509, 2005; [5] Wray J.J., et al., 44th LPSC, abs. n.3065, 2013; [6] Rogers A.D. & Nekvasil H., Geophys. Res. Lett., 42, 2619-2626, 2015; [7] Carli C. and Sgavetti M.,Icarus, 211, 1034–1048, 2011; [7] Carli C. et al., SGL, doi 10.1144/SP401.19, 2015.

  20. SHRIMP zircon U-Pb age and significance of Early Paleozoic volcanic rocks in East Kunlun orogenic belt, Qinghai Province, China


    Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419±5 Ma, and that of the meta-volcanic slice is 401± 6 Ma. These ages directly testify that there existed Early Paleozoic ocean-continent transform in East Kunlun, the basalt slice was formed in an extensional mid-ocean ridge setting and the meta-volcanic rock slice was formed in an extrusion subduction and collision setting. The inherited zircon age of 1734 Ma in volcanic rocks reflects that the base of East Kunlun may be Middle Proterozoic.

  1. Formation Conditions and Distribution Regularities of Oil—gas Pools in Tertiary Volcanic Rocks in Western Huimin Depression,Shandong Province

    刘泽容; 王永杰; 等


    The formation conditions and distribution regularities of oil-gas pools in volcanic rocks in western Huimin Depression have been studied in terms of geolgic,sesmic and well logging information,This paper discusses the types and lithofacies,development and distribution of Tertiary volcanic rocks in the area.The results demonstrate that volcanic activity occurred mainly during the period from the Sha-4 stage to the Guantao episode,i.e.,before the oil-generating period(before the end of the Guantao episode and the Minghuazhen episode).The activity did not destroy oil and gas formation and accumulation,but was favourable for the concentration of organic matter and its conversion to hydrocarbons;besides,volcanic rocks can serve as reservoir rocks and cap rocks,playing a role very similar to that of a syndepositional anticline,The volcanic rocks are distributed near the margins of the oil-generating depression;there are many secondary interstices in the rocks,which are connected with each other.These are the leading conditions for the formation of oil-generating period and their self-sealing or good combination with other cap rocks are important factors for forming volcanic rock-hosted oil and gas pools.The oil-gas pools associated with volcanic rocks in western Huimin are mainly distributed around the deep oil-generating depression,in the central up lift or the high structural levels on the margins of the depression.In particular,the sites where several faults cross are usually locatons where hith-yielding oil-gas pools in volcanic rocks are concentrated.

  2. Age, petrogenesis, and tectonic setting of the Permian bimodal volcanic rocks in the eastern Jiamusi Massif, NE China

    Bi, Jun-Hui; Ge, Wen-Chun; Yang, Hao; Wang, Zhi-Hui; Dong, Yu; Liu, Xi-Wen; Ji, Zheng


    We present new in situ zircon U-Pb and Hf isotope, whole-rock geochemical, and Sr-Nd isotopic data for volcanic rocks from the Jiejinkou and Baoqing areas in the eastern Jiamusi Massif. These volcanic rocks are bimodal and consist of basalts, basaltic andesites, rhyolites, and rhyolitic tuffs that can be subdivided into mafic and silicic groups. Zircon U-Pb dating by LA-ICP-MS indicates that these volcanic rocks were erupted between the Early and Middle Permian (290-267 Ma). The mafic rocks in this area have positive εNd(t) (+0.07 to +6.43) values, and are enriched in light rare earth elements (LREEs) and depleted in heavy REE, Nb, and Ta. From these rocks, the meta-basalt of Jinlu and basaltic andesite of Taipinggou and Haojiatun were derived from parental magmas generated by the partial melting of depleted mantle wedge material that was metasomatized by subduction-related melts. These magmas then underwent variable degrees of fractional crystallization and assimilated insignificant amounts of crustal material. The meta-basalt of Liming likely originated from the metasomatized mantle-derived melts hybridized by the convective asthenosphere during the evolution of the magmas. In comparison, the silicic rocks have negative εNd(t) and variable zircon εHf(t) values, are enriched in the large-ion lithophile elements (LILEs) and LREE, and are depleted in high-field-strength elements (e.g., Nb, Ta, and Ti), yielding arc-like geochemical signatures. The geochemical and zircon εHf(t) characteristics of Jiangfeng and Longtouqiao rhyolites are indicative of formation from magmas generated by the partial melting of mafic lower crustal material, whereas the Liming meta-rhyolite was probably produced from a source involving some depleted mantle components. The bimodal volcanic rocks provide convincing evidence that the Early-Middle Permian volcanism in the Jiamusi Massif occurred in an extensional environment probably associated with slab break-off during the westward

  3. Role of crustal assimilation and basement compositions in the petrogenesis of differentiated intraplate volcanic rocks: a case study from the Siebengebirge Volcanic Field, Germany

    Schneider, K. P.; Kirchenbaur, M.; Fonseca, R. O. C.; Kasper, H. U.; Münker, C.; Froitzheim, N.


    The Siebengebirge Volcanic Field (SVF) in western Germany is part of the Cenozoic Central European Volcanic Province. Amongst these volcanic fields, the relatively small SVF comprises the entire range from silica-undersaturated mafic lavas to both silica-undersaturated and silica-saturated differentiated lavas. Owing to this circumstance, the SVF represents a valuable study area representative of intraplate volcanism in Europe. Compositions of the felsic lavas can shed some new light on differentiation of intraplate magmas and on the extent and composition of potential crustal assimilation processes. In this study, we provide detailed petrographic and geochemical data for various differentiated SVF lavas, including major and trace element concentrations as well as Sr-Nd-Hf-Pb isotope compositions. Samples include tephriphonolites, latites, and trachytes with SiO2 contents ranging between 53 and 66 wt%. If compared to previously published compositions of mafic SVF lavas, relatively unradiogenic 143Nd/144Nd and 176Hf/177Hf coupled with radiogenic 87Sr/86Sr and 207Pb/204Pb lead to the interpretation that the differentiated volcanic rocks have assimilated significant amounts of lower crustal mafic granulites like the ones found as xenoliths in the nearby Eifel volcanic field. These crustal contaminants should possess unradiogenic 143Nd/144Nd and 176Hf/177Hf, radiogenic 87Sr/86Sr, and highly radiogenic 207Pb/204Pb compositions requiring the presence of ancient components in the central European lower crust that are not sampled on the surface. Using energy-constrained assimilation-fractional crystallisation (EC-AFC) model calculations, differentiation of the SVF lithologies can be modelled by approximately 39-47 % fractional crystallisation and 6-15 % crustal assimilation. Notably, the transition from silica-undersaturated to silica-saturated compositions of many felsic lavas in the SVF that is difficult to account for in closed-system models is also well explained by

  4. Mapping local singularities using magnetic data to investigate the volcanic rocks of the Qikou depression, Dagang oilfield, eastern China

    G. Chen


    Full Text Available The spatial structural characteristics of geological anomaly, including singularity and self-similarity, can be analysed using fractal or multifractal modelling. Here we apply the multifractal methods to potential fields to demonstrate that singularities can characterise geological bodies, including rock density and magnetic susceptibility. In addition to enhancing weak gravity and magnetic anomalies with respect to either strong or weak background levels, the local singularity index (α ≈ 2 can be used to delineate the edges of geological bodies. Two models were established to evaluate the effectiveness of mapping singularities for extracting weak anomalies and delineating edges of buried geological bodies. The Qikou depression of the Dagang oilfield in eastern China has been chosen as a study area for demonstrating the extraction of weak anomalies of volcanic rocks, using the singularity mapping technique to analyse complex magnetic anomalies caused by complex geological background. The results have shown that the singularities of magnetic data mapped in the paper are associated with buried volcanic rocks, which have been verified by both drilling and seismic survey, and the S–N and E–W faults in the region. The targets delineated for deeply seated faults and volcanic rocks in the Qikou depression should be further investigated for the potential application in undiscovered oil and gas reservoirs exploration.

  5. Petrography and chemical evidence for multi-stage emplacement of western Buem volcanic rocks in the Dahomeyide orogenic belt, southeastern Ghana, West Africa

    Nude, Prosper M.; Kwayisi, Daniel; Taki, Naa A.; Kutu, Jacob M.; Anani, Chris Y.; Banoeng-Yakubo, Bruce; Asiedu, Daniel K.


    The volcanic rocks of the Buem Structural Unit in the Dahomeyide orogenic belt of southeastern Ghana, constitute a unique assemblage among the monocyclic sedimentary formations of this structural unit. Representative volcanic rock samples were collected from the Asukawkaw, Bowiri-Odumase and Nkonya areas which form a roughly north-south trend. The volcanic rocks comprise spherulitic, amygdaloidal, vesicular, phyric and aphyric varieties. Whole rock geochemistry shows that these volcanic rocks exhibit both alkaline and subalkaline characteristics. The alkaline varieties are relatively enriched in REE and incompatible trace element concentrations, similar to OIB; the subalkaline varieties show E-MORB and N-MORB REE and incompatible element characteristics. The rocks have low La/Nb (<1), low K/Nb (<450) and high Nb/U (averagely 47.3) values, suggesting no significant effect of crustal contamination. The key characteristics of these volcanic rocks are the distinct petrography and geochemistry, shown from the three separate localities, which may suggest source fractionation at different depths or modes of emplacement. The association of volcanic rocks of OIB, E-MORB and N-MORB affinities, with no significant crustal contamination, may suggest mantle derived magma that may have been related to rifting event and eventual emplacement at the eastern passive margin of the West African Craton.

  6. Strontium isotopic geochemistry of the volcanic rocks and associated megacrysts and inclusions from Ross Island and vicinity, Antarctica

    Stuckless, J.S.; Ericksen, R.L.


    Twelve whole-rock samples of volcanic rocks and a composite of 11 basanitoid samples from Ross Island and vicinity, Antarctica show a narrow range of 87Sr/86Sr ratios from 0.70305 to 0.70339. This range is consistent with a model of differentiation from a single parent magma, but the data allow a 30% variation in the 87Rb/86Sr ratio in the source region if the average ratio is less than 0.057 and if the source region has existed as a closed system for 1.5 b.y. Megacrysts of titaniferous augite, kaersutite, and anorthoclase are isotopically indistinguishable from the host volcanic rocks and therefore are probably cogenetic with the volcanic sequence. A single trachyte sample is isotopically distinct from the rest of the volcanic rocks and probably was contaminated with crustal strontium. Ultramafic and mafic nodules found in association with basanitoids and trachybasalts have 87Sr/86Sr ratios ranging from 0.70275 to 0.70575. Several of these nodules exhibit evidence of reaction with the melt and are isotopically indistinguishable from their hosts, but data for seven granulite-facies nodules show an apparent isochronal relationship. Although this isochron may be fortuitous, the resulting age of 158??22 m.y. is similar to ages reported for the voluminous Ferrar Dolerites, and suggests isotopic re-equilibration within the lower crust and upper mantle. These nodules are not genetically related to the Ferrar Dolerites, as evidenced by their lower initial 87Sr/86Sr ratios. Three ultramafic nodules are texturally and isotopically distinct from the rest of the analyzed nodules. These are friable, have larger 87Sr/86Sr ratios, and may represent a deeper sampling of mantle rock than the granulite-facies nodules. They were, however, derived at a shallower depth than the alkalic magma. Thus they are not genetically related to either the magma or the granulite-facies nodules. ?? 1976 Springer-Verlag.

  7. Petrology, mineral chemistry and tectono-magmatic setting of volcanic rocks from northeast Farmahin, north of Arak

    Reza Zarei Sahamieh


    Full Text Available Introduction The study area is a small part of the Urumieh-Dokhtar structural zone in the Markazi province, located in the northeastern part of the Farmahin, north of Arak (Hajian, 1970. The volcanic rocks studied from the area include andesite, dacite, rhyodacite, ignimbrite and tuff of Middle to Late Eocene age (middle Lutetian to upper Lutetian (Ameri et al., 2009. It seems that folding and faulting is caused in sedimentary basin and volcanic activities. On the other hand, except of orogeny maybe rifting had rule in eruption so that this case has seen in the other area such as Taft and Khezrabad in central Iran (Zarei Sahamieh et al., 2008. The oldest formation in the studied area is Triassic limestones. The dominant textures of these rocks are porphyritic, microlite porphyritic, microlitic and rarely sieve-texture. Sieve texture and dusty texture (dusty plagioclases indicates magma mixing. Mineralogically, they contain plagioclases, clinopyroxenes, amphiboles, quartz and biotite as the main constituents and zircon, apatite, and opaque minerals as accessories. Plagioclases in the andesitic and basaltic- andesite rocks are labradorite, bytownite and anorthite (based on electron microprobe .Moreover, plagioclases in andesitic rocks show that H2O is lesser than 2.5 precent. Amphibole is found in both plagioclases and groundmass. Materials and methods In this article are used different analyses methods such as XRF, ICP-MS and EPMA. Whole-rock major and trace element analyses were determined with ICP-MS method. The major and trace element composition of some rock was determined by electron probe micro-analysis (EPMA using a Cameca SX100 instrument in Iran Mineral Processing Research Center (IMPRC. Moreover, whole-rock major and some trace element analyses for some samples were obtained by X-ray fluorescence (XRF, using an ARL Advant-XP automated X-ray spectrometer. Results Chemical data based on electron micro probe studies of minerals indicate

  8. Petrology of the alkaline rocks of the Macau Volcanic Field, NE Brazil

    Ngonge, Emmanuel Donald; de Hollanda, Maria Helena Bezerra Maia; Pimentel, Márcio Martins; de Oliveira, Diógenes Custódio


    The Macau Volcanic Field (MVF) in the Borborema Province, NE Brazil, contains multiple centres of volcanic activity of Early to Late Cenozoic ages. We present element and Sr-Nd-Pb isotope geochemical data for four of the few most prominent basalt types of this volcanic field: Serrote Preto-type, Serra Aguda-type, Pico do Cabugi-type and Serra Preta-type, in order to assess their magmatic history from source to crystallization and the evolution of the mantle beneath the Borborema Province. The basalts are basically sodic nephelinitic-basanitic-alkali olivine basalts enriched in LILE and in Nb-Ta. The Serra Preta, Cabugi and Serra Aguda types demonstrate compositions close to primitive characteristics with 10% < MgO < 15 wt.% and 200 ppm < Ni < 500 ppm, and experienced limited fractional crystallization of olivine-clinopyroxene-plagioclase-oxides with negligible wall-rock assimilation. Rb/Sr and Ba/Rb constraints support the generation of SiO2-undersaturated magmas from mantle melting of amphibole-bearing peridotites with minor phlogopite. The source for the basanites and alkali basalts is estimated to be a garnet-bearing domain around the lithosphere-asthenosphere boundary (80-93 km deep), while the nephelinites are derived from the adiabatic asthenosphere at 105 km with temperatures of 1480 °C. Their incompatible trace element patterns and Sr-Nd-Pb isotopic compositions are similar to FOZO and EM-type OIB magmas. From the comparison of data with those of the Ceará-Mirim dyke swarm we propose that there is a ubiquitous FOZO reservoir in the SCLM beneath the Borborema Province. This FOZO signature characterized the upwelling asthenosphere during the lithospheric extension and thinning at the opening of the Equatorial Atlantic and is clearly represented in the Mesozoic olivine tholeiites of Ceará-Mirim. The upwelled asthenosphere cooled as a rigid SCLM since the Cretaceous and has preserved its FOZO signature evident in the Macau Cenozoic basalts. The EM signatures

  9. Characteristics of Rare Earth Elements of Zircons from Mesozoic Volcanic Rocks in Luanping Region, Hebei

    Zhang Hong; Yuan Hongli; Hu Zhaochu; Liu Xiaoming


    Rare earth elements of the zircons from the Mesozoic volcanic rocks in Luanping region, Hebei, were analyzed the results reflect that the average values of δEu and (Lu/Gd)N are 0.49 and 21.8 respectively in the zircons from the top part of Tiaojishan Formation;but the average values of δEu and (Lu/Gd)N are 0.15, 0.06, 0.09 and 14.51, 15.66, 16.25 respectively in the zircons from the lower, and upper part of the Tuchengzi Formation and the bottom bed of the Zhangjiakou Formation. The results show that the characteristics of the zircons from the Tuchengzi Formation are coincident with those of the zircons from the Zhangjiakou Formation, but are different from those of the zircons from the Tiaojishan Formation, and imply that the Tuchengzi Formation has close relation with the Zhangjiakou Formation. Combining the results above with the former isotopic dating results of the volcanic rocks, the authors draw the conclusions as follows: The Tuchengzi Formation not only has a long interval period with the Tiaojishan Formation, but also is very different from the Tiaojishan Formation in zircon geochemical characteristics. The Tuchengzi Formation not only is nearly continuous with the Zhangjiakou Formation in time, but also is coincident with the Zhangjiakou Formation in geochemistry of zircons. The results imply that the Tuchengzi Formation and the Zhangjiakou Formation were formed in the same geological background, that is, there are not the boundary of the J3-K1 and the interface of the transition of tectonic framework between the Tuchengzi Formation and the Zhangjiakou Formation in the Luanping region. The research shows that the (Lu/Gd)N, δEu are two important parameters which are relatively stable in the analysis of zircons from Crust-source;but the values of ∑LREE of zircons from Crust-source change greatly, especially the abundance of La element, so some ratios of rare earth elements related with La (or ∑LREE) are not usable in determining the characteristics

  10. Geochemistry of meta-volcanic rocks from the Longbohe Cu deposit, Yunnan Province, China: Implications for the genesis and tectonic setting

    CUI Yinliang; QING Dexian; CHEN Yaoguang


    The Longbohe Cu deposit, which is located in the southern part of the Honghe ore-forming zone, Yunnan Province, China, belongs to a typical ore field where volcanic rocks are of wide distribution and are associated with Cu mineralization in time and space. The volcanic rocks in the ore field, which have experienced varying degree of alteration or regional metamorphism, can be divided into three types, i.e., meta-andesite, meta-subvolcanic rock and meta-basic volcanic rock in accordance with their mineral assemblages. These three types of volcanic rocks in the ore field are relatively rich in Na and the main samples plot in the area of alkali basalts in the geochemical classification diagram. With the exception of very few elements, these three types of volcanic rocks are similar in the content of trace elements. In comparison to the basalts of different tectonic settings, the meta-volcanic rocks in the ore field are rich in high field strength elements (HFSE) such as Th, Nb, etc. and depleted in large ion lithophile elements (LILE) such as Sr, Ba, etc. and their primary mantle-normalized trace element patterns show remarkable negative Th and Nb anomalies and negative Sr and Ba anomalies. These three types of volcanic rocks are similar in REE content range and chondrite-normalized REE patterns with the exception of Eu anomaly. Various lines of evidence show that these three types of volcanic rocks in the ore field have the same source but are the products of different stages of magmatic evolution, their original magma is a product of partial melting of the metasomatically enriched mantle in the tensional tectonic setting within the continent plate, and the crystallization differentiation plays an important role in the process of magmatic evolution.

  11. U-Th-He age determinations on volcanic rocks: A new tool for Quaternary geochronology.

    Aciego, S. M.; Kennedy, B. M.; Christensen, J. N.; Depaolo, D. J.; Hutcheon, I. D.


    The U-Th/He system, which has been used successfully for thermochronology, also has potential for dating young volcanic rocks. To test this possibility the age of garnet phenocrysts from the 79 AD eruption of Mt. Vesuvius was measured. The samples consisted of 600-1000 μm diameter andradite-grossular garnet crystals separated from a volcanic tuff previously dated by Ar-Ar (Renne et al, 1997). Our age calculation accounts for diffusive loss of radiogenic He since eruption, loss of He by alpha ejection from the garnet grains, U series disequilibrium, and non-uniform U-Th distribution within the mineral grains. The effect of diffusion was investigated using existing diffusivity and activation energy data for almandine rich garnet (Dunai and Roselieb, 1996), and by step-wise degassing of the Vesuvius garnets. Diffusivity was measured over 7 temperature steps ranging from 800oC to 1300oC with one additional 1500oC step for complete extraction. Even up to the 1300oC step the diffusivity remains linear, indicating an Arrhenius relationship with an activation energy of approximately 427 kJ/mol and a frequency factor, Do/a2 = 0.578 s-1. Alpha emission is accounted for by modeling the garnet grains as spheres with the injection and ejection of alphas from the sphere depending on the concentration and distribution of the U and Th inside and outside of the mineral grain. Distribution of U, Th was determined by ion-probe analysis across garnet grains in thin section. While the overall concentration of U and Th within the garnets is quite high (approximately 16-19ppm and 23-27ppm, respectively), there is a deficiency in 230Th and 226Ra. The 234U/238U and 230Th/238U activity we measure directly, while the 226Ra/238U activity ratio was estimated by assuming Ra follows barium. Due to machine limitations, U, Th concentrations and He concentrations were determined on different aliquots. The measured concentration of 4He in the garnet grains varies from 3.72x10-9 to 4.16x10-9 cc STP

  12. Use of terrestrial laser scanning for engineering geological applications on volcanic rock slopes - an example from Madeira island (Portugal)

    Nguyen, H. T.; Fernandez-Steeger, T. M.; Wiatr, T.; Rodrigues, D.; Azzam, R.


    This study focuses on the adoption of a modern, widely-used Terrestrial Laser Scanner (TLS) application to investigate volcanic rock slopes in Ribeira de João Gomes valley (Funchal, Madeira island). The TLS data acquisition in May and December 2008 provided information for a characterization of the volcanic environment, detailed structural analysis and detection of potentially unstable rock masses on a slope. Using this information, it was possible to determine specific parameters for numerical rockfall simulations such as average block size, shape or potential sources. By including additional data, such as surface roughness, the results from numerical rockfall simulations allowed us to classify different hazardous areas based on run-out distances, frequency of impacts and related kinetic energy. Afterwards, a monitoring of hazardous areas can be performed in order to establish a rockfall inventory.

  13. Petrology of Karoo volcanic rocks in the southern Lebombo monocline, Mozambique

    Melluso, Leone; Cucciniello, Ciro; Petrone, Chiara M.; Lustrino, Michele; Morra, Vincenzo; Tiepolo, Massimo; Vasconcelos, Lopo


    The Karoo volcanic sequence in the southern Lebombo monocline in Mozambique contains different silicic units in the form of pyroclastic rocks, and two different basalt types. The silicic units in the lower part of the Lebombo sequence are formed by a lower unit of dacites and rhyolites (67-80 wt.% SiO 2) with high Ba (990-2500 ppm), Zr (800-1100 ppm) and Y (130-240 ppm), which are part of the Jozini-Mbuluzi Formation, followed by a second unit, interlayered with the Movene basalts, of high-SiO 2 rhyolites (76-78 wt.%; the Sica Beds Formation), with low Sr (19-54 ppm), Zr (340-480 ppm) and Ba (330-850 ppm) plus rare quartz-trachytes (64-66 wt.% SiO 2), with high Nb and Rb contents (240-250 and 370-381 ppm, respectively), and relatively low Zr (450-460 ppm). The mafic rocks found at the top of the sequence are basalts and ferrobasalts belonging to the Movene Formation. The basalts have roughly flat mantle-normalized incompatible element patterns, with abundances of the most incompatible elements not higher than 25 times primitive mantle. The ferrobasalt has TiO 2 ˜ 4.7 wt.%, Fe 2O 3t = 16 wt.%, and high Y (100 ppm), Zr (420 ppm) and Ba (1000 ppm). The Movene basalts have initial (at 180 Ma) 87Sr/ 86Sr = 0.7052-0.7054 and 143Nd/ 144Nd = 0.51232, and the Movene ferrobasalt has even lower 87Sr/ 86Sr (0.70377) and higher 143Nd/ 144Nd (0.51259). The silicic rocks show a modest range of initial Sr-( 87Sr/ 86Sr = 0.70470-0.70648) and Nd-( 143Nd/ 144Nd = 0.51223-0.51243) isotope ratios. The less evolved dacites could have been formed after crystal fractionation of oxide-rich gabbroic cumulates from mafic parental magmas, whereas the most silica-rich rhyolites could have been formed after fractional crystallization of feldspars, pyroxenes, oxides, zircon and apatite from a parental dacite magma. The composition of the Movene basalts imply different feeding systems from those of the underlying Sabie River basalts.

  14. Mineral chemical compositions of late Cretaceous volcanic rocks in the Giresun area, NE Turkey: Implications for the crystallization conditions

    Oǧuz, Simge; Aydin, Faruk; Uysal, İbrahim; Şen, Cüneyt


    This contribution contains phenocryst assemblages and mineral chemical data of late Cretaceous volcanic (LCV) rocks from the south of Görele and Tirebolu areas (Giresun, NE Turkey) in order to investigate their crystallization conditions. The LCV rocks in the study area occur in two different periods (Coniasiyen-Early Santonian and Early-Middle Campanian), which generally consist of alternation of mafic-intermediate (basaltic to andesitic) and felsic rock series (dacitic and rhyolitic) within each period. The basaltic and andesitic rocks in both periods generally exhibit porphyritic to hyalo-microlitic porphyritic texture, and contain phenocrysts of plagioclase and pyroxene, whereas the dacitic and rhyolitic rocks of the volcanic sequence usually show a vitrophyric texture with predominant plagioclase, K-feldspar, quartz and lesser amphibole-biotite phenocrysts. Zoned plagioclase crystals of the mafic and felsic rocks in different volcanic periods are basically different in composition. The compositions of plagioclase in the first-stage mafic rocks range from An52 to An78 whereas those of plagioclase from the first-stage felsic rocks have lower An content varying from An38 to An50. Rim to core profile for the zoned plagioclase of the first-stage mafic rocks show quite abrupt and notable compositional variations whereas that of the first-stage felsic rocks show slight compositional variation, although some of the grains may display reverse zoning. On the other hand, although no zoned plagioclase phenocryst observed in the second-stage mafic rocks, the compositions of microlitic plagioclase show wide range of compositional variation (An45-80). The compositions of zoned plagioclase in the second-stage felsic rocks are more calcic (An65-81) than those of the first-stage felsic rocks, and their rim to core profile display considerable oscillatory zoning. The compositions of pyroxenes in the first- and second-stage mafic-intermediate rocks vary over a wide range from

  15. High-precision thermal ionization mass spectrometry dating of young volcanic rocks by using U-series method


    A high precision thermal ionization mass spectrometric (HP-TIMS) technique is used to determine 238U,234U,232Th,230Th concentrations and their ratios in whole rocks and minerals separated from Quaternary Maanshan,Dayingshan and Heikongshan volcanic rocks of Tengchong volcanic field,Yunnan Province,China.The 238U-230Th isochrons are given,yielding four age values (227± 20) ka (D-1,Dayingshan),(79.6± 5.5) ka (D-7,Dayingshan),(21.9± 3.0) ka (h-1,Heikongshan),and (7.5± 1.0) ka (M-1,Maanshan).The result is not only consistent with but also preciser than those measured by the K-Ar method and the alpha spectrometry U-series method,indicating that the HP-TIMS method is reliable and has high precision.Besides,a procedure of HP-TIMS analysis of young volcanic rocks in China is set up preliminarily.

  16. Cenozoic volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, China: Petrochemical evidence for partial melting of the mantle-crust transition zone

    LAI Shaocong; QIN Jiangfeng; LI Yongfeng; LIU Xin


    Neogene volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, are represented by a typical intermediate-basic and intermediate alkaline rock association, with latite-trachyte as the main rock type. The results of chemical analysis are: SiO2=52%-62%, Al2O3>15%, Na2O/K2O>1 and MgO<3.30%. In addition, the volcanic rocks are LREE-enriched with LREE/HREE=10-13, (La/Yb)N=15-19, and show a weak negative Eu anomaly with δEu=0.71-0.89. The close relationship between Mg# and SiO2 and the co-variation of the magmatophile elements and ultra-magmatophile elements such as La/Sm-La and Cr-Tb indicate that this association of volcanic rocks is the product of comagmatic fractional crystallization. The rock association type and lower Sm/Yb values (Sm/Yb=3.23-3.97) imply that this association of volcanic rocks should have originated from partial melting of spinel lherzolite in the lithospheric mantle. On the other hand, the weak negative Eu anomaly and relative depletion in Nb, Ta and Ti reflect the features of terrigenous magma. So the Neogene Belog Co alkaline volcanic rocks should be the result of partial melting of the special crust-mantle transition zone on the Qinghai-Tibet Plateau.

  17. Petrology and Geochemistry of Hydrothermally Altered Volcanic Rocks in the Iheya North Hydrothermal Field, Middle Okinawa Trough

    Yamasaki, T.


    The Iheya North hydrothermal field is located in the middle Okinawa Trough, a young and actively spreading back-arc basin extending behind the Ryukyu arc-trench system in the southeastern margin of the East China Sea. In this hydrothermal field, two scientific drilling expeditions (IODP Exp 331 and SIP CK14-04) were conducted using a deep-sea drilling vessel "Chikyu," and samples from a total of 27 holes were taken. Through these expeditions, Kuroko-type volcanogenic massive sulfide deposits (VMS), hydrothermally altered volcanic rocks, and pumiceous and pelagic sediments were recovered. The recovered core provided important information about the relationship between hydrothermal activity, alteration, and ore mineralization. Whole-rock major element composition and trace element (TE) patterns of pumices were very similar to those of rhyolites in the middle Okinawa Trough (RMO). However, pumices were relatively enriched in chalcophile elements Sr and Nb, which suggest incipient mineralization. Volcanic rock generally demonstrated strong silicification and was greenish pale gray in color. Regardless of severe alteration, some rock displayed major element composition broadly similar to the RMO. Alteration was evidenced by an increase in the content of SiO2 and MgO, and decrease in Al2O3, Na2O, and K2O content. The most striking geochemical feature of altered volcanic rock was the discordance between texture and the degree of modification of TEs. Some samples showed decussate texture occupied by petal-like quartz with severe silicification, but no prominent disturbance of concentration and patterns of TEs were observed. In contrast, samples with well-preserved igneous porphyritic texture showed very low TE content and modification of TE patterns. These results suggest that the modification of texture and composition of TEs, as well as silicification, do not occur by a uniform process, but several processes. This may reflect the differences in temperature and the

  18. K-Ar ages for the Yahazudake volcanic rocks from southwest Kyushu, Japan; Kyushu nanseibu yahazudake kazanganrui no K-Ar nendai

    Yokose, H.; Kikuchi, W. [Kumamoto Univ. (Japan)] Nagao, K. [Okayama Univ. (Japan)264000] Kodama, K. [Kochi Univ. (Japan)


    Many volcanic rocks, seemed to be erupted during the period from the Pliocene epoch to the Pleistocene epoch, are distributed abounding in Kyushu, Japan. In this study, K-Ar ages determination about the 4 samples which represents the Hisatsu volcanic rocks distributed around Yahazudake and rhyolite distributed in Gesujima placed in the southernmost extremity of Amakusa Shimojima, was conducted. And consideration of time/space distribution of the Hisatsu volcanic rocks upon collecting the data which were reported until now and the data obtained by the present K-Ar age determination, was done. In the result of the present measurement, the absolute age of the Hisatsu volcanic rocks distributed around Minamata-shi became clear. I was clarified that Yahazudake volcanic rocks consisted of andesite, which is comparatively lacking in potassium, were formed during about 100 thousand years from 1.98 to 2.08 Ma, and Ontake volcanic rocks which exists for the bottom erupted at about 2.15 Ma. And, the age value of 2.89 Ma was obtained from Ushibuka rhyolite distributed in Gesujima. 35 refs., 4 figs., 3 tabs.

  19. Geochemistry of the Cenozoic Potassic Volcanic Rocks in the West Kunlun Mountains and Constraints on Their Sources

    ZHANG Zhaochong; XIAO Xuchang; WANG Jun; WANG Yong; LUO Zhaohua


    The geochemical characteristics of the Cenozoic volcanic rocks from the north Pulu, east Pulu and Dahongliutan regions in the west Kunlun Mountains are somewhat similar as a whole. However, the volcanic rocks from the Dahongliutan region in the south belt are geochemically distinguished from those in the Pulu region (including the north and east Pulu) of the north belt. The volcanic rocks of the Dahongliutan region are characterized by relatively low TiO2 abundance, but more enrichment in alkali, much more enrichment in light rare earth elements and large ion lithosphile elements than those from the Pulu region. Compared with the Pulu region, volcanic rocks from the Dahongliutan region have relatively low 87Sr/86Sr ratios, and high εNd, 207pb/204Pb and 208Pb/204Pb. Their trace elements and isotopic data suggest that they were derived from lithospheric mantle, consisting of biotite- and hornblende-bearing garnet lherzolite, which had undertaken metasomatism and enrichment. On the primitive mantle-normalized patterns, they display remarkably negative Nb and Ta anomalies, indicating the presence of early-stage subducted oceanic crust. The metasomatism and enrichment resulted from the fluid released from the crustal materials enclosed in the source region in response to the uplift of asthenospheric mantle. Based on the previous experiments it can be inferred that the thickness of the lithosphere ranges from 75 to 100 km prior to the generation of the magmas. However, the south belt differs from the north one by its thicker lithosphere and lower degree of partial melting. The different thickness of the lithosphere gives rise to corresponding variation of the degree of crustal contamination. The volcanic rocks in the south belt are much more influenced by crustal contamination. In view of the tectonic setting, the generation of potassic magmas is linked with the uplift of asthenosphere resulted from large-scale thinning of the lithosphere after the collision of Indian

  20. Chronology and Geochemistry of Mesozoic Volcanic Rocks in the Linjiang Area, Jilin Province and their Tectonic Implications

    YU Yang; XU Wenliang; PEI Fuping; YANG Debin; ZHAO Quanguo


    Zircon U-Pb ages and geochemical analytical results are presented for the volcanic rocks of the Naozhigou, Ergulazi, and Sidaogou Formations in the Linjiang area, southeastern Jilin Province to constrain the nature of magma source and their tectonic settings. The Naozhigou Formation is composed mainly of andesite and rhyolite and its weighted mean 206Pb/238U age for 13 zircon grains is 2224±1 Ma. The Ergulazi Formation consists of basaltic andesite, basaltic trachyandesite, and andesite, and six grains give a weighted mean 206Pb/238U age of 131±4 Ma. The Sidaogou Formation consists mainly of trachyandesite and rhyolite, and six zircon grains yield a weighted mean 206Pb/238U age of113±4 Ma. The volcanic rocks have SiO2=60.24%-77.46%, MgO=0.36%-1.29% (Mg#=0.32-0.40) for the Naozhigou Formation, SiO2=51.60%-59.32 %, MgO=3.70 %-5.54% (Mg#=0.50-0.60) for the Ergulazi Formation, and SiO2=58.28%-76.32%, MgO=0.07%-1.20% (Mg#=0.14-0.46) for the Sidaogou Formation. The trace element analytical results indicate that these volcanic rocks are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), relative depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs, Nb, Ta, and Ti), and negative Eu anomalies. Compared with the primitive mantle, the Mesozoic volcanic rocks in the Linjiang area have relatively high initial 87Sr/86Sr ratios (0.7053-0.7083) and low eNd(t) values (-8.38 to -2.43), and display an EMII trend. The late Triassic magma for the Naozhigou Formation could be derived from partial melting of a newly accretional crust with the minor involvement of the North China Craton basement and formed under an extensional environment after the collision of the Yangtze Craton and the North China Craton. The Early Cretaceous volcanic rocks for the Ergulazi and Sidaogou Formations could be formed under the tectonic setting of an active continental margin related to the westward snbduction of

  1. Geochemistry of ultrapotassic volcanic rocks in Xiaogulihe NE China: Implications for the role of ancient subducted sediments

    Sun, Yang; Ying, Jifeng; Zhou, Xinhua; Shao, Ji'an; Chu, Zhuyin; Su, Benxun


    The unique eruptions of ultrapotassic volcanic rocks in eastern China reported so far took place in the Xiaogulihe area of western Heilongjiang Province, NE China. These ultrapotassic rocks are characterized by extremely high K2O contents (> 7 wt.%), abnormally unradiogenic Pb isotopic compositions (206Pb/204Pb = 16.44-16.55; 207Pb/204Pb = 15.39-15.46; 208Pb/204Pb = 36.35-36.61), and moderately high 87Sr/86Sr ratios (0.7053-0.7057), which can be basically correlated with those of ultrapotassic igneous rocks distributed widely in northwestern America and Aldan Shield. The positive correlation between 187Os/188Os and 1/Os argues that these ultrapotassic rocks have probably experienced negligible lower continental crust addition (less than 1%) during magma ascent. The high contents of K2O and negative correlation between 87Sr/86Sr and 206Pb/204Pb of these ultrapotassic rocks indicate the presence of a potassic phase, mostly phlogopite, in their mantle source. Their strong fractionation of rare earth elements and lack of Nd-Hf isotopic decoupling reveal a low-degree partial melting of garnet-bearing source rocks. In addition, the low CaO and Al2O3 contents of whole-rock compositions and low Fe/Mn ratios of olivine phenocryst chemistries suggest peridotites rather than pyroxenites as dominant source rocks for the Xiaogulihe ultrapotassic rocks. Based on these distinctive geochemical characteristics, we thus propose that the ultimate mantle source of the Xiaogulihe ultrapotassic volcanic rocks is phlogopite-bearing garnet peridotite within the lower part of the sub-continental lithospheric mantle (SCLM) that had been metasomatized by potassium-rich silicate melts. Combined with the unradiogenic Pb compositions, the most likely source of these potassium-rich silicate melts is the ancient subducted continental-derived sediments (> 1.5 Ga). These ancient subducted sediments, possessing relatively low initial Pb isotopic compositions, had experienced large U/Pb fractionation

  2. New results for Palaeozoic volcanic phases in the Prague Basin – magnetic and geochemical studies of Lištice, Czech Republic

    Tiiu Elbra


    Full Text Available Palaeo-, rock magnetic and geochemical studies were conducted on volcanic samples from the Lištice area to improve the knowledge of Palaeozoic volcanic evolution in the Prague Basin. The magnetic data display no significant differences between two studied localities, indicating one magnetizing event for both localities. Geochemical data suggest that Lištice basalt could have originated from deep melting of the garnet peridotite mantle source during the attenuation and rifting of the continental lithosphere connected with asthenospheric mantle upwelling. The dataset furthermore supports the evidence of syn- or post-intrusive fluid interactions and low-temperature stages of alteration. The Ti-magnetite within amygdales of the samples was found to be carrying the characteristic remanent magnetization and reflects probably the Permo-Carboniferous remagnetization of volcanic phases.

  3. Isotopic Ages of the Carbonatitic Volcanic Rocks in the Kunyang Rift Zone in Central Yunnan,China

    ZHANG Yongbei; WANG Guilan; NIE Jianfeng; ZHAO Chongshun; XU Chengyan; QIU Jiaxiang; Wang Hao


    The Mesoproterozoic Kunyang rift, which is located on the western margin of the Yangtze platform and the southern section of the Kangdian axis, is a rare massive Precambrian iron-copper polymetallic mineralization zone in China. The Mesoproterozoic Wulu (Wuding(Lufeng) basin in the middle of the rift is an elliptic basin controlled by a ring fracture system. Moreover, volcanic activities in the basin display zonation of an outer ring, a middle ring and an inner ring with carbonatitic volcanic rocks and sub-volcanic dykes discovered in the outer and middle rings. The Sm-Nd isochron ages have been determined for the outer-ring carbonatitic lavas (1685 Ma) and basaltic porphyrite of the radiating dyke swarm (1645 Ma) and the Rb-Sr isochron ages for the out-ring carbonatitic lavas (893 Ma) and the middle-ring dykes (1048 Ma). In combination of the U-Pb concordant ages of zircon (1743 Ma) in trachy-andesite of the corresponding period and stratum (1569 Ma) of the Etouchang Formation, as well as the Rb-Sr isochron age (1024 Ma) and K-Ar age (1186 Ma) of the dykes in the middle ring, the age of carbonatites in the basin is preliminarily determined. It is ensured that all of these carbonatites were formed in the Mesoproterozoic period, whereby two stages could be identified as follows: in the first stage, carbonatitic volcanic groups, such as lavas, pyroclastic rocks and volcaniclastic sedimentary rocks, were formed in the outer ring; in the second stage, carbonatitic breccias and dykes appeared in the middle ring. The metamorphic age of the carbonatitic lavas in the outer ring was determined to be concurrent with the end of the first stage of the Neoproterozoic period, corresponding to the Jinning movement in central Yunnan.

  4. Geology and petrology of Tertiary volcanic rocks of Sarbisheh perlite mine area (eastern Iran and industrial applications

    Seayad Sayid Mohammadi


    Full Text Available The study area is located at northwest of Sarbisheh in South Khorasan province and eastern border of Lut block. In this area, Tertiary (Eocene-Oligocene to Pliocene volcanic rocks consisting of basaltic andesite, dacite, rhyodacite, vitreous rhyolite (perlitic in some parts, tuff and ignimbrite are exposed. In the Daghar Mountain, A, B and C perlite layers with thicknesses of 102, 7 and 58 meters respectively, occur alternatively with volcanic-pyroclastic rocks. Zoning, sieve texture and embayment of plagioclase and roundness of minerals in the lavas indicate disequilibrium conditions during magma crystallization. Chemically, these are meta-aluminous, medium to high-K calc-alkaline, enriched in LILE and negative anomaly for Nb and Ti. Chondrite-normalized Rare Earth Elements (REE plots indicate enrichment of light REE in comparison with heavy REE, (La/YbN of 9.14-12.64, low negative anomaly for Eu in basaltic andesite (Eu/Eu*=0.91 and dacite (Eu/Eu*=0.78-0.87 and strong negative anomaly for Eu in the rhyolites (Eu/Eu*=0.18-0.35. Negative anomaly for Eu indicates calc-alkaline nature for these rocks. On the basis of chemical characteristics and magnetic susceptibility, these are I-type. Tectonically, the rocks belong to subduction zone and active continental margin and their parental magma originated from partial melting of enriched mantle and then crustal contamination during differentiation process. Dacites have an initial 87Sr/86Sr between 0.7048 and 0.7050 (average 0.7049 that confirm mantle source for the magma. Volcanic glasses of rhyolitic composition altered by hydration (likely meteoric water and formed perlite. Physical tests and chemical analyses show that perlite of Sarbisheh is suitable as raw material for production of expanded perlite.

  5. Geochemical Signature of Mesozoic Volcanic and Granitic Rocks in Madina Regency Area, North Sumatra, Indonesia, and its Tectonic Implication

    Iskandar Zulkarnain


    Full Text Available samples consisting of two Permian-Triassic basalts, two Triassic-Jurassic granitic rocks, and a Miocene andesite were collected from the Madina Regency area in North Sumatra that is regionally situated on the West Sumatra Block. Previous authors have proposed three different scenarios for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, namely an island-arc, subduction related continental margin arc, and continental break-up. Petrographic analysis of the Mesozoic basaltic samples indicates that they are island-arcs in origin; however their trace element spider diagram patterns (Rock/MORB ratio also show the character of back-arc marginal basin, besides the island-arc. Furthermore, their REE spider diagram patterns (Rock/ Chondrite ratio clearly reveal that they were actually generated in a back-arc marginal basin tectonic setting. Meanwhile, the two Mesozoic granitic rocks and the Miocene andesite reflect the character of an active continental margin. Their spider diagram patterns show a significant enrichment on incompat- ible elements, usually derived from fluids of the subducted slab beneath the subduction zone. The high enrichment on Th makes their plots on Ta/Yb versus Th/Yb diagram are shifted to outside the active continental margin field. Although the volcanic-plutonic products represent different ages, their La/Ce ratio leads to a probability that they have been derived from the same magma sources. This study offers another different scenario for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, where the magmatic activities started in a back-arc marginal basin tectonic setting during the Permian-Triassic time and changed to an active continental margin during Triassic to Miocene. The data are collected through petrographic and chemical analyses for major, trace, and REE includ- ing literature studies.  

  6. Geochemical Signature of Mesozoic Volcanic and Granitic Rocks in Madina Regency Area, North Sumatra, Indonesia, and its Tectonic Implication

    Iskandar Zulkarnain


    Full Text Available samples consisting of two Permian-Triassic basalts, two Triassic-Jurassic granitic rocks, and a Miocene andesite were collected from the Madina Regency area in North Sumatra that is regionally situated on the West Sumatra Block. Previous authors have proposed three different scenarios for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, namely an island-arc, subduction related continental margin arc, and continental break-up. Petrographic analysis of the Mesozoic basaltic samples indicates that they are island-arcs in origin; however their trace element spider diagram patterns (Rock/MORB ratio also show the character of back-arc marginal basin, besides the island-arc. Furthermore, their REE spider diagram patterns (Rock/ Chondrite ratio clearly reveal that they were actually generated in a back-arc marginal basin tectonic setting. Meanwhile, the two Mesozoic granitic rocks and the Miocene andesite reflect the character of an active continental margin. Their spider diagram patterns show a significant enrichment on incompat- ible elements, usually derived from fluids of the subducted slab beneath the subduction zone. The high enrichment on Th makes their plots on Ta/Yb versus Th/Yb diagram are shifted to outside the active continental margin field. Although the volcanic-plutonic products represent different ages, their La/Ce ratio leads to a probability that they have been derived from the same magma sources. This study offers another different scenario for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, where the magmatic activities started in a back-arc marginal basin tectonic setting during the Permian-Triassic time and changed to an active continental margin during Triassic to Miocene. The data are collected through petrographic and chemical analyses for major, trace, and REE includ- ing literature studies.  

  7. The origin of volcanic rock fragments in Upper Pliocene Grad Member of the Mura Formation, North-Eastern Slovenia

    Polona Kralj


    Full Text Available Fresh-water, coarse-grained and detritus-dominated Mura Formation in North Eastern Slovenia includes pyroclastic and volcaniclastic deposits originating from Upper Pliocene volcanic activity of basaltic geochemical character. Although localized in occurrence at the hamlet Grad, these pyroclastic and volcaniclastic sediments forma distinctive depositional unit, for which the term “Grad Member” is proposed and introduced in this paper.In the Grad area no lavas or cinder cones are preserved, and the origin of volcaniclastic fragments still uncertain. For this reason, chemical composition of basaltic rock fragments from the Grad Member volcaniclastics has been studied and compared with basaltic rocks from the neighboring locations at Klöch, Kindsberg, Dölling and Neuhaus. The Grad Member pyroclastic and volcaniclastic deposits seem to be fed from the same source which is different from the occurrences in Austria. That supports the idea about the existence of a local volcanic centre in the present Grad area. The old volcanic edificeswerepossiblydestroyed by the late-stage hydrovolcanic eruptions, and pyroclastic and volcaniclastic deposits subjected to constant reworking by fluvial currents in a dynamic sedimentary environment of alluvial fan and braided river systems.

  8. Potassic volcanic rocks and adakitic intrusions in southern Tibet: Insights into mantle-crust interaction and mass transfer from Indian plate

    Liu, Dong; Zhao, Zhidan; DePaolo, Donald J.; Zhu, Di-Cheng; Meng, Fan-Yi; Shi, Qingshang; Wang, Qing


    Elucidating geodynamic processes at depth relies on a correct interpretation of petrological and geochemical features in magmatic records. In southern Tibet, both potassic volcanic rocks and adakitic intrusions exhibit high Sr/Y and La/Yb, and low Y and Yb concentrations. But these two rock types have contrasting temporal-spatial distributions and isotopic variations. Here we present a systematic study on the postcollisional potassic and adakitic rocks in order to investigate their petrogenetic links with the coeval mantle-derived ultrapotassic rocks and shed light on the potential input from underthrusted Indian continental crust. We found that adakitic intrusions with higher K2O/Na2O tend to display lower Y and higher SiO2, suggesting that the mantle-derived ultrapotassic melts, showing relatively high Y and Yb concentrations, only played a minor role in adakitic magmatism. Therefore, the unradiogenic 143Nd/144Nd and the dramatic decrease of zircon εHf(t) values since 35 Ma shown by postcollisional adakites should be interpreted as reflecting the crustal input from Indian plate. Unlike adakitic intrusions in southern Lhasa subterrane, potassic volcanic rocks share similar spatial distributions with ultrapotassic rocks, and their isotopic discrepancy is diminishing with volcanic activity becomes younger and migrates eastward. Evidence from whole-rock Pb and zircon Hf isotopes further indicates that potassic volcanic rocks are more likely to originate from partial melting of the overthickened and isotopically heterogeneous Lhasa terrane crust rather than the underthrusted Indian continental crust. The elevated Rb/Sr and varying Sr/CaO in potassic volcanic rocks provide an argument for sanidine + plagioclase + clinopyroxene as the major fractionating phases during magmatic differentiation. These findings not only highlight the significance of potassic and adakitic rocks in providing constraints on the geodynamic processes beneath southern Tibet, but also imply that

  9. Dating and source determination of volcanic rocks from Khunik area (South of Birjand, South Khorasan using Rb-Sr and Sm-Nd isotopes

    Somayeh Samiee


    Full Text Available The Khunik area is located in the south of Birjand, Khorasan province, in the eastern margin of Lut block. Tertiary volcanic rocks have andesite to trachy-andesite composition. Dating analyzing by Rb-Sr method on plagioclase and hornblende as well as whole-rock isochron method was performed on pyroxene-hornblende andesite rock unit. On this basis the emplacement age is Upper Paleocene (58±11 Ma. These rocks have initial 87Sr/86Sr and εNd 0.7046-0.7049 and 2.16-3.12, respectively. According to isotopic data, volcanic rocks originated from depleted mantle and have the least crust contamination while it was fractionated. Geochemically, Khunik volcanic rocks have features typical of calk-alkaline to shoshonite and are metaluminous. Enrichment in LILEs and typical negative anomalies of Nb and Ti are evidences that the volcanic rocks formed in a subduction zone and active continental margin. Modeling suggests that these rocks were derived dominantly from 1–5% partial melting of a mainly spinel garnet lherzolite mantle source that is metasomatized by slab-derived fluid.

  10. Age of Tertiary volcanic rocks on the West Greenland continental margin

    Larsen, Lotte M.; Pedersen, Asger K.; Tegner, Christian


    Radiometric ages for undated parts of the volcanic succession and intrusions in West Greenland were obtained by the 40Ar–39Ar incremental heating method. Acceptable crystallization ages were obtained for 27 samples. Combined with published results the new data provide a volcanic stratigraphy corr...

  11. Seismic properties of volcanic rocks from Montagne Pelée (Martinique, Lesser Antilles) and their relations to transport properties

    Bernard, M.-L.; Zamora, M.


    Numerous laboratory and theoretical studies on the physical properties of rocks and their relationships - lead mainly in the framework of petroleum exploration - show that rock physics is necessary for an accurate quantitative interpretation of geophysical observations. Moreover joint inversion of different geophysical datasets is emerging as an important tool to enhance resolution and decrease inversion artifacts in imaging of structurally complex areas such as volcanoes. In many cases, the coupling between the inverted parameters is based on empirical or theoretical relationships derived from laboratory data. Consequently rock physics can be used to: interpret simultaneously several geophysical datasets on volcanoes when they are available, improve the imaging of volcano structures, and better understand the coupled processes that can occur during volcanic unrest. It's in this context that we lead a laboratory study on the transport properties (permeability, thermal and electrical conductivities) and seismic properties (velocity and attenuation of P and S waves) of volcanic rocks representative of Montagne Pelée (Martinique) deposits. In this presentation we will focus on (1) the seismic properties and (2) the relations between seismic and transport properties. The 43 samples collected are representative of the main lithological units of this volcano: vesicular lava blocks and indurated ashed from indurated block-and-ash flows also called breccias, vesicular lava blocks from "Pelean nuee ardente" flows, scoriae from scoria flows, pumices from ash-and-pumices flows, and dense lava blocks from lava flows and lava domes. Their total porosity varies over a wide range from 4 to 73%. Since the samples present similar chemical and mineralogical compositions (andesites), the main difference between the samples comes from their pore structure and reflects differences in the mechanisms of magma degassing and vesiculation during their formation (Bernard et al., 2007). This

  12. Experiments and Spectral Studies of Martian Volcanic Rocks: Implications for the Origin of Pathfinder Rocks and Soils

    Rutherford, Malcolm J.; Mustard, Jack; Weitz, Catherine


    The composition and spectral properties of the Mars Pathfinder rocks and soils together with the identification of basaltic and andesitic Mars terrains based on Thermal Emission Spectrometer (TES) data raised interesting questions regarding the nature and origin of Mars surface rocks. We have investigated the following questions: (1) are the Pathfinder rocks igneous and is it possible these rocks could have formed by known igneous processes, such as equilibrium or fractional crystallization, operating within SNC magmas known to exist on Mars? If it is possible, what P (depth) and PH2O conditions are required? (2) whether TES-based interpretations of plagioclase-rich basalt and andesitic terrains in the south and north regions of Mars respectively are unique. Are the surface compositions of these regions plagioclase-rich, possibly indicating the presence of old AI-rich crust of Mars, or are the spectra being affected by something like surface weathering processes that might determine the spectral pyroxene to plagioclase ratio?

  13. Halogen content in Lesser Antilles arc volcanic rocks : exploring subduction recycling

    Thierry, Pauline; Villemant, Benoit; Caron, Benoit


    Halogens (F, Cl, Br and I) are strongly reactive volatile elements which can be used as tracers of igneous processes, through mantle melting, magma differentiation and degassing or crustal material recycling into mantle at subduction zones. Cl, Br and I are higly incompatible during partial melting or fractional cristallization and strongly depleted in melts by H2O degassing, which means that no Cl-Br-I fractionation is expected through magmatic differenciation [current thesis]. Thus, Cl/Br/I ratios in lavas reflect the halogen content of their mantle sources. Whereas these ratios seemed quite constant (e.g. Cl/Br =300 as seawater), recent works suggest significant variations in arc volcanism [1,2]. In this work we provide high-precision halogen measurements in volcanic rocks from the recent activity of the Lesser Antilles arc (Montserrat, Martinique, Guadeloupe, Dominique). Halogen contents of powdered samples were determined through extraction in solution by pyrohydrolysis and analysed by Ion Chromatography for F and Cl and high performance ICP-MS (Agilent 8800 Tripe Quad) for Cl, Br and I [3,4]. We show that lavas - and mantle sources - display significant vraiations in Cl/Br/I ratios along the Lesser Antilles arc. These variations are compared with Pb, Nd and Sr isotopes and fluid-mobile elements (Ba, U, Sr, Pb etc.) compositions which vary along the arc from a nothern ordinary arc compositions to a southern 'crustal-like' composition [5,6]. These characteristics are attributed to subducted sediments recycling into the mantle wedge, whose contribution vary along the arc from north to south [7,8]. The proportion of added sediments is also related to the distance to the trench as sediment melting and slab dehydration may occur depending on the slab depth [9]. Further Cl-Br-I in situ measurements by LA-ICP-MS in Lesser Antilles arc lavas melt inclusions will be performed, in order to provide better constraints on the deep halogen recycling cycle from crust to

  14. Geology, thermal maturation, and source rock geochemistry in a volcanic covered basin: San Juan sag, south-central Colorado

    Gries, R.R. [Priority Oil & Gas, Denver, CO (United States); Clayton, J.L. [Geological Survey, Denver, CO (United States); Leonard, C. [Platte River Associates, Denver, CO (United States)


    The San Juan sag, concealed by the vast San Juan volcanic field of south-central Colorado, has only recently benefited from oil and gas wildcat drilling and evaluations. Sound geochemical analyses and maturation modeling are essential elements for successful exploration and development. Oil has been produced in minor quantities from an Oligocene sill in the Mancos Shale within the sag, and major oil and gas production occurs from stratigraphically equivalent rocks in the San Juan basin to the southwest and in the Denver basin to the northeast. The objectives of this study were to identify potential source rocks, assess thermal maturity, and determine hydrocarbon-source bed relationships. Source rocks are present in the San Juan sag in the upper and lower Mancos Shale (including the Niobrara Member), which consists of about 666 m (2184 ft) of marine shale with from 0.5 to 3.1 wt. % organic carbon. Pyrolysis yields (S{sub 1} + S{sub 2} = 2000-6000 ppm) and solvent extraction yields (1000-4000 ppm) indicate that some intervals within the Mancos Shale are good potential source rocks for oil, containing type II organic matter, according to Rock-Eval pyrolysis assay.

  15. Experimental and textural constraints on mafic enclave formation in volcanic rocks

    Coombs, M.L.; Eichelberger, J.C.; Rutherford, M.J.


    We have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at Southwest Trident volcano, Alaska. Andesite enclaves are present throughout the four dacite lava flows produced by the eruption, and resemble mafic enclaves commonly found in other volcanic rocks. Our experiments replicate the pressure-temperature-time path taken by enclave-forming andesite magma as it is engulfed in dacite during magma mixing. Pressure and temperature information for the andesite and dacite are from [Coombs et al., Contrib. Mineral. Petrol. 140 (2000) 99-118]. The andesite was annealed at 1000??C, and then cooled to 890??C at rates of 110??C h1, 10??C h1 and 2??C h1. Once cooled to 890??C, andesite was held at this lower temperature from times ranging from 1 to 40 h. The andesite that was cooled at the slower rates of 2??C h1 and 10??C h1 most resembles enclave groundmass texturally and compositionally. Based on simple thermal calculations, these rates are more consistent with cooling of the andesite groundmass below an andesite-dacite interface than with cooling of enclave-sized spheres. If enclaves do crystallize as spheres, post-crystallization disaggregation must occur. Calculations using the MELTS algorithm [Ghiorso and Sack, Contrib. Mineral. Petrol. 119 (1995) 197-212] show that for incoming andesite to become less dense than the dacite to become less dense ???34 volume % of its groundmass must crystallize to undergo~18 volume % vesiculation; these values are similar to those determined for Southwest Trident enclaves. Thus such crystallization may lead to 'flotation' of enclaves and be a viable mechanism for enclave formation and dispersal. The residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments. Decompression experiments on Southwest Trident dacite suggest an average ascent rate for the eruption of ???2

  16. Preparation of Al-Si Master Alloy by Electrochemical Reduction of Volcanic Rock in Cryolite Molten Salt

    Liu, Aimin; Shi, Zhongning; Xu, Junli; Hu, Xianwei; Gao, Bingliang; Wang, Zhaowen


    Volcanic rock found in the Longgang Volcano Group in Jilin Province of China has properties essentially similar to Apollo lunar soils and previously prepared lunar soil simulants, such as Johnson Space Center Lunar simulant and Minnesota Lunar simulant. In this study, an electrochemical method of preparation of Al-Si master alloy was investigated in 52.7 wt.%NaF-47.3 wt.%AlF3 melt adding 5 wt.% volcanic rock at 1233 K. The cathodic electrochemical process was studied by cyclic voltammetry, and the results showed that the cathodic reduction of Si(IV) is a two-step reversible diffusion-controlled reaction. Si(IV) is reduced to Si(II) by two electron transfers at -1.05 V versus platinum quasi-reference electrode in 52.7 wt.%NaF-47.3 wt.%AlF3 molten salt adding 5 wt.% volcanic rock, while the reduction peak at -1.18 V was the co-deposition of aluminum and silicon. In addition, the cathodic product obtained by galvanostatic electrolysis for 4 h was analyzed by means of x-ray diffraction, x-ray fluorescence, scanning electron microscopy and energy dispersive spectrometry. The results showed that the phase compositions of the products are Al, Si, Al5FeSi, and Al3.21Si0.47, while the components are 90.5 wt.% aluminum, 4.4 wt.% silicon, 1.9 wt.% iron, and 0.2 wt.% titanium.

  17. Petrology and chemistry of late Cretaceous volcanic rocks from the southernmost segment of the Western Cordillera of Colombia (South America)

    Spadea, P.; Espinosa, A.


    This paper presents new data on the petrology and chemistry of the igneous rocks composing the Mesozoic basement of southernmost Western Cordillera of Colombia along the Ricaurte-Altaquer section. The studied sequence includes variably metamorphosed submarine lavas, breccias, tuffs and dykes of basalt to andesite composition, and minor, shallow quartz microdiorite intrusives. A Campanian age is recorded by radiolarian faunas from chert strata capping the lavas. Two different tholeiitic suites and a younger calc-alkaline suite, represented by hornblende andesite, are distinguished. One tholeiitic suite, represented by plagioclase and pyroxene phyric lavas, evolves from basalt to basaltic andesite. It is characterized by the occurrence of diopsidic pyroxene as early crystallising phase, by depletion in high-field strength elements, particularly Nb and by relative enrichment in light REE and Th. The second tholeiitic suite, which includes aphyric or poorly phyric lavas of basalt to dacite composition, differs from the first group in having moderately low {FeO tot}/{MgO} ratio and lower P 2O 5 content for a given SiO 2, and higher {Ti}/{Zr}and{Y}/{Zr} ratios. The pyroxene chemistry of the two suites also differs. The primary geochemical characteristics of the two suites suggest a similarity with tholeiitic suites generated in island-arc environment. The hornblende andesite has mineralogical and chemical characteristics of calc-alkaline lavas erupted in an oceanic setting in an evolved island-arc. Petrologic and geochemical evidence suggests that the volcanic rocks from the Ricaurte-Altraquer section are similar to the island-arc tholeiite volcanics from the upper Macuchi Formation of northern Ecuador and can be correlated partly with this unit. Conversely, they are petrochemically dissimilar from the typical Diabase Group volcanic rocks, characterized by transitional MORB lavas, extensively present to the north in the Western Cordillera of Colombia.

  18. The Early Andean Magmatic Province (EAMP): 40Ar/ 39Ar dating on Mesozoic volcanic and plutonic rocks from the Coastal Cordillera, northern Chile

    Oliveros, Verónica; Féraud, Gilbert; Aguirre, Luis; Fornari, Michel; Morata, Diego


    The Early Andean Magmatic Province (EAMP), consists of about 150 000 km 3 of volcanic and plutonic units in the Coastal Cordillera of northern Chile and southern Peru and represents a major magmatic Mesozoic event in the world, for which the precise age of the thick volcanic series was unknown. Thirty 40Ar/ 39Ar analyses were carried out on primary mineral phases of volcanic and plutonic rocks from northern Chile (18°30'-24°S). Reliable plateau and "mini plateau" ages were obtained on plagioclase, amphibole and biotite from volcanic and plutonic rocks, despite widespread strong alteration degree. In the Arica, Tocopilla and Antofagasta (700 km apart) regions, the ages obtained on lava flows constrain the volcanic activity between 164 and 150 Ma and no N-S migration of volcanism is observed. The uppermost lava flows of the volcanic sequence at the type locality of the La Negra Formation extruded at ca. 153-150 Ma, suggesting the end of the volcanic activity of the arc at that time. The oldest volcanic activity occurred probably at ca. 175-170 Ma in the Iquique area, although no plateau age could be obtained. The plutonic bodies of the same regions were dated between ca. 160 and 142 Ma, indicating that they were partly contemporaneous with the volcanic activity. At least one volcanic pulse around 160 Ma is evidenced over the entire investigated reach of the EAMP, according to the ages found in Arica, Tocopilla, Michilla and Mantos Blancos regions. The episodic emplacement of huge amounts of subduction related volcanism is observed throughout the whole Andean history and particularly during the Jurassic (southern Peru, northern Chile and southern Argentina). These events probably correspond to periodic extensional geodynamic episodes, as a consequence of particular subduction conditions, such as change of obliquity of the convergence, change in the subduction angle, slab roll back effect or lower convergence rate, that remain to be precisely defined.

  19. Along-arc geochemical and isotopic variations in Javanese volcanic rocks: 'crustal' versus 'source' contamination at the Sunda arc, Indonesia

    Handley, H.; Blichert-Toft, J.; Turner, S.; Macpherson, C. G.


    Understanding the genesis of volcanic rocks in subduction zone settings is complicated by the multitude of differentiation processes and source components that exert control on lava geochemistry. Magma genesis and evolution at the Sunda arc is controlled and influenced by 1) along arc changes in the composition and thickness of the overriding Eurasian plate, 2) the variable age of the subducting oceanic crust and, 3) changes in the type and amount of sediment deposited on the subducting plate. Along-arc changes in geochemistry have long been recognised in the Sunda arc (Whitford, 1975), but debate still prevails over the cause of such variations and the relative importance of shallow (crustal) versus deep (subduction) contamination at the Sunda arc, Indonesia. Detailed study of individual Sunda arc volcanic centres is, therefore, a prerequisite in order to establish the relative importance and contributions of various potential source components and composition modifying differentiation processes at individual volcanoes, prior to an along arc comparative petrogenetic investigation. We present new radiogenic isotope data for Javanese volcanoes, which is combined with our recently published (Handley et al., 2007; Handley et al., 2008, Handley et al., 2010; Handley et al., 2011) geochemical and isotopic data of Javanese volcanic rocks along with data from other detailed geochemical studies to establish whether variable contributions from the subducting slab, or a change in crustal architecture of the overriding plate, best explain along-arc variations in isotope ratios and trace element characteristics. In West and Central Java Sr isotope ratios of the volcanic rocks broadly correlate with inferred lithospheric thickness implicating a shallow level control on isotopic composition. However, key trace element ratios combined with Hf isotope data indicate that the subducted slab and slab thermal regime also exert major control on the composition of the erupted Javanese

  20. Composition of plagioclases in volcanic rocks of King George Island, Antarctica with reference to the petrogenetic significance


    Plagioclases occur mainly as phenocrysts in volcanic rocks of King George Island, South Shetland Islands, West Antarctica. In basaltic andesites and andesites of Keller Peninsula and Ullman Spur (Admiralty Bay), they are high structure state labradorite-andesines; and in high-A1 basalts and basaltic andesites of Barton and Weaver peninsulas (Maxwell Bay), they are high structure state bytownite-anorthites.∑REE, La/Yb ratios and δEu values of plagioclases from Admiralty Bay are higher than those from Maxwell Bay. All plagioclases have rather identical chondritenormalized transitional element distribution patterns, probably reflecting that crystal structure rather than composition of plagioclase controls their diversity. Compositions of plagioclases depend chiefly on those of their host rocks, compositional differences of plagioclases reveal that basaltic magmas in the Admiralty Bay area are more evolved than in the Maxwell Bay area.

  1. The Ediacaran volcanic rocks and associated mafic dykes of the Ouarzazate Group (Anti-Atlas, Morocco): Clinopyroxene composition, whole-rock geochemistry and Sr-Nd isotopes constraints from the Ouzellarh-Siroua salient (Tifnoute valley)

    Belkacim, Said; Gasquet, Dominique; Liégeois, Jean-Paul; Arai, Shoji; Gahlan, Hisham A.; Ahmed, Hassan; Ishida, Yoshito; Ikenne, Moha


    Belonging to the huge Ouarzazate volcanic Group that covered the whole Anti-Atlas during the late Ediacaran (580-545 Ma), the Tifnoute valley volcanic formations are mainly pyroclastic and show a large composition, from trachybasalt to rhyolite and are crosscut by dolerite dykes. The Tifnoute valley volcanic rocks are located within a rigid salient of the Anti-Atlas that gives them special extreme characteristics. Due to the heavy greenschist alteration that affects this volcanic group, we focused the more immobile elements, but as REE can also be affected, we used the composition of unaltered clinopyroxene crystals to determine the nature of these volcanic rocks. The clinopyroxene is an augite diopside in the basalt, an augite in the andesite and an augite-salite in the dolerite. Petrography of the Tifnoute mafic volcanic rocks and clinopyroxene compositions indicate the presence of two magmatic series: (i) older high-K calc-alkaline (alkali-calcic) andesite and basalt characterized by the early crystallization of Fe-Ti oxides and of the late fractionation of plagioclase, the modal proportion of the latter increasing from the basalt to the andesite and (ii) younger alkalic dolerite dykes. With clinopyroxene trace element compositions obtained using laser ablation ICP-MS, we calculated the composition of the melts in equilibrium with the pyroxenes. The volcanic rocks of the Tifnoute Valley have positive εNd570 (+1.7 to +5.0), low Sri (volcanic rocks emplaced in a Pan-African transtensive post-collisional environment that evolved towards the major rifting event that will give rise to the Rheic ocean, in a similar way to what occurred just after the Variscan orogeny during the Triassic period that evolved to the Tethys ocean opening.

  2. Geochemistry of the volcanic rocks from Bioko Island (“Cameroon Hot Line”: Evidence for plume-lithosphere interaction

    Fadimatou Ngounouno Yamgouot


    Full Text Available Bioko Island (3008 m a.s.l is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine (83% < Fo < 87% and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase (An62-67Ab35-32Or3-1. Major element variation diagrams show an increase in SiO2, Al2O3, Na2O and K2O with increasing MgO for the studied rock groups. The rocks are characterized by low (86Sr/87Sri ratios (0.70320–0.70406, high ɛNd(t values (2.56–4.33 and high (206Pb/204Pbi ratios (20.032–20.035 values. Basalts are enriched in LILE and LREE, and have (Hf/SmN = 0.57–1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.

  3. Hydrothermal alteration in volcanic rocks, eastern part of the Lukavice Group, Železné Hory Mountains, Czech Republic

    Pertold, Z.; Watkinson, D. H.; Novotný, L.


    Many rocks mapped as felsic metavolcanics in the eastern part of the Lukavice Group are shown to be altered mafic metavolcanics, similar to those in the Noranda and Flin Flon-Snow Lake mining districts, Canada. The relatively fresh rocks of the Lukavice Group are rhyolite, dacite-andesite, and andesite-basalt of calcalka-line character. Assuming no substantial volume change during alteration, Ti, P, La, Ce, Yb, Lu, Th (partly), Sc and V contents remained unchanged. Altered rocks are enriched in (Fe + Mg), K and Si and depleted in Na, Ca and Zr. Some elements show both increased and decreased contents in altered rocks (Mg, Ba, Sm, eu, Tb and Hf). Although hydrothermal alteration in the Lukavice Group is of large extent, it is of the proximal Kuroko style and not of regional ‘Amulet Rhyolite’ style. Implications for a large hydrothermal system within a volcanic pile are discussed in relation to the Ordovician Lukavice Group and its mineral deposits and to some other parts of the Bohemian Massif with volcanosedimentary sequences of the same age.

  4. Ar-Ar geochronology of Late Mesozoic volcanic rocks from the Yanji area,NE China and tectonic implications

    LI ChaoWen; GUO Feng; FAN WeiMing; GAO XiaoFeng


    Ar-Ar dating results of late Mesozoic-Cenozoic volcanic rocks from the Yanji area, NE China provide a new volcano-sedimentary stratigraphic framework. The previously defined "Triassic-Jurassic" volcanic rocks(including those from Sanxianling, Tuntianying, Tianqiaoling and Jingouling Fms.)were erupted during 118-106Ma,corresponding to Early Cretaceous. The new eruption age span is slightly younger than the main stage(130-120 Ma)of the extensive magmatism in the eastern Central Asian Orogenic Belt and its adjacent regions. Subduction-related adakites occurring in the previously defined Quanshuicun Fm. Were extruded at ca.55 Ma. Based on these new Ar-Ar ages, the late Mesozoic to Palaeocene volcano-sedimentary sequences is rebuilt as:Tuopangou Fm., Sanxianling/Tuntianying Fm.(118-115 Ma),Malugou/Tianqiaoling Fm.(K1),Huoshanyan/Jingouling Fm.(108-106 Ma),Changcai Fm.(K2),Quanshuicun Fm.(~55 Ma)and Dalazi Fm. Our results suggest that subduction of the Palaeo-Pacific Ocean beneath the East Asian continental margin occurred during 106to 55 Ma. Consistent with the paleomagnetic observations and magmatic records which indicated that the Izanagi-Farallon ridge subduction beneath the southwestern Japan took place during 95-65 Ma.

  5. Ar-Ar geochronology of Late Mesozoic volcanic rocks from the Yanji area, NE China and tectonic implications


    Ar-Ar dating results of late Mesozoic-Cenozoic volcanic rocks from the Yanji area, NE China provide a new volcano-sedimentary stratigraphic framework. The previously defined “Triassic-Jurassic” volcanic rocks (including those from Sanxianling, Tuntianying, Tianqiaoling and Jingouling Fms.) were erupted during 118―106 Ma, corresponding to Early Cretaceous. The new eruption age span is slightly younger than the main stage (130―120 Ma) of the extensive magmatism in the eastern Central Asian Orogenic Belt and its adjacent regions. Subduction-related adakites occurring in the previously defined Quanshuicun Fm. were extruded at ca. 55 Ma. Based on these new Ar-Ar ages, the late Mesozoic to Palaeocene volcano-sedimentary sequences is rebuilt as: Tuopangou Fm., Sanxianling/Tuntianying Fm. (118―115 Ma), Malugou/Tianqiaoling Fm. (K1), Huoshanyan/Jingouling Fm. (108―106 Ma), Changcai Fm. (K2), Quanshuicun Fm. (~55 Ma) and Dalazi Fm. Our results suggest that subduction of the Pa- laeo-Pacific Ocean beneath the East Asian continental margin occurred during 106 to 55 Ma, consistent with the paleomagnetic observations and magmatic records which indicated that the Izanagi-Farallon ridge subduction beneath the southwestern Japan took place during 95―65 Ma.

  6. Syntectonic remagnetization in the southern Methow block: Resolving large displacements in the southern Canadian Cordillera

    Enkin, R.J.; Mahoney, J.B.; Baker, J.; Kiessling, M.; Haugerud, R.A.


    The Upper Cretaceous Ventura Member of the Goat Wall unit in the southern Methow block of southern British Columbia and northern Washington State holds a syntectonic magnetization. Eight new sites from Manning Park in British Columbia give a mean direction of D = 27.5??, I = 60.1??, k = 304.7, ??95 = 3.2?? after optimal partial tilt correction. Of five groups of bedded sites from farther south in the basin reported by Bazard et al. [1990], four have a syntectonic remanence with a direction similar to what we observe. The exception is one group which has optimal concentration of remanence directions on >100% untilting and an abherent direction which must be rejected. Combining the accepted sites, the optimal differential syntilting direction is D = 11.8??, I = 61.5??, k = 39.3, ??95 = 3.4?? (N = 47), giving a mean pole of 79.8??N, 359.2??E, K = 19.5, and A95 = 4.8??. The age of the remagnetization is constrained to be between 88 and 80 Ma. Compared to cratonic North America, this result indicates that the southern Methow block was displaced from the south by 1800 ?? 500 km, meaning it lay south of the Sierra Nevada subduction zone but well north of other paleomagnetically constrained Cretaceous rock units from the Insular superterrane, including correlative strata of the Mount Tatlow area in the northern Methow block. Among several possibilities to reconcile this discrepancy, the most plausible has the whole Methow block translated coherently but with the southern Methow block strata remagnetized during transit.

  7. Geochemistry and tectonic setting of alkaline volcanic rocks in the Antarctic Peninsula: A review

    Smellie, J. L.


    The numerous Miocene-Recent alkaline volcanic outcrops in the Antarctic Peninsula form a substantial volcanic province, the least well-known part of a major belt of alkaline volcanism that extends between South America and New Zealand. The outcrops consists mainly of aa and pahoehoe lavas and hyaloclastites which locally contain accidental nodules of spinel lherzolite and other mantle-derived lithologies. The province is predominantly basaltic with two major differentiation lineages: (1) a sodic series of olivine and alkali basalt, hawaiite, mugearite, trachy-phonolite and trachyte; and (2) a relatively potassic, highly undersaturated series of basanite, tephrite and phono-tephrite. All the lavas show varying effects of fractionation by crystallization of olivine and clinopyroxene, joined by plagioclase in the hawaiites to trachytes. Fractional crystallization can probably explain most of the chemical variation observed within each outcrop, but variable partial melting is necessary to account for the differences in incompatible element enrichment between the two series, and between the individual outcrops. The degree of partial melting may not have exceeded 3%, as is the case for many other alkaline magmas. The volcanism is an intraplate phenomenon but there is no correlation in timing between the cessation of subduction and the inception of alkaline volcanism. The activity cannot be related to the passage of the coupled Pacific-Antarctic plate over a stationary mantle hot-spot. Although the precise causal relationship with tectonic setting is unknown, regional extension was a prerequisite for giving the magmas rapid access to the surface.

  8. Chronological dating and tectonic implications of late Cenozoic volcanic rocks and lacustrine sequence in Oiyug Basin of southern Tibet


    Reconstruction of uplift history of the Tibetan Plateau is crucial for understanding its environmental impacts. The Oiyug Basin in southern Tibet contains multiple periods of sedimentary sequences and volcanic rocks that span much of the Cenozoic and has great potential for further studying this issue. However, these strata were poorly dated. This paper presents a chronological study of the 145 m thick and horizontally-distributed lacustrine sequence using paleomagnetic method as well as a K-Ar dating of the underlying volcanic rocks. Based on these dating results, a chronostratigraphic framework and the basin-developmental history have been established for the past 15 Ma, during which three tectonic stages are identified. The period of 15-8.1 Ma is characterized by intense volcanic activities involving at least three major eruptions. Subsequently, the basin came into a tectonically quiescent period and a lacustrine sedimentary sequence was developed. Around 2.5 Ma, an N-S fault occurred across the southern margin of the basin, leading to the disappearance of the lake environment and the development of the Oiyug River. The Gyirong basin on northern slope of the Himalayas shows a similar basin developmental history and thus there is a good agreement in tectonic activities between the Himalayan and Gangdise orogenic belts. Therefore, the tectonic evolution stages experienced by the Oiyug Basin during the past 15 Ma could have a regional significance for southern Tibet. The chronological data obtained from this study may provide some constraints for further studies with regard to the tectonic processes and environmental changes in southern Tibetan Plateau.

  9. Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

    Temizel, İrfan; Arslan, Mehmet; Yücel, Cem; Abdioğlu, Emel; Ruffet, Gilles


    40Ar-39Ar age, whole-rock chemical, and Sr-Nd isotope data are presented for the post-collisional, Eocene (51.3-44.1 Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal, and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded, and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K, and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr, and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042-0.7051), high positive εNd(t) values (+ 0.20 to + 3.32), and depleted mantle Nd model ages (TDM1 = 0.63-0.93 Ga, TDM2 = 0.58-0.84 Ga). The bulk-rock chemical and Sr-Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element

  10. Zircon SHRIMP U-Pb dating for the Cangshuipu volcanic rocks and its implications for the lower boundary age of the Nanhua strata in South China

    WANG Jian; LI Xianhua; DUAN Taizhong; LIU Dunyi; SONG Biao; LI Zhongxiong; GAO Yonghua


    The continental volcanic rocks and volcaniclastic sedimentary conglomerates of the Cangshuipu Formation occur well in Yiyang of Hunan Province, consisting of a wedge-shaped succession of Neoproterozoic strata that overlie with high-angle unconformity the flysch turbidites of the Lengjiaxi Group in the Upper Mesoproterozoic Eonothem. SHRIMP zircon U-Pb dating gives a weighted mean age of 814 ( 12 Ma for the dacitic volcanic agglomerates from the lowest part of the volcanic rocks in the Cangshuipu Formation. This age is younger than previously reported values of 921-933 Ma for the volcanic rocks from the Cangshuipu Formation. Our new dating represents the lower boundary age of the Neoproterozoic System in the studied area. The younger age for the Cangshuipu volcanic rocks is supported by the following observations: (1) Lower Neoproterozoic strata (814-1000 Ma) are missing in the studied area; (2) the Nanhua rift system was initiated at about 820 Ma; and (3) an age of ~820 Ma may be taken as the lower boundary timing of the Nanhua System (even Neoproterozoic) in South China.

  11. Rare Earths, Trace Element Characteristics of High-Mg Volcanic Rocks of Yixian Formation in Sihetun West Liaoning Province and Its Apocalypse

    Zhang Hong; Zhang Qi


    The high-Mg volcanic rocks of the Yixian Formation in the Sihetun area have the obvious characteristics of mantle-derived lava in rare earth, trace element characteristics with high Mg# (62~70) and high content of compatible elements. In the meantime, the volcanic rocks also have the obvious characteristics of Crust-source material in rare earth, trace element characteristics with high ∑REE (158.78×10-6~359.66×10-6), high (La/Yb)N (14.61~29.60), high La/Nb(2.37~7.52) and high Ba/Nb(67.58~205.96), obvious positive anomaly of Pb and negative anomaly of Nb, Ta in trace element spider-gram. In Sr-Nd-Pb isotope the (87Sr/86Sr)i ratio is higher than 0.706 and the εNd(t) ratio is from -3.4 to -13, both reflect enriched Mantle characteristics. The characteristics above of the volcanic rocks combined with the content of Sr, Ba, Y, Yb and the ratio of Sr/Y show that the volcanic rocks have the property of the Sanukite rocks in Setouchi Japan beside subduction zone, and illuminate that the Sanukite rocks can be formed not only in island-arc near subduction zone but also in intro-plate. The analysis indicates that the high-Mg volcanic rocks in the Sihetun area result from the collective function of mantle-derived lava and crust-source materials. The result illuminates that the West Liaoning region is very special in tectonic geochemical background in Cretaceous in East China, and is an ideal region for us to further study the characteristics of magmatic activity as well as the process of Crust-Mantle interaction in Eastern China.

  12. Searching for Rich Uranium Layers of Volcanic Rocks by Measuring Potential Difference%测定电位差圈定火山岩富铀层位

    袁富蕴; 刘峰


    (2.岩石的电位差(△Eh)控制着变价元素的地球化学行为,用差减电位法对330铀矿区393个火山岩△Eh值测量结果表明:火山岩的△Eh值可以准确地圈出铀的富集层位,铀矿化只产在△Eh值高的岩石中。%The potential difference(P.D) of rocks controls geochemical behavior of the element whose valence can change.393volcanic rocks are survied in the 330 uranium ore district by the potential difference method. The result shows: volcanic rocks P.D. accuratly the concetration place of uranium accumtly and uranium mineralize only in the rocks where P.D. value is high.

  13. Mineral chemistry of clinopyroxene: guidance on geo- thermobarometry and tectonomagmatic setting of Nabar volcanic rocks, South of Kashan

    Rezvan Mehvari


    Full Text Available Introduction The Nabar area that is a part of the Urumieh- Dokhtar volcano- plutonic belt is located in the south of Kashan. Research works such as Emami (Emami, 1993 and Abbasi (Abbasi, 2012 have been done about the geology of this area. Rock units in the study area contain middle- upper Eocene intermediate to acidic lavas and pyroclastic rocks, green marl, shale and sandy marls of Oligo- Miocene, limestones of Qom formation, intrusive granitoids with Oligo- Miocene age and quaternary travertine and recent alluvium (Emami, 1993. The volcanic and sub volcanic rocks of this area are composed of andesite, trachyandesite, dacite, rhyolite and porphyric pyroxene diorite along with pyroclastic rocks. Materials and methods In order to achieve the aims of this work, at first field surveying and sampling were done. Then, thin and polished thin sections were prepared. Some of the samples were selected for microprobe analysis and clinopyroxene minerals were analyzed by using JEOL- JXA-8800 analyzer with a voltage of 20 Kv and a current of 12 nA in the Kanazava University of Japan and Cameca-Sx100 analyzer with a voltage of 15 Kv and a current of 15 nA in the Iranian mineral processing research center, Karaj. Discussion On the basis of petrographic investigations, porphyritic, porphyroid, fluidal, amygdaloidal and porphyry with microlitic groundmass are common textures of these rocks. Also plagioclase, clinopyroxene, amphibole, biotite, sanidine and quartz are essential minerals, opaque, zircon and apatite as accessory minerals are observed in the studied rocks. Clinopyroxenes are observed with corona texture that resulted during the uralitization process. On the basis of minerals’ chemistry, pyroxenes are Fe- Mg- Ca type in composition (Morimoto et al., 1988. These clinopyroxenes are augite. Investigations indicate that mineral composition of clinopyroxene can be effectively used to evaluation the P-T conditions during crystallization. Previous research

  14. The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)

    Busquets, P.; Méndez-Bedia, I.; Gallastegui, G.; Colombo, F.; Cardó, R.; Limarino, O.; Heredia, N.; Césari, S. N.


    The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the

  15. Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

    Federico, C.; Aiuppa, A.; Allard, P.; Bellomo, S.; Jean-Baptiste, P.; Parello, F.; Valenza, M.


    We report in this paper a systematic investigation of the chemical and isotopic composition of groundwaters flowing in the volcanic aquifer of Mt. Vesuvius during its current phase of dormancy, including the first data on dissolved helium isotope composition and tritium content. The relevant results on dissolved He and C presented in this paper reveal that an extensive interaction between rising magmatic volatiles and groundwaters currently takes place at Vesuvius. Vesuvius groundwaters are dilute (mean TDS ˜ 2800 mg/L) hypothermal fluids ( mean T = 17.7°C) with a prevalent alkaline-bicarbonate composition. Calcium-bicarbonate groundwaters normally occur on the surrounding Campanian Plain, likely recharged from the Apennines. δD and δ 18O data evidence an essentially meteoric origin of Vesuvius groundwaters, the contribution from either Tyrrhenian seawater or 18O-enriched thermal water appearing to be small or negligible. However, the dissolution of CO 2-rich gases at depth promotes acid alteration and isochemical leaching of the permeable volcanic rocks, which explains the generally low pH and high total carbon content of waters. Attainment of chemical equilibrium between the rock and the weathering solutions is prevented by commonly low temperature (10 to 28°C) and acid-reducing conditions. The chemical and isotope (C and He) composition of dissolved gases highlights the magmatic origin of the gas phase feeding the aquifer. We show that although the pristine magmatic composition may vary upon gas ascent because of either dilution by a soil-atmospheric component or fractionation processes during interaction with the aquifer, both 13C/ 12C and 3He/ 4He measurements indicate the contribution of a magmatic component with a δ 13C ˜ 0‰ and R/R a of ˜2.7, which is consistent with data from Vesuvius fumaroles and phenocryst melt inclusions in olivine phenocrysts. A main control of tectonics on gas ascent is revealed by data presented in this paper. For example

  16. Geochemistry and tectonomagatic setting of Tertiary volcanic rocks of the Kangan area, northeast of Sarbisheh, southern Khorasan

    Mahshid Malekian Dastjerdi


    Full Text Available Introduction The study area is located 12km away from the north east of Sarbisheh at the eastern border of the Lut block (Karimpour et al., 2011; Richards et al., 2012. The magmatic activity in the Lut blockhas begun in the middle Jurassic (165-162 Ma and reached its peak in the Tertiary age (Jung et al., 1983; Karimpour et al., 2011. Volcanic and subvolcanic rocks in the Tertiary age cover over half of the Lut block with up to 2000 m thickness and they were formed due to subduction prior to the collision of the Arabian and Asian plates (Jung et al., 1983; Karimpour et al., 2011. In the Kangan area, the basaltic lavas cropped out beyond the above intermediate to acid volcanic rocks. In this area, bentonite and perlite deposits have an economic importance. The main purpose of this paper is to present a better understanding of the tectono-magmatic settings of volcanic rocks in the northeast of Sarbisheh, east of Iran based on their geochemical characteristics. Materials and methods Fifteen samples were analyzed for major elements by inductively coupled plasma (ICP technologies and trace elements by using inductively coupled plasma mass spectrometry (ICP-MS, following a lithium metaborate/tetraborate fusion and nitric acid total digestion, at the Acme laboratories, Vancouver, Canada. Results The Kangan area is located at the northeast of Sarbishe, Southern Khorasan and the eastern border of the Lut block. In this area, basaltic lavas have cropped out above intermediate to acid lavas such as andesite, dacite, rhyolite (sometimes perlitic .The main minerals in the basalt are plagioclase, olivine and pyroxene, in andesite contain plagioclase, pyroxene, biotite and amphibole and in acid rocks include plagioclase, quartz, sanidine, biotite and amphibole. Intermediate to acid rocks have medium to high-K calc-alkaline nature and basalt is alkaline. Enrichment in LREE relative to HREE (Ce/Yb= 21.14-28.7, high ratio of Zr/Y(4.79- 10.81, enrichment in LILE

  17. Thermal and geotectonic setting of Cretaceous volcanic rocks near Ica, Peru, in relation to Andean crustal thinning

    Atherton, M. P.; Aguirre, L.


    The Cretaceous volcanic rocks of the Ica region, Peru, were deposited in the Cañete marginal basin developed on the Precambrian Arequipa Massif, which was split and thinned along southwardpropagating faults. They are compositionally bimodal, calc-alkaline, high-Al 2O 3 rocks with shoshonitic affinities and a pronounced enriched within-plate character. This contrasts markedly with the equivalent rocks of the Huarmey Basin to the north, which lie directly on mantle-derived material and are chiefly low-K, thleiitic basalts and basaltic andesites, with trace-element abundances related to subduction and/or asthenospheric components. The Ica rocks also show a characteristic very low-grade, nondeformational prehnite-pumpellyite to zeolite facies metamorphism formed under moderate thermal gradients. This contrasts strongly with the high thermal gradient of the rocks to the north, which relates to the more extensive crustal splitting and/or thinning in that direction. These metamorphic patterns, together with the lava chemistry, are important elements in modeling the thermal and geotectonic setting, which has close affinities to a model of the Icelandic rifting system. The source of the Ica rocks is thought to be old enriched mantle beneath the Precambrian Arequipa Massif, in contrast to the mantle beneath the basin to the north, which is much younger and less enriched. This change along the axis of the basin is similar to the K-h relationships suggested at subduction zones. However, the direction of subduction along this sector has been approximately E-W, and the enrichment, which is a right angles to this, relates to the history of the subcontinental mantle along the Andean margin. It is also associated with an abrupt compositional change in the Peruvian Coastal Batholith rocks of slightly younger age, marking a major segmental difference in Mesozoic magma composition along the axis of the Andes. Crustal contributions are not considered to be important, certainly not

  18. Laser step-heating 40Ar/39Ar dating on young volcanic rocks

    WANG Fei; HE Huaiyu; ZHU Rixiang; YANG Liekun; SANG Haiqing; WANG Yinglan


    An attempt was made to use CO2 laser step-heating method to date the late Pleistocene basaltic groundmass (DF-2) from Tengchong volcanic field. Among the fourteen heating steps, ten define a good inverse isochron (MSWD = 1.4) with an age of 32.2 ± 7.1 ka (2 σ).The inverse isochron also shows that the initial argon isotopic ratio is 297.1±2.0 (2σ)which is the same as the atmospheric argon at the 2σ error level. Study indicates that it is a useful means to date young volcanic groundmass with low K content by using CO2 laser step-heating 40Ar/39Ar method.

  19. Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

    Ashima Saikia; Bibhuti Gogoi; Mansoor Ahmad; Talat Ahmad


    The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, crosscutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

  20. Geochemical constraints on the evolution of mafic and felsic rocks in the Bathani volcanic and volcano-sedimentary sequence of Chotanagpur Granite Gneiss Complex

    Saikia, Ashima; Gogoi, Bibhuti; Ahmad, Mansoor; Ahmad, Talat


    The Bathani volcanic and volcano-sedimentary (BVS) sequence is a volcanic and volcano-sedimentary sequence, best exposed near Bathani village in Gaya district of Bihar. It is located in the northern fringe of the Chotanagpur Granite Gneiss Complex (CGGC). The volcano-sedimentary unit comprises of garnet-mica schist, rhyolite, tuff, banded iron formation (BIF) and chert bands with carbonate rocks as enclaves within the rhyolite and the differentiated volcanic sequence comprises of rhyolite, andesite, pillow basalt, massive basalt, tuff and mafic pyroclasts. Emplacement of diverse felsic and mafic rocks together testifies for a multi-stage and multi-source magmatism for the area. The presence of pillow basalt marks the eruption of these rocks in a subaqueous environment. Intermittent eruption of mafic and felsic magmas resulted in the formation of rhyolite, mafic pyroclasts, and tuff. Mixing and mingling of the felsic and mafic magmas resulted in the hybrid rock andesite. Granites are emplaced later, cross-cutting the volcanic sequence and are probably products of fractional crystallization of basaltic magma. The present work characterizes the geochemical characteristics of the magmatic rocks comprising of basalt, andesite, rhyolite, tuff, and granite of the area. Tholeiitic trend for basalt and calc-alkaline affinities of andesite, rhyolite and granite is consistent with their generation in an island arc, subduction related setting. The rocks of the BVS sequence probably mark the collision of the northern and southern Indian blocks during Proterozoic period. The explosive submarine volcanism may be related to culmination of the collision of the aforementioned blocks during the Neoproterozoic (1.0 Ga) as the Grenvillian metamorphism is well established in various parts of CGGC.

  1. Geochemistry and petrology of the Early Miocene lamproites and related volcanic rocks in the Thrace Basin, NW Anatolia

    Ersoy, Yalçın E.; Palmer, Martin R.; Uysal, İbrahim; Gündoğan, İbrahim


    The extensional Thrace basin (NW Anatolia) contains an association of early Miocene diopside-leucite-phlogopite (Doğanca) and diopside-phlogopite (Korucuköy) lamproites with Oligocene medium-K calc-alkaline andesites (Keşan volcanics), early Miocene shoshonitic rocks (Altınyazı trachyte) and middle Miocene Na-alkaline basalts (Beğendik basalts). The Doğanca lamproite (K2O = 5.1-5.5 wt.%; K/Na = 2.78-2.89; MgO = 11.4-11.8 wt.%) consists of olivine (Fo71-86), diopside (Al2O3 = 1.0-5.0, Na2O = 0.2-0.6), phlogopite (TiO2 = 1.1-9.4, Al2O3 = 11.1-13.9), spinel (Mg# = 22.9-32.6; Cr# = 64-83.4), leucite, apatite, zircon, Fe-Ti-oxides and magnetite in a poikilitic sanidine matrix. The potassic volcanic units (lamproites and trachytes) in the region have similarly high Sr and low Nd isotopic compositions (87Sr/86Sr(i) = 0.70835-0.70873 and 143Nd/144Nd(i) = 0.51227-0.51232). The major and trace element compositions and Sr-Nd-Pb isotopic ratios of the shoshonitic, ultrapotassic and lamproitic units closely resemble those of other Mediterranean ultrapotassic lamproites (i.e., orogenic lamproites) from Italia, Serbia, Macedonia and western Anatolia. The Beğendik basalts show intraplate geochemical signatures with an Na-alkaline composition, an absence of Nb negative anomalies on primitive mantle-normalized multi-element diagrams, as well as low Sr (~ 0.70416) and high Nd (0.51293) isotopic ratios; and include olivine (Fo72-84), diopside, spinel, Fe-Ti-oxides and magnetite. The Oligocene Keşan volcanics were emplaced in the earlier stages of extension in Thrace, and represent the typical volcanic products of post-collisional volcanism. The continental crust-like trace element abundances and isotopic compositions of the most primitive early Miocene ultrapotassic rocks (Mg# up to 74) indicate that their mantle sources were intensely contaminated by the continental material. By considering the geodynamic evolution of the region, including oceanic subduction, crustal

  2. Stable isotope compositions and water contents of boninite series volcanic rocks from Chichi-jima, Bonin Islands, Japan

    Dobson, P.F.; O'Neil, J.R.


    Measurements of stable isotope compositions and water contents of boninite series volcanic rocks from the island of Chichi-jima, Bonin Islands, Japan, confirm that a large amount (1.6-2.4 wt.%) of primary water was present in these unusual magmas. An enrichment of 0.6??? in 18O during differentiation is explained by crystallization of 18O-depleted mafic phases. Silicic glasses have elevated ??18O values and relatively low ??D values indicating that they were modified by low-temperature alteration and hydration processes. Mafic glasses, on the other hand, have for the most part retained their primary isotopic signatures since Eocene time. Primary ??D values of -53 for boninite glasses are higher than those of MORB and suggest that the water was derived from subducted oceanic lithosphere. ?? 1987.

  3. Volcanism and Oil & Gas In Northeast China

    Shan Xuanlong


    Based on study on the relation with volcanic rock and oil & gas in Songliao Basin and Liaohe Basin in northeast China, author proposes that material from deep by volcanism enrichs the resources in basins, that heat by volcanism promotes organic matter transforming to oil and gas, that volcanic reservoir is fracture, vesicular, solution pore, intercrystal pore.Lava facies and pyroclastic facies are favourable reservoir. Mesozoic volcanic reservoir is majority of intermediate, acid rock,but Cenozoic volcanic reservoir is majority of basalt. Types of oil and gas pool relating to volcanic rock include volcanic fracture pool, volcanic unconformity pool, volcanic rock - screened pool, volcanic darpe structural pool.

  4. Mineralogy of Rock Flour in Glaciated Volcanic Terrains: An Analog for a Cold and Icy Early Mars

    Rampe, E. B.; Horgan, B.; Scudder, N.; Smith, R. J.; Rutledge, A. M.


    Geomorphological and mineralogical data from early Martian surfaces indicate liquid water was present on ancient Mars. The relative surface temperatures, however, remain a subject of debate. Was early Mars warm and wet or cold and icy with punctuated periods of warmth and ice melt? By characterizing the mineralogy and geochemistry of modern icy mafic terrains on Earth, we can search for these characteristics in early Martian terrains to better constrain the early Martian climate. Here, we describe the mineralogy of glacial flour in a modern glaciated volcanic terrain in Oregon, USA. We are particularly interested in secondary phases that form in these environments, and we hypothesize that poorly crystalline phases may preferentially form in these terrains because of the low temperatures and the seasonality of melt water production. A description of the mineralogy of the moraines, the composition of the amorphous materials, and the geochemistry of the glacial melt waters are presented elsewhere. Glacial flour is made up of silt- and clay-sized particles that form from the physical weathering of rock underlying a wet-based glacier as the glacier slides over it. Flour is usually transported from underneath a glacier by melt water streams. The geochemistry of glacial melt water streams has been studied extensively and has been used to infer weathering reactions within glacial systems. However, the mineralogy of these environments, especially on mafic volcanic terrains, is not well studied. Rock flour is a ubiquitous physical weathering product in glaciated terrains and, therefore, affects microbial habitats, stream and lake chemistry, and chemical weathering processes. and by studying the mineralogy of glacial flour, we can better understand geochemical and microbiological processes in subglacial and proglacial terrains.

  5. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

    Huang, Wentao; Dupont-Nivet, Guillaume|info:eu-repo/dai/nl/313092559; Lippert, Peter C.; Van Hinsbergen, Douwe J J; Dekkers, Mark J.|info:eu-repo/dai/nl/073463744; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong

  6. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; Van Hinsbergen, Douwe J J; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Gr

  7. Geochronology and correlation of Tertiary volcanic and intrusive rocks in part of the southern Toquima Range, Nye County, Nevada

    Shawe, Daniel R.; Snee, Lawrence W.; Byers, Frank M.; du Bray, Edward A.


    Extensive volcanic and intrusive igneous activity, partly localized along regional structural zones, characterized the southern Toquima Range, Nevada, in the late Eocene, Oligocene, and Miocene. The general chronology of igneous activity has been defined previously. This major episode of Tertiary magmatism began with emplacement of a variety of intrusive rocks, followed by formation of nine major calderas and associated with voluminous extrusive and additional intrusive activity. Emplacement of volcanic eruptive and collapse megabreccias accompanied formation of some calderas. Penecontemporaneous volcanism in central Nevada resulted in deposition of distally derived outflow facies ash-flow tuff units that are interleaved in the Toquima Range with proximally derived ash-flow tuffs. Eruption of the Northumberland Tuff in the north part of the southern Toquima Range and collapse of the Northumberland caldera occurred about 32.3 million years ago. The poorly defined Corcoran Canyon caldera farther to the southeast formed following eruption of the tuff of Corcoran Canyon about 27.2 million years ago. The Big Ten Peak caldera in the south part of the southern Toquima Range Tertiary volcanic complex formed about 27 million years ago during eruption of the tuff of Big Ten Peak and associated air-fall tuffs. The inferred Ryecroft Canyon caldera formed in the south end of the Monitor Valley adjacent to the southern Toquima Range and just north of the Big Ten Peak caldera in response to eruption of the tuff of Ryecroft Canyon about 27 million years ago, and the Moores Creek caldera just south of the Northumberland caldera developed at about the same time. Eruption of the tuff of Mount Jefferson about 26.8 million years ago was accompanied by collapse of the Mount Jefferson caldera in the central part of the southern Toquima Range. An inferred caldera, mostly buried beneath alluvium of Big Smoky Valley southwest of the Mount Jefferson caldera, formed about 26.5 million years

  8. Petrology and Geochemistry of Boninite Series Volcanic Rocks,Chichi-jima, Bonin Islands, Japan

    Dobson, Patrick F.; Blank, Jennifer G.; Maruyama, Shigenori; Liou, J.G.


    An Eocene submarine boninite series volcanic center isexposed on the island of Chichi-jima, Bonin Islands, Japan. Five rocktypes, boninite, bronzite andesite, dacite, quartz dacite, and rhyolite,were distinguished within the boninite volcanic sequence on the basis ofpetrographic and geochemical observations. Boninite lavas contain highmagnesium, nickel, and chromium contentsindicative of primitive melts,but have high silica contents relative to other mantle-derived magmas.All boninite series lavas contain very low incompatible elementconcentrations, and concentrations of high-field strength elements inprimitive boninite lavas are less than half of those found in depletedmid-ocean ridge basalts. Abundances of large-ion lithophile elements arerelatively high in boninite series lavas, similar to the enrichmentsobserved in many island arc lavas. Trends for both major and traceelement data suggest that the more evolved lavas of the boninite magmaseries were derived primarily through high-level fractionalcrystallization of boninite. Textural features, such as resorption andglomeroporphyrocrysts, and reverse chemical zonations suggest that magmamixing contributed to the development of the quartz dacitelavas.

  9. Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block: New constraints from Hf isotopes and Fe/Mn ratios

    LI Xianhua; QI Changshi; LIU Ying; LIANG Xirong; TU Xianglin; XIE Liewen; YANG Yueheng


    High-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block. These volcanic rocks have variable εHf(T) values and Fe/Mn ratios. The relatively primitive basalts have high Fe/Mn ratios and high Hf-Nd isotopic compositions, indicating that they were generated by partial melting of garnet clinopyroxene in mantle plume at high pressure. Thus, the Suxiong basalts are genetically related to the proposed Neoproterozoic superplume. On the contrary, a few differentiated basalts have low Fe/Mn ratios and low Hf-Nd isotopic compositions. They are likely to experience assimilation-fractional crystallization process. The Suxiong rhyolites have consistent Hf and Nd model ages of 1.3-1.4 Ga. They are likely generated by shallow dehydration melting of pre-existing young arc igneous rocks associated with the basaltic underplating/intrusion in a continental rift.

  10. Petrogenesis of Volcanic Rocks in the Khabr-Marvast Tectonized Ophiolite: Evidence for Subduction Processes in the South-Western Margin of Central Iranian Microcontinent



    The Late Cretaceous Khabr-Marvast tectonized ophiolite is located in the middle part of the Nain-Baft ophiolite belt, at the south-western edge of the central Iranian microcontinent. Although all the volcanic rocks in the study area indicate subduction-related magmatism (e.g. high LILE (large ion lithophile elements)/ HFSE (high field strenght elements) ratios and negative anomalies in Nb and Ta), geological and geochemical data clearly distinguish two distinct groups of volcanic rocks in the tectonized association: (1) group 1 is comprised of hyaloclustic breccias, basaltic pillow iavas, and andesite sheet flows. These rocks represent the Nain-Baft oceanic crust; and (2) group 2 is alkaline iavas from the top section of the ophiolite suite. These lavas show shoshonite affinity, but do not support the propensity of ophiolite.

  11. Insights from Pb and O isotopes into along-arc variations in subduction inputs and crustal assimilation for volcanic rocks in Java, Sunda arc, Indonesia

    Handley, Heather K.; Blichert-Toft, Janne; Gertisser, Ralf; Macpherson, Colin G.; Turner, Simon P.; Zaennudin, Akhmad; Abdurrachman, Mirzam


    New Pb isotope data are presented for Gede Volcanic Complex, Salak and Galunggung volcanoes in West Java, Merbabu and Merapi volcanoes in Central Java and Ijen Volcanic Complex in East Java of the Sunda arc, Indonesia. New O isotope data for Merbabu and new geochemical and radiogenic isotope data (Sr-Nd-Hf-Pb) for three West Javanese, upper crustal, Tertiary sedimentary rocks are also presented. The data are combined with published geochemical and isotopic data to constrain the relative importance of crustal assimilation and subducted input of crustal material in petrogenesis in Java. Also discussed are the significance of limestone assimilation in controlling the geochemical and isotopic characteristics of erupted Javanese rocks and the geochemical impact upon central and eastern Javanese arc rocks due to the subduction of Roo Rise between 105 and 109°E. The negative correlation between Pb isotopes and SiO2, combined with mantle-like δ18O values in Gede Volcanic Complex rocks, West Java, are most likely explained by assimilation of more isotopically-primitive arc rocks and/or ophiolitic crust known to outcrop in West Java. The negative Pb isotope-SiO2 trend cannot be explained by assimilation of the known compositions of the upper crustal rocks. A peak in δ18O whole-rock and mineral values in Central Javanese volcanic rocks (Merbabu and Merapi) combined with along-arc trends in Sr isotope ratios suggest that a different or additional crustal assimilant exerts control on the isotopic composition of Central Javanese volcanic rocks. This assimilant (likely carbonate material) is characterised by high δ18O and high Sr isotope ratio but is not particularly elevated in its Pb isotopic ratio. Once the effects of crustal assimilation are accounted for, strong East to West Java regional variations in Ba concentration, Ba/Hf ratio and Pb isotopic composition are evident. These differences are attributed to heterogeneity in the subducted source input component along the

  12. Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar

    Melluso, Leone; Sheth, Hetu C.; Mahoney, John J.;


    , clinopyroxene, orthopyroxene and opaque oxide, Moderate enrichment in the incompatible elements, (e.g. Zr 580 720 ppm, Nb 43 53 ppan, La/Yb-a 0.9 7.2), relatively low initial Sr-87/Sr-86 (0.7052 0.7055) and near-chondritic initial Nd-143/Nd-144 (0.51248 0.51249), They have mineral chemical, whole-rock chemical...

  13. Geochronology and geochemistry of Early Jurassic volcanic rocks in the Erguna Massif, northeast China: Petrogenesis and implications for the tectonic evolution of the Mongol-Okhotsk suture belt

    Wang, Wei; Tang, Jie; Xu, Wen-Liang; Wang, Feng


    The Mongol-Okhotsk suture belt played an important role in the tectonic evolution of northeast Asia during the Mesozoic. However, few studies have examined the influence of this tectonic belt on the geological evolution of northeast China. In this paper, we present zircon U-Pb geochronology, major and trace element geochemistry, and zircon Hf-O isotopic data for Early Jurassic volcanic rocks in the Erguna Massif of northeast China, with the aim of constraining the evolution of the Mongol-Okhotsk suture belt and its influence on the tectonic history of China during the Early Jurassic. Zircon U-Pb dating indicates that the trachybasalt and basaltic andesite in the study area were erupted between 193 ± 5 Ma and 181 ± 9 Ma (i.e., in the Early Jurassic). These Early Jurassic volcanic rocks belong to the high-K calc-alkaline series and are enriched in large ion lithophile elements and light rare earth elements, as well as being depleted in heavy rare earth elements and high field strength elements such as Nb and Ta. The rocks show a small negative Eu anomaly. The zircon εHf (182 Ma) values of the volcanic rocks range from - 1.9 to + 5.1, corresponding to TDM1 values of 640-901 Ma and TDM2 values of 901-1345 Ma. Zircons from two volcanic rocks yield δ18O values of 7.2‰ ± 1.5‰ (n = 19) and 6.6‰ ± 0.7‰ (n = 35). Geochemically, these Early Jurassic volcanic rocks are similar to those from active continental margin settings, and their primary magmas could have been derived from the partial melting of a lithospheric mantle wedge modified by fluid from a subducted slab. The discovery of Early Jurassic calc-alkaline volcanic rocks in the Erguna Massif, together with the coeval porphyry Cu-Mo deposits, indicates that an active continental margin existed in the Erguna area during the Early Jurassic. Taken together, we conclude that southward subduction of the Mongol-Okhotsk oceanic plate took place beneath the Erguna Massif during the Early Jurassic.

  14. New records of rare lichenicolous and lichen-forming fungi from volcanic rocks in SW Poland

    Katarzyna Szczepańska


    Full Text Available Records of two lichenicolous and nine lichen-forming fungi found in the southwestern part of Poland are presented. All of the reported species are very rare and they have only a few scattered localities in the country. One of them, Lecanora pannonica, is reported for the second time from Poland. Additionally, the new, contemporary records of Cercidospora macrospora, Rhizocarpon disporum, R. viridiatrum and Stereocaulon pileatum in Lower Silesia were noted. These species were known only from historical collections in the study area. Furthermore, Lecidea fuscoatra has been found a new host for Sagediopsis barbara. All of the localities of recorded species were found on natural outcrops of basalt rocks.

  15. Water-rock interaction during mineral carbonation and volcanic ash weathering

    Helgi Arnar Alfreðsson 1984


    The reduction of atmospheric carbon dioxide (CO2) is considered one of the greatest challenges of this century. Carbon capture and storage (CCS) is one of the means proposed to lower the atmospheric CO2 content. The aim of the CarbFix project in Iceland was to design and test a CO2 re-injection system, in which CO2 from the Hellisheidi geothermal power plant was injected, fully dissolved in water, into basaltic rocks. In this way the carbon is mineralized upon basalt dissolution by the precip...

  16. 226Ra or 226Ra/Ba dating of Holocene volcanic rocks: application to Mt. Etna and Merapi volcanoes

    Condomines, M.; Gauthier, P. J.; Tanguy, J. C.; Gertisser, R.; Thouret, J. C.; Berthommier, P.; Camus, G.


    This paper shows how 226Ra- 230Th disequilibria can be used to date Holocene volcanic rocks from some well selected volcanoes. A systematic study of these disequilibria on historical or well-dated volcanic samples is indeed first required to test the applicability of this method. Two examples are described here to illustrate its potential. In the case of Mt. Etna, the good correlation observed between ( 226Ra) 0 activities at the time of eruption and Th contents in lava flows from the last two millennia [M. Condomines, J.C. Tanguy, V. Michaud, Magma dynamics at Mt. Etna: constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas, Earth Planet. Sci. Lett. 132 (1995) 25-41] is used to infer the ages of several newly analysed lava flows. The calculated ages are in good agreement with those deduced from the archaeomagnetic curve describing the variation of the geomagnetic field direction in southern Italy [J.C. Tanguy, I. Bucur, J.F.C. Thompson, Geomagnetic secular variation in Sicily and revised ages of historic lavas from Mt. Etna, Nature 318 (1985) 453-455, J.C. Tanguy, M. Le Goff, V. Chillemi, A. Paiotti, C. Principe, S. La Delfa, G. Patane, Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires, C. R. Acad. Sci. Paris 329 (1999) 557-564, J.C. Tanguy, M. Le Goff, C. Principe, S. Arrighi, V. Chillemi, A. Paiotti, S. La Delfa, G. Patane, Archaeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits. Earth Planet. Sci. Lett. 211 (2003) 111-124]. We also present a whole set of new U-series data on historical, recent, and older samples from Merapi (Indonesia), and show that the ( 226Ra)/Ba ratio has probably maintained a quasi-steady state value during at least the past four millennia, and can be used to infer the ( 226Ra) 0/Ba ratio of old volcanics at the time of eruption, and thus their ages. Comparison with

  17. How temperature-dependent elasticity alters host rock/magmatic reservoir models: A case study on the effects of ice-cap unloading on shallow volcanic systems

    Bakker, Richard R.; Frehner, Marcel; Lupi, Matteo


    In geodynamic numerical models of volcanic systems, the volcanic basement hosting the magmatic reservoir is often assumed to exhibit constant elastic parameters with a sharp transition from the host rocks to the magmatic reservoir. We assess this assumption by deriving an empirical relation between elastic parameters and temperature for Icelandic basalts by conducting a set of triaxial compression experiments between 200 °C and 1000 °C. Results show a significant decrease of Young's modulus from ∼38 GPa to less than 4.7 GPa at around 1000 °C. Based on these laboratory data, we develop a 2D axisymmetric finite-element model including temperature-dependent elastic properties of the volcanic basement. As a case study, we use the Snæfellsjökull volcanic system, Western Iceland to evaluate pressure differences in the volcanic edifice and basement due to glacial unloading of the volcano. First, we calculate the temperature field throughout the model and assign elastic properties accordingly. Then we assess unloading-driven pressure differences in the magma chamber at various depths in models with and without temperature-dependent elastic parameters. With constant elastic parameters and a sharp transition between basement and magma chamber we obtain results comparable to other studies. However, pressure changes due to surface unloading become smaller when using more realistic temperature-dependent elastic properties. We ascribe this subdued effect to a transition zone around the magma chamber, which is still solid rock but with relatively low Young's modulus due to high temperatures. We discuss our findings in the light of volcanic processes in proximity to the magma chamber, such as roof collapse, dyke injection, or deep hydrothermal circulation. Our results aim at quantifying the effects of glacial unloading on magma chamber dynamics and volcanic activity.

  18. Geochemistry of the Ophiolite and Island-Arc Volcanic Rocks in the Mianxian-Lueyang Suture Zone,Southern Qinling and Their Tectonic Significance


    Ultrabasic rocks in the Mianxian-Lueyang ophiolitic melange zone include harzburgite and dunite which exhibit LREE depletion with remarkable positive Eu anomaly.The diabase dike swarm shows LREE enrichment but slightly negative Eu anomaly.Metamorphosed volcanic rocks can be divided into two groups in terms of their REE geochemistry and trace element ratios of Ti/V,Th/Ta,Th/Yb and Ta/Yb.One is ths MORB-type basalt with LREE depletion,representing the fragments of oceanic crust and implying an association of the MORB-type ophiolite and an ancient ocean basin between the Qinling and Yangtze plates during the Middle Paleozoic-Early Mesozoic era.The oter comprises the island-arc volcanic rocks including tholeiitic basalt and a large amount of calc-alkaline intermediate-acic volcanic rock,which could not be the component of the ancient oceanic crust but the result of magmatism at the continental margin.This indicates that the Mianxian-Lueyang limited ocean basin had undergone a whole process of development,evolution and vanishing from Devonian-Cretaceous to Permian.And the Qinling area had becone an independent lithospheric microplate,on the southern side of which there were exhibited the tectonic characteristics of active continental margins during the Late Paleozoic-Early Mesozoic.That is to say.the Qinling cannot be simply considered as a result of collision between the Yangtze and North China plates.

  19. Geochemical and isotopic characteristics of volcanic rocks from the northern East China Sea shelf margin and the Okinawa Trough

    ZENG Zhigang; YU Shaoxiong; WANG Xiaoyuan; FU Yongtao; YIN Xuebo; ZHANG Guoliang; WANG Xiaomei; CHEN Shuai


    Volcanic rocks both from the northern East China Sea (NECS) shelf margin and the northern Okinawa Trough are subalkaline less aluminous,and lower in High Field Strength Elements (HFSE).These rocks are higher in Large Ion Lithophile Elements (LILE),thorium and uranium contents,positive lead anomalies,negative Nb-Ta anomalies,and enrichment in Light Rare Earth Elements (LREE).Basalts from the NECS shelf margin are akin to Indian Ocean Mid-Ocean Ridge Basalt (MORB),and rhyolites from the northern Okinawa Trough have the highest 207Pb/204Pb and 208Pb/204Pb ratios.The NECS shelf margin basalts have lower 87Sr/86Sr ratios,εNd and σ18O than the northern Okinawa Trough silicic rocks.According to 40K-40Ar isotopic ages of basalts from the NECS shelf margin,rifting of the Okinawa Trough may have been active since at least 3.65-3.86 Ma.The origin of the NECS shelf margin basalt can be explained by the interaction of melt derived from Indian Ocean MORB-like mantle with enriched subcontinental lithosphere.The basalts from both sides of the Okinawa Trough may have a similar origin during the initial rifting of the Okinawa Trough,and the formation of basaltic magmas closely relates to the thinning of continental crust.The source of the formation of the northern Okinawa Trough silicic rocks was different from that of the middle Okinawa Trough,which could have been generated by the interaction of basaltic melt with an enriched crustal component.From the Ryukyu island arc to East China,the Cenozoic basalts have apparently increasing trends of MgO contents and ratios of LREE to Heavy Rare Earth Elements (HREE),suggesting that the trace element variabilities of basalts may have been influenced by the subduction of the Philippine Sea plate,and that the effects of subduction of the Philippine Sea plate on the chemical composition of basaltic melts have had a decreasing effect from the Ryukyu island arc to East China.

  20. Relative Roles of Source Composition, Fractional Crystallization and Crustal Contamination in the Petrogenesis of Andean Volcanic Rocks

    Thorpe, R. S.; Francis, P. W.; O'Callaghan, L.


    There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt--andesite--decite--rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt--trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of `within-plate' character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust. Active Andean volcanoes of the calc-alkaline basalt--andesite--dacite rhyolite association occur within a northern (n.v.z.), central (c.v.z.) and southern volcanic zone (s.v.z.) (figure 9). Alkaline volcanic rocks occur within and to the east of these zones. The n.v.z. and s.v.z. lavas have chemical and isotope characteristics consistent with an origin by

  1. Paleomagnetic investigation of late Neogene vertical axis rotation and remagnetization in central coastal California

    Horns, Daniel M.; Verosub, Kenneth L.


    Outcrops of shallow marine sedimentary rocks of the Neogene Purisima Formation and Santa Cruz Mudstone occur throughout the Santa Cruz Mountains and along the coast of Santa Cruz and San Mateo counties in California. Within the Santa Cruz Mountains, the Purisima Formation has been extensively folded and offset by reverse faults. Along the coast, the Purisima Formation and Santa Cruz Mudstone are only gently folded although the Purisima Formation is cut by the San Gregorio Fault Zone. Previous paleomagnetic studies have shown that vertical axis rotation has occurred within some parts of the San Gregorio Fault Zone. As part of a kinematic study of the San Gregorio Fault Zone, this paleomagnetic investigation of the Purisima Formation and Santa Cruz Mudstone was conducted to determine the extent of the vertical axis rotation. The characteristic paleomagnetic directions of the Purisima Formation and Santa Cruz Mudstone, which we interpret as the primary directions, indicate that a clockwise vertical axis rotation of 35 deg to 60 deg has occurred throughout the San Gregorio Zone, whereas no rotation has occurred away from the fault zone. This result suggests that vertical axis rotation is fundamentally related to shear across the San Gregorio Fault Zone. Our work also indicates that while the Purisima Formation and Santa Cruz Mudstone along the coast have retained their primary magnetizations, the Purisima Formation within the Santa Cruz Mountains has been remagnetized after folding.

  2. Geochronology and geochemistry of the Early Jurassic Yeba Formation volcanic rocks in southern Tibet: Initiation of back-arc rifting and crustal accretion in the southern Lhasa Terrane

    Wei, Youqing; Zhao, Zhidan; Niu, Yaoling; Zhu, Di-Cheng; Liu, Dong; Wang, Qing; Hou, Zengqian; Mo, Xuanxue; Wei, Jiuchuan


    Understanding the geological history of the Lhasa Terrane prior to the India-Asia collision ( 55 ± 10 Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene ( 18-10 Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183-174 Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr-Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I-S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane.

  3. A Clinopyroxene-Plagioclase Geobarometer for A-type Silicic Volcanic Rocks

    Wolff, J.; Iveson, A. A.; Davis, K.; Johnson, T. A.; Gahagan, S.; Ellis, B. S.


    Constraining the crustal storage depths of magmas is important in understanding volcanism. The reaction: anorthite (pl) = Ca-Tschermak's (cpx) + silica (Q or liq) has a large volume change and hence offers potential as a geobarometer, but has not been extensively exploited as such. One of the chief barriers to its wide application is consistent estimation of melt silica activity for assemblages that lack quartz. We have skirted this problem by confining attention to metaluminous silicic compositions (SiO2 > 60% by weight), for which silica activity during crystallization is presumed to be close to 1, and calibrated the barometer for the range 0 - 2 GPa using the LEPR database and additional experiments from the literature. Additional improvement is obtained by excluding hydrous phase-bearing assemblages. Despite the analytical uncertainties present in older experimental investigations, with knowledge of temperature, and clinopyroxene, plagioclase and host melt compositions, pressures for amphibole- and biotite-free dacites and rhyolites can be estimated to ±0.17 GPa (1 sigma). The limitations of the barometer render it most applicable to intraplate, A-type rhyolites. Application to one such system, the Snake River Plain rhyolites, indicate that both melt-hosted phenocrysts and clinopyroxene-plagioclase aggregate grains found in these rhyolites formed at low pressures, <0.5 GPa. This is consistent with isotopic evidence for a shallow crustal origin for Snake River Plain rhyolites.

  4. Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii

    Kurz, M.D.; Jenkins, W.J.; Hart, S.R.; Clague, D.


    Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3He 4He(RA) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the 3He/4He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower 3He 4He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio ( 30 RA) helium with some (variable) component of lithospheric contamination added during "breakthrough", while the later stages are characterized by a relaxation toward lithospheric 3He 4He ratios (??? 8 RA) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in 3He 4He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume. ?? 1983.

  5. 3.5 billion years of glass bioalteration: Volcanic rocks as a basis for microbial life?

    Staudigel, Hubert; Furnes, Harald; McLoughlin, Nicola; Banerjee, Neil R.; Connell, Laurie B.; Templeton, Alexis


    Alteration textures in volcanic glass from the seafloor fall into two classes, one suggestive of abiotic/diffusive hydration and chemical exchange, and another likely to be caused by microbial, cavity-forming, congruent dissolution. Glass bioalteration is common in submarine lavas throughout the world's ocean, dominant in the upper 300 m of the oceanic crust, and found in all well-preserved ophiolites and greenstone belts dating back to 3.5 Ga. It may yield a significant fraction of the global biomass and geochemical fluxes and is relevant to the development of the earliest life on Earth. We present a critical review concerning these glass bioalteration textures and present new data on their microchemical environment. We explore arguments for their biogenicity and further develop the prevalent model for their formation by relating corrosion morphology to the mechanism of microbial dissolution. Biological alteration produces conspicuous micron-scale granular and tubular textures. Granular glass alteration is well explained by colonizing microbes that selectively dissolve the glass in their contact area, forming a sponge-like interconnected network of micron-sized cavities along glass surfaces. Tubular alteration meanwhile, is more likely to be caused by filamentous cell extensions in a process similar to fungal tunneling of soil feldspars and marine carbonates. While we see clear functional similarities to fungal dissolution behavior, we do not know whether fungal or prokaryotic organisms are involved. However, this functional constraint may eventually help to identify potential microbes responsible for these features, potentially including eukaryotic or prokaryotic organisms. Yet, we caution that these organisms may be difficult to identify and to study, because they are likely to be sparsely distributed, slow growing, and difficult to cultivate.

  6. Petrochemical Results for Volcanic Rocks recovered from SHINKAI 6500 diving on the Bonin Ridge (27°15'N-28°25'N): submarine extension of Ogasawara forearc volcanism

    Bloomer, S. H.; Kimura, J.; Stern, R. J.; Ohara, Y.; Ishii, T.; Ishizuka, O.; Haraguchi, S.; Machida, S.; Reagan, M.; Kelley, K.; Hargrove, U.; Wortel, M.; Li, Y. B.


    Four SHINKAI 6500 submersible dives (dive #823 to #826) were performed along the Bonin Ridge escarpment west of Ogasawara (Bonin) Islands in the West Pacific during May 2004, in the hopes of finding exposures of lower crust of the IBM forearc. The Ogasawara Islands are located on the Bonin ridge, exposing 48-40 Ma boninites on Chichi-jima and depleted arc tholeiite lavas of the same age on Haha-jima. These extremely depleted lavas are believed to have been generated when subduction began beneath the Izu-Bonin-Mariana oceanic arc system. Subsequent rifting (35-30 Ma) formed the Bonin Trough and a 350 km long N-S trending eastern escarpment (Bonin Ridge), where we concentrated our dives. We observed lavas and volcaniclastic sequences by the four SHINKAI dives along the escarpment, and 16 fresh basaltic to andesitic lava samples have been recovered. The first three dives appear to have sampled volcanic constructs, of presumed Oligocene age, along the escarpment, whereas the last dive sampled exposures similar to Eocene rocks of the Bonin islands, including nummulitic limestone. The lava samples were analyzed by ICP-MS at Shimane University for 30 incompatible trace elements. All samples show arc-like chemical signatures, including elevated concentrations of LIL elements, depletions in Ta and Nb, and spikes in Pb, Sr, and Li. All samples show modest enrichments in LREE. A lava sample from the northernmost dive #824 is identical with the depleted tholeiite from Haha-jima Islands at the southernmost end of the Bonin Ridge in terms of trace element characteristics. Other lava samples from northern three dives (#823, #824, #825) have tholeiitic affinities with more elevated highly incompatible elements. This suggests derivation of the series of lavas by different degree of partial melting of a similar source mantle. Samples from southernmost dive site #826, immediately northwest of Chichi-jima Islands, are boninites with U-shaped REE patterns and relatively enriched Zr and

  7. Rock magnetism and magnetic anisotropy in folded sills and basaltic flows: A case study of volcanics from the Taimyr Peninsula, Northern Russia

    ZHANG ShuWei; J. Harald WALDERHAUG; YANG YueJun


    Magnetic measurements were performed on apparently deformed igneous rocks of 23 sites from the southeastern part of the Taimyr Peninsula. Rock magnetism and reflected light microscopy analyses reveal that fine-grained titanomagnetites up to pure magnetites mainly carry the majority of magnetic fabrics in the sills, and that the slightly coarser Ti-poor or-medium titanomagnetites carry most mag-netic fabrics in the basaltic flows. Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) on 180 unheated samples and 128 samples that had been pre-viously heated to 600℃ during a paleomagnetic study to detect heating effects on the anisotropy of magnetic susceptibility (AMS) properties of volcanic rocks. Laboratory heating significantly affects anisotropy variations of these igneous rocks corresponding to the mineralogical changes during the heat treatment.

  8. Geochemistry, geochronology, and tectonic setting of Early Cretaceous volcanic rocks in the northern segment of the Tan-Lu Fault region, northeast China

    Ling, Yi-Yun; Zhang, Jin-Jiang; Liu, Kai; Ge, Mao-Hui; Wang, Meng; Wang, Jia-Min


    We present new geochemical and geochronological data for volcanic and related rocks in the regions of the Jia-Yi and Dun-Mi faults, in order to constrain the late Mesozoic tectonic evolution of the northern segment of the Tan-Lu Fault. Zircon U-Pb dating shows that rhyolite and intermediate-mafic rocks along the southern part of the Jia-Yi Fault formed at 124 and 113 Ma, respectively, whereas the volcanic rocks along the northern parts of the Jia-Yi and Dun-Mi faults formed at 100 Ma. The rhyolite has an A-type granitoid affinity, with high alkalis, low MgO, Ti, and P contents, high rare earth element (REE) contents and Ga/Al ratios, enrichments in large-ion lithophile (LILEs; e.g., Rb, Th, and U) and high-field-strength element (HFSEs; e.g., Nb, Ta, Zr, and Y), and marked negative Eu anomalies. These features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment. The basaltic rocks are enriched in light REEs and LILEs (e.g., Rb, K, Th, and U), and depleted in heavy REEs, HFSEs (e.g., Nb, Ta, Ti, and P), and Sr. These geochemical characteristics indicate that these rocks are calc-alkaline basalts that formed in an intraplate extensional tectonic setting. The dacite is a medium- to high-K, calc-alkaline, I-type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc. Therefore, the volcanic rocks along the Jia-Yi and Dun-Mi faults were formed in an extensional regime at 124-100 Ma (Early Cretaceous), and these faults were extensional strike-slip faults at this time.

  9. Petrogenesis of Middle-Late Triassic volcanic rocks from the Gangdese belt, southern Lhasa terrane: Implications for early subduction of Neo-Tethyan oceanic lithosphere

    Wang, Chao; Ding, Lin; Zhang, Li-Yun; Kapp, Paul; Pullen, Alex; Yue, Ya-Hui


    The Gangdese belt is dominantly composed of igneous rocks that formed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane and has played a crucial role in understanding the pre-collisional evolution of southern Tibet. This paper presents new geochronological and geochemical (whole-rock major and trace element and Sr-Nd and zircon Hf isotope) data for recently identified volcanic rocks exposed in Changguo area, southernmost part of the Lhasa terrane. Zircon U-Pb dating from six samples yields consistent ages of 237.1 ± 1.1 Ma to 211.7 ± 1.5 Ma for magma emplacement through volcanic eruption, showing the Middle-Late Triassic magmatic activity in the southernmost Gangdese Belt. The Changguo volcanic rocks are mainly composed of basaltic and andesitic rocks and exhibit LILE enrichment and HFSE depletion. They also exhibit relatively uniform Nd-Hf isotopic compositions (εNd(t) = + 5.20 to + 7.74 and εHf(t)zircon = + 10.2 to + 15.9). The basaltic magmas were likely sourced from partial melting of sub-arc mantle wedge that was metasomatized by not only the aqueous fluid derived from subducting altered oceanic crust but also hydrous melt derived from subducting seafloor sediments, and subsequently experienced fractional crystallization and juvenile crustal contamination during ascent. The andesitic magmas were generated by partial melting of mafic-ultramafic metasomes through melt/fluid-peridotite reaction at slab-mantle interface. Taking into account the temporal and spatial distribution of the Early Mesozoic magmatic rocks and regional detrital zircon data, we further propose that the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane commenced by Middle Triassic.

  10. Iron and Zinc isotope fractionation during magmatism in the continental crust: Evidence from bimodal volcanic rocks from Hailar basin, NE China

    Xia, Ying; Li, Shuangqing; Huang, Fang


    This study presents Fe-Zn isotope data for a suite of well-characterized bimodal volcanic rocks from Hailar Basin, northeast China to understand the mechanism of Fe isotope fractionation in highly differentiated igneous rocks. The samples range from basaltic trachyandesites to trachytes-rhyodacites, and rhyolites. The δ56Fe values increase with increasing SiO2 contents with the rhyolites having the highest δ56Fe (up to 0.64 ± 0.02‰) among the previously reported data for igneous rocks at a similar SiO2. The lack of correlation between δ56Fe and Rb/La argues against the effect of fluid exsolution on Fe isotopes. The δ56Fe do not show a clear correlation with δ66Zn and radiogenic isotopes, suggesting that thermal diffusion or crustal contamination cannot produce the high δ56Fe in Hailar volcanic rocks. Fe isotopic variation in Hailar volcanic rocks can be explained by two steps of magmatism. During the first step, partial melting of basaltic trachyandesites with an average δ56Fe of 0.09 ± 0.14‰ produced trachytes-rhyodacites with an average δ56Fe of 0.24 ± 0.27‰. Modelling using rhyolite-MELTS shows that Fe isotopes can be fractionated by preferential partitioning of isotopically different Fe3+ and Fe2+ between the solid residue and partial melt. The second step involves formation of rhyolites with significantly high δ56Fe through partial melting or extensive crystallization of crust materials, during which isotopically heavy Fe preferentially partition into the rhyolitic melt. Therefore, fractionation of Fe isotopes between melts and minerals can result in high δ56Fe in SiO2-rich igneous rocks and apparent Fe isotope heterogeneity within the continental crust.

  11. Late sodic metasomatism evidences in bimodal volcanic rocks of the Acampamento Velho Alloformation, Neoproterozoic III, southern Brazil

    Delia Del Pilar M. de Almeida


    Full Text Available A mineralogical study was carried out in mafic and felsic volcanic rocks of the Acampamento Velho Alloformation at Cerro do Bugio, Perau and Serra de Santa Bárbara areas (Camaquã Basin in southern Brazil. The Acampamento Velho bimodal event consists of two associations: lower mafic at the base and upper felsic at the top. Plagioclase and alkali-feldspar were studied using an electronic microprobe, and magnetite, ilmenite, rutile, illite and alkali-feldspar were investigated through scanning electron microscopy. The rocks were affected by a process of late sodic autometasomatism. In mafic rocks, Ca-plagioclase was transformed to albite and pyroxenes were altered. In felsic rocks, sanidine was partially pseudomorphosed, generating heterogeneous alkali-feldspar. In this association, unstable Ti-rich magnetite was replaced by rutile and ilmenite. In mafic rocks, the crystallization sequence was: (1 Ti-rich magnetite (?, (2 pyroxene and Ca-plagioclase, (3 albite (alteration to Ca-plagioclase, (4 sericite, chlorite and calcite (alteration to pyroxene, and kaolinite (alteration to plagioclase/albite. In felsic rocks: (1 zircon, (2 Ti-rich magnetite, (3 sanidine, (4 quartz. The introduction of late Na-rich fluids, generated the formation of (5 heterogeneous alkali-feldspar, (6 ilmenite and rutile from the Ti-rich magnetite, (7 albite in the spherulites. Finally, alteration of sanidine, vitroclasts and pumice to (8 illite.Um estudo mineralógico de detalhe foi realizado nas rochas vulcânicas da Aloformação Acampamento Velho nos Cerros do Bugio, Perau e Serra de Santa Bárbara (Bacia do Camaquã, sudeste do Brasil. Este evento bimodal é constituído por duas associações: máfica inferior na base e félsica superior no topo. Foram estudados grãos de plagioclásio e feldspato alcalino com o uso de microssonda eletrônica, sendo que, magnetita,ilmenita, rutilo e ilita além de feldspato alcalino foram pesquisados através do microscópio eletr

  12. Geochemical discrimination of the geotectonic environment of basaltic-andesitic volcanic rocks associated with the Laochang polymetallic ore deposit at Lancang, Yunnan

    GAO Jianguo


    The Laochang polymetallic ore deposit at Lancang is one of the well known ancient ore deposits associated with volcanic rocks in the Sanjiang (Tri-river) region of Southwest China. Volcanic rocks are dominated by alkali basalt and trachyte basalt. There has long been a controversy on the environment of formation of basalts. Some scholars hold that the basalts were formed in a continental environment, some thought they were formed in an oceanic environment and others considered that the basalts were emplaced in a back-arc basin. This study focuses on the geochemical characteristics of the basalts on the basis of their major elements, REEs and trace elements. At the same time, strongly incompatible elements such as Ta, Th and Hf and their ratios were used to differentiate the geotectonic settings of basalts. The results showed that the basalts in the region studied were formed in a continental rift environment.

  13. Petrography and geochemistry of volcanic rocks in the east of Nabar (SW of Kashan with emphasis on the role of crustal contamination

    Seyed Mohsen Tabatabaei Manesh


    Full Text Available The studied area is located in the east of Nabar village and southwest of Kashan, a part of the Urumieh – Dokhtar magmatic arc. The volcanic rocks belonging to Eocene age, are composed of pyroxene andesites, andesites, dacites and rhyolites. Porphyritic, glomeroporphyric, microlitic, and sieved textures are the most common textures of these rocks. Plagioclase, clinopyroxene and amphibole are the predominant minerals in the pyroxene andesites and andesites, whereas dacites and rhyolites are characterized by the presence of plagioclase, amphibole, quartz, biotite, and K-feldspar. Inequilibrium textures including embayed plagioclases and quartz with rounded margins, and oscillatory zoning in the plagioclases, sieved texture, and dusty rims are evidences of magma mixing. The enrichment in LREE and LILE and the HREE and HFSE depletion in the chondrite and primitive mantle normalized diagrams point to calc-alkaline nature of the rocks studied and they are related to volcanic arcs setting. High ratio of La/Nb (2-4.36 and negative Ti and Nb anomalies in spider diagrams can support crustal contamination hypotheses of these rocks. Also, low ratio of Nb/La (0.23-0.5 and high ratio of Sr/Ce (8.4-19 indicate contamination of parental magma with crustal materials. The rocks studied are formed from magma which is derived from enriched-mantle with 1-5 percent partial melting of spinel-lehrzolite.

  14. Isotope and trace element systematics in a spinel-lherzolite-bearing suite of basanitic volcanic rocks from San Luis Potosi, Mexico

    Pier, J.E.G.


    Lherzolite-bearing basanitic magmas of Quaternary age have erupted to form maars, lava/cinder cones and lava flows in two volcanic fields (Ventura and Santo Domingo) in the central Mexican state of San Luis Potosi. The systematics of the radiogenic isotopes of Sr, Nd, and Pb and the relationship between these parameters and elemental compositions are used to investigate the petrogenesis of the volcanic rocks and the nature of their mantle sources. Sr and Nd isotopic data are presented for 19 basanitic rocks, 5 kaersutites, and 6 lherzolitic xenoliths; Pb data presented for the same 19 volcanic rocks and 4 of the 5 kaersutites. The isotopic compositions for all of these samples fall within the mantle range defined by MORBs and OIBs. The basanites generally plot within the OIB field on isotopic diagrams; most of the kaersutites are displaced to slightly more-depleted (i.e. MORB-like) values than the volcanic samples and the xenoliths, with one exception, are significantly more-depleted than either of these sample-types. As crustal contamination is considered unlikely for most of the volcanic samples, these trends are thought to arise from mixing multiple mantle components. The absence of similar isotopic elemental relationships for Epsilon Nd and the lack of correlation between {sup 206}Pb/{sup 204}Pb and the other Pb isotopes require a mixture of at least three mantle reservoirs: a depleted reservoir analogous to that of the MORBs, a St. Helena-type component, and a third component, which primarily affects Sr and {sup 208}Pb/{sup 204}Pb composition. This third component carries relatively radiogenic Sr and {sup 208}Pb/{sup 204}Pb and appears to be correlated with the degree of melting.

  15. The behavior of biogenic silica-rich rocks and volcanic tuffs as pozzolanic additives in cement

    Fragoulis, Dimitris; Stamatakis, Michael; Anastasatou, Marianthi


    Cements currently produced, include a variety of pozzolanic materials, aiming for lower clinker addition and utilization of vast deposits of certain raw materials and/or mining wastes and byproducts. The major naturally occurring pozzolanic materials include glassy tuffs, zeolitic tuffs, diatomites and volcanic lavas rich in glassy phase, such as perlites. Therefore, based on the available raw materials in different locations, the cement composition might vary according to the accessibility of efficient pozzolanic materials. In the present investigation, the behavior of pozzolanic cements produced with representative samples of the aforementioned materials was studied, following the characterization of the implemented pozzolanas with respect to their chemical and mineralogical characteristics. Laboratory cements were produced by co-grinding 75% clinker, 5% gypsum and 20% pozzolana, for the same period of time (45 min). Regarding pozzolanic materials, four different types of pozzolanas were utilized namely, diatomite, perlite, zeolite tuff and glassy tuff. More specifically, two diatomite samples originated from Australia and Greece, with high and low reactive silica content respectively, two perlite samples originated from Turkey and from Milos Island, Greece, with different reactive silica contents, a zeolite tuff sample originated from Turkey and a glassy tuff sample originated from Milos Island, Greece. The above pozzolana samples, which were ground in the laboratory ball mill for cement production performed differently during grinding and that was reflected upon the specific surface area (cm2/gr) values. The perlites and the glassy tuff were the hardest to grind, whereas, the zeolite tuff and the Australian diatomite were the easiest ones. However, the exceedingly high specific surface area of the Australian diatomite renders cement difficult to transport and tricky to use for concrete manufacturing, due to the high water demand of the cement mixture. Regarding

  16. Paleointensities of the Auckland Excursion from Volcanic Rocks in New Zealand

    Mochizuki, N.; Tsunakawa, H.; Shibuya, H.; Cassidy, J.; Smith, I. E.


    Shibuya et al. (1992) reported the Auckland excursion from several basaltic lava flows of monogenetic volcanic centers (forced to introduce correction for thermal alterations in laboratory heating, using low temperature part of the Arai plot. We, therefore, applied the double heating technique (DHT) of Shaw method (Tsunakawa and Shaw, 1994), which was capable of detecting inappropriate results by the ARM correction, to the samples. The low temperature demagnetization (LTD) was combined with DHT (Yamamoto et al., submitted) before AF demagnetization and samples were heated in a vacuum of 10-100 Pa. Sixty-one samples from the five lava flows were subjected to the LTD-DHT Shaw method. Twenty-three of these samples yielded successful results passing the selection criteria. Five out of six paleointensities from the Crater Hill lava were consistent with each other. A mean paleointensity was given to be 10.9+/- 1.9 μ T (N=5) for the Crater Hill lava. Five out of seven paleointensities from the Wiri lava, were consistent and a mean was 10.8+/- 1.2 μ T (N=5). Three samples from the Puketutu lava gave a mean paleointensity of 11.4+/- 0.8 μ T (N=3). These three lava flows, Crater Hill, Wiri and Puketutu lava, all recorded the north-down paleodirection and gave almost the same paleointensities of ~ 11 μ T. This concordance of paleointensities and paleodirections supports the reliability of the paleointensity determination. Four paleointensities were obtained from the Hampton Park lava of the west paleodirection, and gave a mean paleointensity of 10.1+/- 1.1 μ T (N=4). The field strength was comparable to that of the north-down group. Three samples from the McLennan Hills lava of the south paleodirection gave quite low paleointensities, a mean of which was calculated to be 2.4+/- 0.6 μ T (N=3). These five paleointensities from the Auckland excursion are no more than one-fifth of the present-field intensity. The corresponding VDMs range from 0.6x1022 to 2.3*E22Am2, which

  17. Segmentation of the Cascade Arc Based on Compositional and Sr and Nd Isotopic Variations in Primitive Volcanic Rocks

    Schmidt, M. E.; Grunder, A. L.


    We define four segments in the Cascade Volcanic Arc based on 87Sr/86Sr and 143Nd/144Nd of primitive volcanic rocks: 1) The North segment extends 450 km from Mt. Meager to Glacier Peak; 2) the 350-km Columbia segment includes volcanoes from Mt. Rainier to Mt. Jefferson; 3) the 250 km Central segment comprises the portion of the arc between the Three Sisters and Crater Lake; and 4) the 350-km South segment includes Mt. Shasta to Mt. Lassen. Isotopic data were compiled for primitive bulk composition (MgO concentrations >8 wt.% MgO) as a fingerprint mantle sources. The North segment has a range in 87Sr/86Sr of 0.7030-0.7037 and is distinguished by the predominance of calcalkaline basalts (CAB) and few low K tholeiites (LKT). The North segment lies on the North Cascade craton where convergence is near orthogonal. Oblique subduction occurs beneath the Columbia, Central, and South segments. The Columbia segment (87Sr/86Sr of 0.7028-0.7037) has both LKT and CABs as well as enriched ocean island-like basalts (OIB) that are found both on the arc axis and, especially at the Simcoe Volcanic Field, behind the arc. This segment lies primarily on the accreted Tertiary oceanic plateau terrane of the Columbia Embayment. The Central segment is dominated by LKT with lesser CAB and has the most restricted Sr isotopic range (0.7034- 0.7038). Like the South segment, the Central segment mainly overlies accreted terranes stitched by Mesozoic plutons and has Basin and Range (B&R) extension behind as well as locally within the arc. Medicine Lake Volcano, on the margin of the B&R behind Mt. Shasta is also dominated by LKT and has a narrow isotopic range like the Central segment. This suggests that the LKT's are related to extension in the arc. The South segment is distinguished by the widest Sr isotopic range (0.7028-0.7042) and the presence of high Mg basaltic andesite and andesite compositions in addition to LKT and CABs. These arc segments broadly correspond to physical segments that were

  18. Zircon age and geochemistry of the Tost bimodal volcanic rocks: Constraints on the Early Carboniferous tectonic evolution of the South Mongolia

    Yang, Shunhu; Miao, Laicheng; Zhang, Fochin; Meng, Qingren; Zhu, Mingshuai; Baatar, Munkhtsengel; Anaad, Chimedtseren


    SIMS zircon U-Pb dating, geochemical and Sr-Nd isotopic data are presented for the Late Paleozoic volcanic rocks from Tost area in Mongolia, the southern portion of the Central Asian Orogenic Belt (CAOB). The Tost volcanic rocks show a bimodal feature characterized by a mafic member of basalt and a felsic component of rhyolite, which are temporally and spatially related each other, implying a genetic relationship. Zircon U-Pb isotopic data of the rhyolite constrain the Tost bimodal magmatism occurring from 355 Ma to 320 Ma. The Tost basalt is characterized by high abundances in Th, U and Pb, slightly enriched LREE patterns and low HFSE/LREE ratios. These features, together with their OIB-like isotopic signature ((87Sr/86Sr)i = 0.7039378-0.704397, εNd(t) = 3.55-5.02), suggest that they were likely derived from low-degree partial melting of a metasomatized asthenospheric mantle source with subordinate input of subduction components. The Tost rhyolite, which displays an intimate affinity to Tost basalt, with enrichment in Th, U and Pb, depleted in Nb, Ta and Ti, and gently right-tilted REE patterns, is inferred to be generated by partial melting of a juvenile lower crustal source heated by underplating mafic magmas which rise from asthenosphere during continued rifting. The Tost bimodal volcanic rocks are comparable both in age and composition with those in the East Tianshan, which together constitute an E-W-oriented belt of bimodal volcanic rocks, marking an Early Carboniferous rifting event. Considering regional geology, we propose that the rifting took place in a back-arc extensional setting, probably induced by the subduction of the Dzungaria Ocean between the East Tianshan and Junggar-Kazakhstan plate during the Early Carboniferous.

  19. Vulnerability of shallow ground water and drinking-water wells to nitrate in the United States: Model of predicted nitrate concentration in shallow, recently recharged ground water -- Input data set for basalt and volcanic rocks (gwava-s_vrox)

    U.S. Geological Survey, Department of the Interior — This data set represents the presence or absence of basalt and volcanic rocks in the conterminous United States. The data set was used as an input data layer for a...

  20. Geochronological and geochemical constraints on the petrogenesis of late Cretaceous volcanic rock series from the eastern Sakarya zone, NE Anatolia-Turkey

    Aydin, Faruk; Oǧuz, Simge; Şen, Cüneyt; Uysal, İbrahim; Başer, Rasim


    New SHRIMP zircon U-Pb ages and whole-rock geochemical data as well as Sr-Nd-Pb and δ18O isotopes of late Cretaceous volcanic rock series from the Giresun and Artvin areas (NE Anatolia, Turkey) in the northern part of the eastern Sakarya zone (ESZ) provide important evidence for northward subduction of the Neo-Tethyan oceanic lithosphere along the southern border of the ESZ. In particular, tectonic setting and petrogenesis of these subduction-related volcanites play a critical role in determining the nature of the lower continental crust and mantle dynamics during late Mesozoic orogenic processes in this region. The late Cretaceous time in the ESZ is represented by intensive volcanic activities that occurred in two different periods, which generally consist of alternation of mafic-intermediate (basaltic to andesitic) and felsic rock series (dacitic to rhyolitic) within each period. Although there is no geochronological data for the lower mafic-intermediate rock series of the first volcanic period, U-Pb zircon dating from the first cycle of felsic rocks yielded ages ranging from 88.6±1.8 to 85.0±1.3 Ma (i.e. Coniacian-Early Santonian). The first volcanic period in the region is generally overlain by reddish biomicrite-rich sedimentary rocks of Santonian-Early Campanian. U-Pb zircon dating for the second cycle of mafic-intermediate and felsic rocks yielded ages varying from 84.9±1.7 to 80.8±1.5Ma (i.e. Early to Middle Campanian). The studied volcanic rocks have mostly transitional geochemical character changing from tholeiitic to calc-alkaline with typical arc signatures. N-MORB-normalised multi-element and chondrite-normalised rare earth element (REE) patterns show that all rocks are enriched in LILEs (e.g. Rb, Ba, Th) and LREEs (e.g. La, Ce) but depleted in Nb and Ti. In particular, the felsic samples are characterised by distinct negative Eu anomalies. The samples are characterized by a wide range of Sr-Nd-Pb isotopic compositions (initial ɛNd values from -7

  1. Matrix diffusion coefficients in volcanic rocks at the Nevada test site: Influence of matrix porosity, matrix permeability, and fracture coating minerals

    Reimus, Paul W.; Callahan, Timothy J.; Ware, S. Doug; Haga, Marc J.; Counce, Dale A.


    Diffusion cell experiments were conducted to measure nonsorbing solute matrix diffusion coefficients in forty-seven different volcanic rock matrix samples from eight different locations (with multiple depth intervals represented at several locations) at the Nevada Test Site. The solutes used in the experiments included bromide, iodide, pentafluorobenzoate (PFBA), and tritiated water ( 3HHO). The porosity and saturated permeability of most of the diffusion cell samples were measured to evaluate the correlation of these two variables with tracer matrix diffusion coefficients divided by the free-water diffusion coefficient ( Dm/ D*). To investigate the influence of fracture coating minerals on matrix diffusion, ten of the diffusion cells represented paired samples from the same depth interval in which one sample contained a fracture surface with mineral coatings and the other sample consisted of only pure matrix. The log of ( Dm/ D*) was found to be positively correlated with both the matrix porosity and the log of matrix permeability. A multiple linear regression analysis indicated that both parameters contributed significantly to the regression at the 95% confidence level. However, the log of the matrix diffusion coefficient was more highly-correlated with the log of matrix permeability than with matrix porosity, which suggests that matrix diffusion coefficients, like matrix permeabilities, have a greater dependence on the interconnectedness of matrix porosity than on the matrix porosity itself. The regression equation for the volcanic rocks was found to provide satisfactory predictions of log( Dm/ D*) for other types of rocks with similar ranges of matrix porosity and permeability as the volcanic rocks, but it did a poorer job predicting log( Dm/ D*) for rocks with lower porosities and/or permeabilities. The presence of mineral coatings on fracture walls did not appear to have a significant effect on matrix diffusion in the ten paired diffusion cell experiments.

  2. Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

    Basu, A.R.; Wang, Junwen; Huang, Wankang; Xie, Guanghong; Tatsumoto, M.


    Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb Sr and Nd Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb 204Pb vs 206Pb 204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb 204Pb vs 206Pb 204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components-a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle

  3. Geochronology and geochemistry of Permian bimodal volcanic rocks from central Inner Mongolia, China: Implications for the late Palaeozoic tectonic evolution of the south-eastern Central Asian Orogenic Belt

    Zhang, Zhicheng; Chen, Yan; Li, Ke; Li, Jianfeng; Yang, Jinfu; Qian, Xiaoyan


    Zircon U-Pb ages, geochemical data and Sr-Nd isotopic data are presented for volcanic rocks from the lower Permian Dashizhai Formation. These rocks are widely distributed in the south-eastern Central Asian Orogenic Belt in central Inner Mongolia, China. The volcanic rocks mainly consist of basaltic andesite and rhyolite, subordinate dacite and local andesite, and exhibit bimodal geochemical features. The results of zircon U-Pb dating indicate that the volcanic rocks formed during the early Permian (292-279 Ma). The mafic volcanic rocks belong to low-K tholeiitic to medium-K calc-alkaline series. These mafic volcanic rocks are also characterised by moderately enriched light rare earth element (LREE) patterns; high abundances of Th, U, Zr and Hf; negative Nb, Ta and Ti anomalies; initial 87Sr/86Sr ratios of 0.70514-0.70623; and positive εNd(t) values (+1.9 to +3.8). These features indicate that the mafic volcanic rocks were likely derived from the high-percentage partial melting of subduction-related metasomatised asthenospheric mantle. The felsic rocks show an A-type affinity, with enrichments in alkalis, Th, U and LREEs. The felsic rocks are depleted in Ba, Sr, Nb, Ta and Ti and exhibit moderately LREE-enriched patterns (LaN/YbN = 2.09-6.45) and strongly negative Eu anomalies (Eu/Eu∗ = 0.04-0.25). These features, along with the positive εNd(t) values (+2.6 to +7.7) and young TDM2 ages (TDM2 = 435-916 Ma), indicate that the felsic rocks were likely derived from a juvenile crustal source that mainly consisted of juvenile mid-ocean ridge basalt-related rocks. The volcanic association in this study and in previously published work widely distributed in central Inner Mongolia. The observations in this study suggest that the lower Permian volcanic rocks formed in an identical tectonic environment. The regional geological data indicate that the bimodal volcanic rocks from the lower Permian Dashizhai Formation in the study area formed in an extensional setting that was

  4. Age, geochemical and isotopic variations in volcanic rocks from the Coastal Range of Taiwan: Implications for magma generation in the Northern Luzon Arc

    Lai, Yu-Ming; Song, Sheng-Rong; Lo, Ching-Hua; Lin, Te-Hsien; Chu, Mei-Fei; Chung, Sun-Lin


    This paper reports the first systematic analysis of age and geochemical variations in volcanic rocks from the Coastal Range of Taiwan, the Northern Luzon Arc. The rocks, recovered from four main volcanoes, vary from low-K tholeiitic to medium-K calc-alkaline basalts to dacites. The rocks are typical of arc magmatic products, exhibiting enrichment in the large ion lithophile elements and depletion in the high field strength elements. Our new 40Ar/39Ar age data constrain the youngest eruption time in each of the four volcanoes, i.e., from north to south, at 7.2 Ma (Yuemei), 4.2 Ma (Chimei), 6.2 Ma (Chengkuang'ao) and 8.5 Ma (Tuluanshan), respectively. These data indicate that volcanism in the Northern Luzon Arc did not cease progressively from north to south, as previously alleged. The high and broadly uniform Nd isotope ratios [εNd = + 10.1 to + 8.8] and trace element characteristics of the rocks suggest a principal magma source from the depleted mantle wedge. Their overall geochemical variations are ascribed to magma chamber processes. The effects of magmatic differentiation and crustal contamination differ among each volcano, most likely owing to the discrepancy of residence time in individual magma chambers. Consequently, we propose a binary mixing model for the magma generation that involves arc magmas sourced from the depleted mantle wedge and up to 5% crustal contamination with a continental fragment split off from the Eurasian margin.

  5. Numerical modelling of gas-water-rock interactions in volcanic-hydrothermal environment: the Ischia Island (Southern Italy) case study.

    Di Napoli, R.; Federico, C.; Aiuppa, A.; D'Antonio, M.; Valenza, M.


    Hydrothermal systems hosted within active volcanic systems represent an excellent opportunity to investigate the interactions between aquifer rocks, infiltrating waters and deep-rising magmatic fluids, and thus allow deriving information on the activity state of dormant volcanoes. From a thermodynamic perspective, gas-water-rock interaction processes are normally far from equilibrium, but can be represented by an array of chemical reactions, in which irreversible mass transfer occurs from host rock minerals to leaching solutions, and then to secondary hydrothermal minerals. While initially developed to investigate interactions in near-surface groundwater environments, the reaction path modeling approach of Helgeson and co-workers can also be applied to quantitative investigation of reactions in high T-P environments. Ischia volcano, being the site of diffuse hydrothermal circulation, is an ideal place where to test the application of reaction-path modeling. Since its last eruption in 1302 AD, Ischia has shown a variety of hydrothermal features, including fumarolic emissions, diffuse soil degassing and hot waters discharges. These are the superficial manifestation of an intense hydrothermal circulation at depth. A recent work has shown the existence of several superposed aquifers; the shallowest (near to boiling) feeds the numerous surface thermal discharges, and is recharged by both superficial waters and deeper and hotter (150-260°C) hydrothermal reservoir fluids. Here, we use reaction path modelling (performed by using the code EQ3/6) to quantitatively constrain the compositional evolution of Ischia thermal fluids during their hydrothermal flow. Simulations suggest that compositions of Ischia groundwaters are buffered by interactions between reservoir rocks and recharge waters (meteoric fluids variably mixed - from 2 to 80% - with seawater) at shallow aquifer conditions. A CO2 rich gaseous phase is also involved in the interaction processes (fCO2 = 0.4-0.6 bar

  6. Origin and age of the Volcanic Rocks of Tláloc Volcano, Sierra Nevada, Central Mexico

    Meier, M.; Grobéty, B.; Arce, J. L.; Rueda, H.


    The Tláloc volcano (TV) is a 4125 m high stratovolcano of the Trans Mexican Volcanic Belt (TMVB) and is located in the northern end of the N-S trending Sierra Nevada, 30 km NE of Mexico City. Few data on the petrological and temporal evolution of TV have been published to date. Recently dated deposits gave ages between 32'000 and 34'500±500 years BP (Huddart and Gonzalez, 2004). Mapping and sampling of extrusive rocks in the summit region of TV revealed a dome structure with radiating lava flows consisting of dacitic rocks containing plagioclase and hornblende phenocrysts. Some flows, however, seem to be associated with a collapse structure E of the main summit. Crossing relationships indicate that this structure is older (“Paleo Tláloc”). A stratigraphy of the pyroclastic deposits was established along the northern slope of TV. From the numerous pyroclastic flows, separated by paleosoils and fluviatile deposits, only two pumice and one block and ash flow (BAF) have regional extent. Their thickness - distance relationship and their granulometry point to major explosive events. A carbonized wood sample from the BAF deposit gave ages similar to the previous ages (33'180±550 yr BP and 23'170±270 yr BP), a sample from a pyroclastic flow gave even a younger age (16'620±110 yr BP), suggesting that TV remained active also after the volcanoes Iztaccíhuatl and Popocatépetl further to the South started their activity. Based on these preliminary data it may be necessary to reconsider the accepted scenario of the temporal evolution of the central section of the TMVB, which assumes that the activity migrates from North to South with time. Huddart, D. and Gonzalez, S., 2004. Pyroclastic flows and associated sediments, Tláloc-Telapón, piedmont fringe of the eastern basin of Mexico. In: G.J. Aguirre-Diaz, Macías, J.L., and Siebe, C., (Editor), Penrose Conference. UNAM, Metepec, Puebla, Mexico, pp. 35.

  7. Immiscibility of high salinity fluids in volcanic rocks and the mechanism of magma degassing in the Dongying sag, eastern China

    CHEN Yong; ZHOU Yaoqi; XIAO Huanqin; REN Yongjun; SUN Xinian; WANG Qiang; YAN Shiyong; LIU Chaoying


    Fluid inclusions that bear halite daughter min- erals were discovered in volcanic rocks at Pingnan area in the Dongying sag. The samples of the fluid inclusions collected from the BGX-15 well drill cores are hosted in quartz of diorite-porphyrite. The daughter minerals are identified as NaCl crystals after being observed under a microscope and analyzed by in situ Raman spectroscopy at -185℃. The results of micro-thermal analysis show that the homogeniza- tion temperatures of primary fluid inclusions are between 359 and 496℃, and the salinities of fluid inclusions are from 43.26 to 54.51 wt-%. All fluid inclusions in the studied sam- ples can be divided into five types including primary fluid inclusions and secondary fluid inclusions. The fact that five types of fluid inclusions were symbiotic in the same quartz grain implies that immiscibility happened in magma. Due to the decrease in temperature and pressure during the ascent of magma, the fluids became intensively immiscible. This pro- cess accelerates the degassing of CO2 from magma, but the remnant fluids with high salinity are preserved in fluid inclu- sions. Thus, the primary fluid inclusions are mainly in NaCl- H2O fluids and poor in CO2. The results of our study indicate that the degassing of magma and accumulation of CO2 gas at the Pingnan area are relative to the immiscibility of high salinity fluids. This discovery is important because it can help us have a further understanding of the mechanism of magma degassing and accumulation of the inorganic CO2 in eastern China.

  8. The Quaternary volcanic rocks of the northern Afar Depression (northern Ethiopia): Perspectives on petrology, geochemistry, and tectonics

    Hagos, Miruts; Koeberl, Christian; van Wyk de Vries, Benjamin


    The northern Afar Depression is one of the most volcano-tectonically active parts of the East African Rift system, a place where oceanic rifting may be beginning to form an incipient oceanic crust. In its center, over an area that is ∼80 km long and ∼50 km wide, there are seven major NNW-SSE-aligned shield volcanoes/volcanic edifices surrounded by compositionally distinct fissure-fed basalts. The Quaternary lavas in this area range from transitional to tholeiitic basalts, with significant across-axis variation both in mineralogy and chemistry. The variation in the contents of the major elements (TiO2, Al2O3, and Fe2O3), incompatible trace elements (Nd, Hf, Th, Ta), and the contents and ratios of the rare earth elements (REE) (e.g., (La/Yb)n = 5.3-8.9) indicate some variation in the petrogenetic processes responsible for the formation of these basalts. However, the variation in isotopic compositions of the mafic lavas is minimal (87Sr/86Sr = 0.7036-0.7041, 143Nd/144Nd = 0.51286-0.51289), which suggests only one source for all the Danakil Depression basalts. These basalts have isotope and incompatible trace element ratios that overlap with those of the Oligocene High-Ti2 flood basalts from the Ethiopian Plateau, interpreted as being derived from the last phase/tail of the Afar mantle plume source. Moreover, the Ce/Pb, Ba/U ratios indicate that the involvement of continental crust in the petrogenesis of the basaltic rocks is minimal; instead, both depth and degree of melting of the source reservoir underneath the northern Afar Depression played a major role for the production of incompatible element-enriched basalts (e.g., AleBagu Shield basalts) and the incompatible element-depleted tholeiitic basalts (e.g., Erta'Ale and Alu Shield basalts).

  9. Zircon U-Pb geochronology and geochemistry of low-grade metamorphosed volcanic rocks from the Dantazi Complex: Implications for the evolution of the North China Craton

    Ge, Songsheng; Zhai, Mingguo; Li, Tiesheng; Peng, Peng; Santosh, M.; Shan, Houxiang; Zuo, Pengfei


    The late Neoarchean witnessed the cratonization of the North China Craton (NCC) through amalgamation of several micro-blocks to form a coherent basement. The Archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at ∼2500 Ma. Recently, a suite of low-grade metamorphosed (greenschist to lower amphibolite facies) volcanic rocks was discovered in the late Neoarchean Dantazi Complex in northern Hebei province. These meta-volcanic rocks consist of bimodal basalt-andesite and trachyte-dacite with a SiO2 gap between 54.4 wt.% and 60.7 wt.%. Here we report SHRIMP zircon U-Pb ages of 2490 ± 19 Ma (MSWD = 2.0) and 2502 ± 8 Ma (MSWD = 0.83) from the meta-mafic and meta-felsic volcanics, respectively, representing the timing of igneous activity. All the meta-mafic volcanic rocks display coherent trace element and REE patterns which are characterized by enriched LILE and LREE but depleted HFSE and HREE ((La/Yb)N = 6.29-15.10). Combining these trace element features with the positive zircon εHf(t) values (+1.3 to +6.6), we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle. In the primitive mantle-normalized diagram, the felsic rocks display uniform patterns enriched in LILE but depleted in Nb and Ta, similar to those of lower crust. Furthermore, their strongly fractionated REE ((La/Yb)N = 15.24-61.20), lower HREE concentrations (Yb = 0.47-1.65 ppm) and positive zircon εHf(t) values (+1.6 to +5.3) suggest that they were derived from partial melting of the lower crust with garnet in the residue. This coeval occurrence of metasomatized mantle-derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late Neoarchean to earliest Paleoproterozoic following the cratonization of the NCC. Our new data, combined with previous

  10. Origin of the ca. 90 Ma magnesia-rich volcanic rocks in SE Nyima, central Tibet: Products of lithospheric delamination beneath the Lhasa-Qiangtang collision zone

    Wang, Qing; Zhu, Di-Cheng; Zhao, Zhi-Dan; Liu, Sheng-Ao; Chung, Sun-Lin; Li, Shi-Min; Liu, Dong; Dai, Jin-Gen; Wang, Li-Quan; Mo, Xuan-Xue


    Bulk-rock major and trace element, Sr-Nd-Hf isotope, zircon U-Pb age, and zircon Hf isotopic data of the Late Cretaceous Zhuogapu volcanic rocks in the northern Lhasa subterrane provide a new insight into tectonic processes following the collision of the terrane with the Qiangtang zone. SHRIMP zircon U-Pb dating reveals that the Zhuogapu volcanic rocks crystallized at ca. 91 Ma, postdating the development of a regional angular unconformity between the Upper Cretaceous and the underlying strata in the Lhasa-Qiangtang collision zone. Compared to the Andean arc-type andesites and dacites, the Zhuogapu volcanic rocks are characterized by higher MgO of 2.78-5.86 wt.% and Mg# of 54-64 for andesites and MgO of 2.30-2.61 wt.% and Mg# of 55-58 for dacites. Eight andesite samples have whole-rock (87Sr/86Sr)i of 0.7054-0.7065, εNd(t) of - 3.2 to - 1.7, and εHf(t) of + 3.8-+ 6.4, similar to those of the three dacite samples with (87Sr/86Sr)i = 0.7056-0.7060, εNd(t) of - 2.7 to - 2.2, and εHf(t) of + 5.6-+ 7.0. Thirteen analyses from a dacite sample give positive zircon εHf(t) of + 5.6 to + 8.7. These signatures indicate that the Zhuogapu Mg-rich andesites were most likely derived from partial melting of a delaminated mafic lower crust (including the lowermost crust straddling the northern and central Lhasa subterranes) that led to the generation of the Zhuogapu primary melts with adakitic signatures and small negative εNd(t). Such melts subsequently experienced interaction of melt-asthenospheric mantle peridotite followed by the modification of highly fractionated magmas in shallow crustal magma chamber. Hornblende-controlled fractionation results in the change of geochemical composition from Mg-rich andesitic to Mg-rich dacitic magmas. Field observations, together with geochronological and geochemical data, indicate that the Zhuogapu Mg-rich volcanic rocks and coeval magmatism in the northern Lhasa subterrane may be the result of thickened lithospheric delamination

  11. Petrogenesis and its significance to continental dynamics of the Neogene high-potassium calc-alkaline volcanic rock association from north Qiangtang, Tibetan Plateau

    赖绍聪; 刘池阳; S.Y.O’Reilly


    Detailed studies indicate that the main rock type of the Neogene high-potassium calc-alkaline volcanic rock association from north Qiangtang is andesite, dacite and rhyolite. They belong to typical crust-generation magmatic system and originate from the special thickened crust of the Tibetan Plateau by dehydration melting. This group of rocks exhibits LREE enrichment but no remarkable Eu anomaly that shows their source region should be a thickened deep crust consisting of eclogitic mass group, implying that the crust had been thickened and an eclogitic deep crust had been formed during the Neogene period in Qiangtang area. This understanding is important and significant to making further discussion on the uplift mechanism and continental dynamics of the Tibetan Plateau.

  12. The Maar-Diatreme System in a Mixed "Hard/Soft-Rock" Setting: an Example from the Pali Aike Volcanic Field, Argentina

    Delpit, S.; Ross, P.


    The eruptive processes in diatremes remain poorly understood compared to those at other volcano types, because these processes occur at depth. Except for maar-diatreme volcanoes formed during kimberlitic eruptions, volcanologists agree that these systems are of phreatomagmatic origin. The origin of kimberlitic diatremes is more contentious, but studying non kimberlitic equivalents can be a good approach to better understand kimberlitic diatremes considering their numerous common characteristics. The geometry of maar-diatreme systems is strongly influenced by their setting in "hard-rock" or "soft-rock" environments (Lorenz, 2003, Geolines 15:72-83). Formation of maar-diatreme systems in "hard-rock" environments, like in the West Eifel Volcanic Field of Germany, is largely described in the literature but emplacement in "soft-rock" environments or mixed settings is not. In the case of "hard-rock" environments external water is provided by fracture aquifers. The eruption products are juvenile clasts and country rock fragments. The inner crater walls of the maar, and the diatreme walls, have steep slopes. In the case of "soft- rock" environments, water is contained in the sediment pores and the walls tend to be at lower angles. We recently conducted field work on maars, cinder cones and spatter rings of the Pali Aike Volcanic Field of southern Argentina as part of the Potrok Aike Maar Lake Sediment Archive Drilling Project (PASADO). These Quaternary monogenetic volcanoes were emplaced in a mixed "hard/soft-rock" environment containing young glacial sediments, basaltic lava flows, partly consolidated fluviatile sediments, and older indurated sedimentary rocks. The mixed environment of emplacement is reflected in a phreatomagmatic deposit on the inner slope of a tephra ring exposing some lapilli-tuff layers. The lapilli fraction comprises approximately 40% lithics on average (visual estimate): at least half of the fraction is composed of basaltic lava derived from a pre

  13. Volcanic rock-hosted gold and base-metal mineralization associated with neoproterozoic-early Paleozoic back-arc extension in the Carolina terrane, southern Appalachian Piedmont

    Feiss, P.G. (Univ. of North Carolina, Chapel Hill (United States)); Vance, R.K. (Georgia Southern Univ., Statesboro (United States)); Wesolowski, D.J. (Oak Ridge National Lab., TN (United States))


    Volcanogenic mineral deposits in the Carolina terrane, southern Appalachian Piedmont, include Kuroko-type polymetallic massive sulfide deposits and disseminated gold-pyrite deposits associated with propylitic, silicic, argillic, and advanced argillic alteration. Host rocks are metavolcaniclastic and metaepiclastic rocks of a Neoproterozoic-Early Cambrian magmatic arc. The favorable gold horizon is the transition from a lower succession of andesitic and rhyolitic pyroelastic rocks with basal mafic lavas to an upper sequence of epiclastic sedimentary units and minor lava and ash flows. Kuroko-type deposits are associated with mafic to bimodal volcanic rocks in the upper sequence. Whole-rock oxygen isotope analyses indicate that gold mineralization is associated with a transition from hydrothermal systems dominated by isotopically relatively light ([delta][sup 18]O = -6% to -10%) waters, typical of high-latitude subaerial systems, to seawater ([delta][sup 18]O = 0%). Plots of [delta][sup 18]O vs. SiO[sub 2] of the host rocks show a compositional gap associated with mineralization at the subaerial to submarine transition. Values of [delta][sup 18]O for the hydrothermal waters, lithostratigraphic analyses, and tectonic models of the Carolina terrane demonstrate that mineralization coincided with extension in a rifted arc. 34 refs., 3 figs.

  14. Discovery of double-peaking potassic volcanic rocks in Langshan Group of the Tanyaokou hydrothermal-sedimentary deposit, Inner Mongolia, and its indicating significance

    PENG; Runmin; ZHAI; Yusheng; WANG; Zhigang; HAN; Xuefeng


    It is revealed that the protolith of gray-light brown potash-feldspar-leucogranulites and granulites in the 2nd formation of the LG in Tanyaokou deposit are quartz kerotophyre of synsedimentary eruption based on the following facts and features: (1) The rocks look compact and homogeneous without obvious crystals with naked eyes; (2) they contain blastoporphyritic or glomeroporphyritic and blasto-crystalloclastic crystals consisting of quartz with wavy extinction and albite with obvious alteration and deformation; (3) they also contain radiated and fibrous blasto-microspherulitic texture and swallow-tailed bifurcate and blasto-hollow-skeleton crystal texture, representing the rapid cooling characteristic of the magma during submarine volcanic eruption; (4) the major chemical compositions of the rocks are: SiO2 = 70.80%―76.00%, K2O (4.83%―6.22%)>Na2O(2.78%―3.80%), and K2O+Na2O = 8.63%―9.00%; and (5) their petrochemical diagrams indicate that they are volcanic rocks. Together with the characteristic that they occur in the same sequence with potassic spilite (SiO2 = 46.12%―50.68%, K2O = 4.23%―5.93%>Na2O = 2.15%―3.14%, K2O+Na2O = 6.51%―8.08%), it can be confirmed that the volcanics occurring in the 2nd Formation of the LG in Tanyaokou district are double-peaking potassic volcanic rocks. The discovery, together with the tuffs with ore minerals and the distribution of lead isotopic as well as the value of Co/Ni of pyrites >1 showing the obvious endogenic metalization, can prove that the Tanyaokou deposit is an untypical SEDEX-type deposit formed in the extension fault basin in the Mesoproterozonic aulacogen of the northern margin of the North China Platform, and its metallogenesis is related to the synsedimentary volcanic activities and the hydrothermal exhalation, and both the ore-forming material source and volcanics came from mantle or lower crust. These facts mentioned above, together with the meta-volcanic rocks (double-peaking) found in the

  15. The anatomy of a cinder cone: preliminary paleomagnetic, rock magnetic, structural, and petrologic data from the La Cienega volcano, Cerros del Rio volcanic field, northern New Mexico

    Petronis, M. S.; Foucher, M.; Lineline, J.; Van Wyk de Vries, B.


    The Cerros del Rio volcanic field is one of several middle Pliocene to Pleistocene basaltic volcanic fields of the axial Rio Grande Rift in central and northern New Mexico. It is a monogenetic volcanic field that comprises about 60 cinder-spatter cones, occupies ~ 700 km2, and ranges in age from 2.7 Ma to 1.1 Ma. Eruptive centers are typically central vent volcanoes, ranging from low-relief shields to steep-sided, breached cinder and spatter cone remnants. They represent short eruptive events that likely were derived from rapidly evolving reservoir-conduit systems. Mining activity has exposed the volcanic plumbing system of the Cienega Mine cinder cone, just west of Santa Fe, NM. Here, geologists from France and USA have been investigating the exposed roots of this eviscerated Pliocene volcano to investigate magma conduit geometry, magma flow structures, and eruption patterns. We are testing models for magma transport and volcano construction using a variety of field and laboratory tools. Common models of volcanic construction envision the magma feeder as a dike or pipe-like conduit transporting molten rock from a deep reservoir to the eruptive vent. We posit that small volcanic pluming systems are inherently more complex and actually involve numerous feeder geometries throughout the volcano lifespan. Our preliminary work suggests that the simple exteriors of some cinder cones hide a long life and complex history, both of which would change the appreciation of the related volcanic hazards in active systems. The Cienega Mine cinder cone consists of several meter- to decimeter-wide intrusions that connect to eruptive centers. These intrusions show a continuity of brittle to ductile structures from their margins to interiors. We have collected samples across each intrusion as well as along strike for anisotropy of magnetic susceptibility (AMS) and petrographic analysis in order to establish magma flow patterns. AMS results yield a remarkably consistent dataset that

  16. Geochemistry of the late Holocene rocks from the Tolbachik volcanic field, Kamchatka: Quantitative modelling of subduction-related open magmatic systems

    Portnyagin, Maxim; Duggen, Svend; Hauff, Folkmar; Mironov, Nikita; Bindeman, Ilya; Thirlwall, Matthew; Hoernle, Kaj


    We present new major and trace element, high-precision Sr-Nd-Pb (double spike), and O-isotope data for the whole range of rocks from the Holocene Tolbachik volcanic field in the Central Kamchatka Depression (CKD). The Tolbachik rocks range from high-Mg basalts to low-Mg basaltic trachyandesites. The rocks considered in this paper represent mostly Late Holocene eruptions (using tephrochronological dating), including historic ones in 1941, 1975-1976 and 2012-2013. Major compositional features of the Tolbachik volcanic rocks include the prolonged predominance of one erupted magma type, close association of middle-K primitive and high-K evolved rocks, large variations in incompatible element abundances and ratios but narrow range in isotopic composition. We quantify the conditions of the Tolbachik magma origin and evolution and revise previously proposed models. We conclude that all Tolbachik rocks are genetically related by crystal fractionation of medium-K primary magmas with only a small range in trace element and isotope composition. The primary Tolbachik magmas contain ~ 14 wt.% of MgO and ~ 4% wt.% of H2O and originated by partial melting (~ 6%) of moderately depleted mantle peridotite with Indian-MORB-type isotopic composition at temperature of ~ 1250 °C and pressure of ~ 2 GPa. The melting of the mantle wedge was triggered by slab-derived hydrous melts formed at ~ 2.8 GPa and ~ 725 °C from a mixture of sediments and MORB- and Meiji-type altered oceanic crust. The primary magmas experienced a complex open-system evolution termed Recharge-Evacuation-Fractional Crystallization (REFC). First the original primary magmas underwent open-system crystal fractionation combined with periodic recharge of the magma chamber with more primitive magma, followed by mixing of both magma types, further fractionation and finally eruption. Evolved high-K basalts, which predominate in the Tolbachik field, and basaltic trachyandesites erupted in 2012-2013 approach steady-state REFC

  17. Geochemistry and geochronology of late Mesozoic volcanic rocks in the northern part of the Eastern Pontide Orogenic Belt (NE Turkey): Implications for the closure of the Neo-Tethys Ocean

    Özdamar, Şenel


    This paper presents 40Ar/39Ar and U-Pb age data, Sr-Nd isotopes, whole-rock and mineral compositions of Upper Cretaceous volcanic rocks from the Ordu area of the Eastern Pontide Orogenic Belt (EPOB) in northeastern Turkey. The volcanic rocks exhibit a wide compositional range: basalt, basaltic-andesites, andesites and a rhyodacite suite; they are characterized by subparallel light rare earth element (LREE)-enrichment, relatively flat heavy rare earth element (HREE) patterns with Eu anomalies and moderate fractionation [average (La/Yb)N = 8.55]. The geochemical results show that the volcanic rocks have calc-alkaline affinity consistent with arc volcanic rocks erupted in an active continental margin. Initial 87Sr/86Sr values vary between 0.70569 and 0.70606, while initial 143Nd/144Nd values lie between 0.51244 and 0.51249. Crustal contamination affected the mantle-originated primary magma, as indicated by increased 87Sr/86Sr and decreased 143Nd/144Nd ratios with increasing SiO2. New precise laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) 206Pb-238U age analyses of zircon and 40Ar/39Ar age data of plagioclase from the volcanics enable a more precise reconstruction of the EBOP. The ages provide insight into the timing of arc formation in this region, constrain the volcanic activity between 86 My (Coniacian) and 75 My (Campanian) and constrain the timing of closure of the Neo-Tethys.

  18. Geology, geochemistry, geochronology, and economic potential of Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes segments of the Archibarca lineament, northwest Argentina

    Richards, J. P.; Jourdan, F.; Creaser, R. A.; Maldonado, G.; DuFrane, S. A.


    This study presents new geochemical, geochronological, isotopic, and mineralogical data, combined with new geological mapping for a 2400 km2 area of Neogene volcanic rocks in northwestern Argentina near the border with Chile, between 25°10‧S and 25°45‧S. The area covers the zone of intersection between the main axis of the Cordillera Occidental and a set of NW-SE-trending structures that form part of the transverse Archibarca lineament. This lineament has localized major ore deposits in Chile (e.g., the late Eocene La Escondida porphyry Cu deposit) and large volcanic centers such as the active Llullaillaco and Lastarría volcanoes on the border between Chile and Argentina, and the Neogene Archibarca, Antofalla, and Cerro Galán volcanoes in Argentina. Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes areas are mostly high-K calc-alkaline in composition, and range from basaltic andesites, through andesites and dacites, to rhyolites. Magmatic temperatures and oxidation states, estimated from mineral compositions, range from ~ 1000 °C and ∆FMQ ≈ 1.0-1.5 in andesites, to ~ 850 °C and ∆FMQ ≈ 1.5-2.0 in dacites and rhyolites. The oldest rocks consist of early-middle Miocene andesite-dacite plagioclase-pyroxene-phyric lava flows and ignimbrites, with 40Ar/39Ar ages ranging from 17.14 ± 0.10 Ma to 11.76 ± 0.27 Ma. Their major and trace element compositions are typical of the Andean Central Volcanic Zone, and show strong crustal contamination trends for highly incompatible elements such as Cs, Rb, Th, and U. These rocks are geochemically grouped as sub-suite 1. This widespread intermediate composition volcanism was followed in the middle-late Miocene by a period of more focused rhyodacitic flow-dome complex formation. These felsic rocks are characterized by less extreme enrichments in highly incompatible elements, and increasing depletion of heavy rare earth elements. These rocks are geochemically grouped as sub-suite 2. The


    V. F. Matyuk


    Full Text Available The features of bipolar pulsed remagnetization of construction medium-carbon steel items for testing the heat treatment temperature and structure of these items are discussed, the methods of bipolar pulse remagnetization providing testing of items of considered steels are suggested.

  20. Discovery of the boninite series volcanic rocks in the Bangong Lake ophiolite mélange,western Tibet,and its tectonic implications

    SHI Rendeng; YANG Jingsui; XU Zhiqin; QI Xuexiang


    The boninite series volcanic rocks, mainly composed of basaltic andesite, andesitic lava breccia and andesite porphyrite, were recognized for the first time in the Bangong Lake ophiolite mé1ange, western Tibet. These rocks have a strong boninitic affinity, with high SiO2 (55.61%-59.23%,weight percent), MgO (6.63%-13.08%, 9.13% on average (weight percent)), A1203/1iO2 ratios (36-54), Mg# (0.61-0.74), Ni (116 ppm on average) and Cr (354 ppm on average)low TiO2 (0.23%-0.39%, weight percent), and strong LILEs enrichment relative to the depleted HFSEs. C1-chondrite normalized (La/Gd)N and (Gd/Yb)N ratios of about 1.70 and 0.83, respectively, produce prominent“U-shaped” normalized REE (rare earth element) patterns. Such a close compositional affinity to boninite indicates that these volcanic rocks were formed in a forearc setting produced by the intra-oceanic subduction.

  1. Geochemistry of the Caledonian Basic Volcanic Rocks at the South Margin of the Qinling Orogenc Belt,and Its Tectonic Implications


    The geochemistry of the basic volcanic rocks at the south margin of the Qinling orogenic belt(SMQOB) suggests that they were formed in an intraplate tectonic setting.The REE distribution patterns show these rocks are strongly enriched in LREE with high ∑REE, and their trace elements geochemistry is similar to that of contimental flood basalt.All the above evidence suggests that the Caledonian basic volcanic rocks in the SMQOB were tholeiitic basalts formed in an intraplate spreading-initial rift tectonic setting.The characteristics of regional geology and geochemistry indicate that there was an intraplate spreading-rift tectonic setting between the South Qingling block and the Yangtze block in the Caledonian epoch.The dynamic spreading in this district began in the Early Caledonian and then the intraplate spreadinginitial rifts were formed in the Late Caledonian.As a result of spreading of the Tethys and geodynamic processes in deep mantle ,the Mianlue-Huashan oceanic basin was formed between the Qinling block and the Yangtze block in Devonian,and the Qinling microplate was separated from the northern part of the Yangtze plate.

  2. Paleomagnetic and rock-magnetic study on volcanic units of the Valsequillo Basin: implications for early human occupation in central Mexico

    Goguitchaichvili, Avto; Pozzo, Ana Lillian Martin-Del; Rocha-Fernandez, Jose Luis; Urrutia-Fucugauchi, Jaime; Soler-Arechalde, Ana Maria


    Alleged human and animal footprints were found within the upper bedding surfaces of the Xalnene volcanic ash layer that outcrops in the Valsequillo Basin, south of Puebla, Mexico (Gonzalez et al, 2005). The ash has been dated at 40 ka by optically stimulated luminescence analysis, thereby providing new evidence that America was colonized earlier than the Clovis culture (about 13.5 Ma). We carried out paleomagnetic and rock magnetic analysis on 18 Xalnene ash block and core samples collected at two distinct localities and 19 standard paleomagnetic cores belonging to nearby monogenetic volcanoes. Our data provide evidence that both the volcanic lava flow and Xalnene ash were emplaced during the Laschamp geomagnetic event spanning from about 45 to 39 ka.

  3. Geochemical and Sr Nd Pb isotopic evidence for a combined assimilation and fractional crystallisation process for volcanic rocks from the Huichapan caldera, Hidalgo, Mexico

    Verma, Surendra P.


    This study reports new geochemical and Sr-Nd-Pb isotopic data for Miocene to Quaternary basaltic to andesitic, dacitic, and rhyolitic volcanic rocks from the Huichapan caldera, located in the central part of the Mexican Volcanic Belt (MVB). The initial Sr and Nd isotopic ratios, except for one rhyolite, range as follows: 87Sr/ 86Sr 0.70357-0.70498 and 143Nd/ 144Nd 0.51265-0.51282. The Sr-Nd-Pb isotopic ratios are generally similar to those for volcanic rocks from other areas of the central and eastern parts of the MVB. The isotopic ratios of one older pre-caldera rhyolite (HP30) from the Huichapan area, particularly its high 87Sr/ 86Sr, are significantly different from rhyolitic rocks from this and other areas of the MVB, but are isotopically similar to some felsic rocks from the neighbouring geological province of Sierra Madre Occidental (SMO), implying an origin as a partial melt of the underlying crust. The evolved andesitic to rhyolitic magmas could have originated from a basaltic magma through a combined assimilation and fractional crystallisation (AFC) process. Different compositions, representing lower crust (LC) and upper crust (UC) as well as a hypothetical crust similar to the source of high 87Sr/ 86Sr rhyolite HP30, were tested as plausible assimilants for the AFC process. The results show that the UC represented by granitic rocks from a nearby Los Humeros area or by Cretaceous limestone (L) rocks outcropping in the northern part of the study area, and the LC represented by granulitic xenoliths from a nearby San Luis Potosı´ (SLP) area are not possible assimilants for Huichapan magmas, whereas a hypothetical crust (HA) similar in isotopic compositions to rhyolite HP30 could be considered a possible assimilant for the AFC process. Chemical composition of assimilant HA, although not well constrained at present, was inferred under the assumption that HP30 type partial melts could be generated from its partial melting. These data were then used to evaluate

  4. Volcanic stratigraphy and geochemical variations in Miocene-age rocks in western and southeastern Fort Irwin, California

    Buesch, D.


    Lava flows and tuffaceous deposits ranging in composition from basalt to rhyolite, including basaltic trachyandesite to trachyte, are exposed in 800 km2 of western Fort Irwin area, California, and form the eastern edge of the Eagle Crags volcanic field (ECVF). The main ECVF has 40Ar/39Ar ages from ~18.7-12.4 Ma (mostly 18.7-18.5 Ma; Sabin et al. 1994), and on Fort Irwin, the ages are from 21.0-15.8 Ma (mostly 18.6-15.8 Ma; Schermer et al. 1996). 68 samples (56 lava flow, 4 dome-collapse breccia, 3 ignimbrite, and 5 fallout tephra) were analyzed for major, minor, and trace elements. Typically, stratigraphic sequences dip cinder cones. A general upward felsic to mafic compositional sequence occurs throughout the area, but is not continuous as B is locally in a R-D sequence and B is at the base of and interstratified with a BA-A sequence. Also, there are compositional variations at different locations along the edges of the field. In the Goldstone Mesa, Pink Canyon, and Stone Ridge areas (~70 km2), B-BA forms the youngest lava flows, but ~21 km to the north in the Garry Owen area (~25 km2), BTA forms the youngest lava flows. Compared to the Stone Ridge area with a D-A-TA-BA trend, ~6 km west in the Pioneer Plateau area is R-TA-D, ~3 km south in the Pink Canyon area is R-B-BA-A, and ~8 km east at Dacite Dome is D only (all areas have slightly different Na2O+K2O in each rock type). A non-ECVF, 5.6 Ma BA flow in SE Fort Irwin also has distinct compositions. Chemical variations indicate the region had similar general evolution of magma sources, but (1) there were numerous small, isolated chambers that fed flows along the edges of the field, (2) several tuffs are similar to local lavas but some differ and might have distant sources, and (3) basalt flows locally encroached into adjacent areas.

  5. Paleomagnetic secular variation study of Ar-Ar dated lavas flows from Tacambaro area (Central Mexico): Possible evidence of Intra-Jaramillo geomagnetic excursion in volcanic rocks

    Peña, Rafael Maciel; Goguitchaichvili, Avto; Guilbaud, Marie-Noëlle; Martínez, Vicente Carlos Ruiz; Rathert, Manuel Calvo; Siebe, Claus; Reyes, Bertha Aguilar; Morales, Juan


    More than 350 oriented paleomagnetic cores were obtained for rock-magnetic and paleomagnetic analysis from radiometrically dated (40Ar-39Ar) magmatic rocks occurring in the southern segment (Jorullo and Tacámbaro areas) of the Michoacán-Guanajuato Volcanic Field in the Trans-Mexican Volcanic Belt. Most of the lavas (37) stem from monogenetic volcanoes dated at less than 4 Ma. Two additional sites were sampled from the plutonic basement dated at 33-30 Ma. Primary remanences carried by low-Ti titanomagnetites allowed to determining 34 reliable site-mean directions of mostly normal (27) but also reversed (7) polarities. The mean directions of these two populations are antipodal, and suggest neither major vertical-axis rotations with respect to the North America craton nor tilting in the region for the last 4 Ma (rotation and flattening of the inclination parameters being less than -5.9 ± 3.8 and 0.1 ± 3.9, respectively). The corresponding paleomagnetic pole obtained for Pliocene-Pleistocene times is PLAT = 83.4°, PLON = 2.4° (N = 32, A95 = 2.7°). Virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation (PSV) investigations at low latitudes from lavas for the last 5 Ma, showing a geomagnetic dispersion value that is in agreement with available PSV models. When comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the Cenozoic geomagnetic polarity time scale (GPTS), a good correlation is observable. This finding underscores the suitability of data obtained on lavas in Central Mexico for contributing to the GPTS. Furthermore, the detection of short-lived geomagnetic features seems possible, since the possible evidence of Intra-Jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks.

  6. Remagnetization in the Monterrey Salient (NE Mexico) and regional tecto-magnetic events in the Mexican Fold-Thrust Belt

    Nemkin, S. R.; Chavez-Cabello, G.; Fitz-Diaz, E.; van der Pluijm, B.; Van Der Voo, R.


    In the 1980's, carbonate remagnetizations became widely recognized, with many units re-analyzed to examine these later magnetization events. In this study we focus on the Lower Cretaceous La Peña-Cupido formations transition from the Monterrey Salient in Northeast Mexico, at the external most part of the Mexican Fold-Thrust Belt in this area. The remagnetization observed is carried by single domain (SD) magnetite, resulting from chemical growth of magnetite in a range of superparamagnetic (SPM) to SD grain sizes. In order to determine the relative remagnetization age, local km-scale folds were sampled for paleomagnetic fold tests. The application resulted in 8 syn-folding (1 a regional test) and 4 pre-folding remagnetizations (1 a regional application). Syn-folding results are found in the N-S to NW-SE trending portion of the Monterrey Salient and pre-folding results in the E-W trending segment of the belt. By combining syn-folding results with Ar/Ar illite ages of folds, a remagnetization age of 51 + 4 Ma is obtained in folded limestone samples. Relatively strong ferrimagnetic signals from sites with syn-folding remagnetizations suggest that more magnetite growth occurred in these folds, as compared to pre-folding sites where paramagnetic and diamagnetic signals are more dominant. Based on the relative timing of remagnetization, deformation progressed from the SW to the NE in northern Mexican Fold-Thrust Belt. Prior results (Nemkin et al., 2015) from the central Sierra Madre Oriental, to the south, showed two remagnetization events; 77 Ma and 44 Ma from W to E. Based on the timing of remagnetization in the new study area, folding and remagnetization in the Monterrey Salient occurred in between these events to the south.

  7. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou Basin, Tibet)

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; Van Hinsbergen, Douwe J.J.; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    International audience; The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou Basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-P...


    张莹; 潘保芝


    针对火山岩储层,从岩石化学成分分类和岩石结构分类两个角度出发,提出了一种利用测井资料识别火山岩岩性的方法.基于取芯薄片鉴定资料获得对应井段的常规测井数据,利用统计学习理论中的支持向量机方法对其处理,得到地质上按岩石化学成分分类的火山岩岩性类别.建立地层微电阻率成像测井图像与不同结构火山岩岩性之间的对应关系,归纳出典型的微电阻率图像模式,从而得到地质上按岩石结构分类的火山岩岩性类别,结合上述两者结论确定最终岩性,实现了运用支持向量机算法处理常规测井资料与微电阻率图像模式相结合的火山岩岩性测井识别新方法.%From the viewpoint of chemical composition categorization and structure classification of rocks, an effective method was proposed to identify the lithology of volcanic rocks by using logging data. On the one hand, the conventional logging data could be obtained by core wafer identification. Thus, after processing the data with Support Vector Machines (SVM) method of statistical theory, we could get the lithologic type of the volcanic rocks, which are classified according to the chemical composition of rocks. On the other hand, the volcanic rocks can be classified as volcanic lava, pyroclastic lava and pyroclastic rock according to the rock structure. Typical formation micro-resistivity imaging logging (FMI) image mode can be concluded by establishing the corresponding relationship between FMI images and lithology of volcanic rocks with different structures. As a result, the lithologic type of the volcanic rock classified by rock structure can be determined. Finally, by combining these two kinds of lithology, the ultimate rock lithology can be determined, too. In this paper, the authors presented a novel method to identify the lithology of volcanic rocks by combining SVM processed logging data and FMI image mode.

  9. Soils Developed from Different Volcanic Rocks from the Fernando de Noronha Island: Rare-Earth Element Patterns and Isotopic Lead Composition

    Sonia Maria Barros de Oliveira


    Full Text Available This study examines the infl uence of pedogenesis on the distribution of rare earth elements in soils derived fromdifferent rock types and formed under tropical humid climate, as well as the possible contribution of airborne Pb to thesesoils. We studied 5 soil profi les developed from different volcanic rocks cropping out in the Fernando de Noronha island.Results show that in the course of weathering, the soils were enriched in REE. The REE patterns of the soils are similarto those of the parent material, except for a slight HREE enrichment. Lead-isotope data indicate the presence of a nonradiogenicanthropogenic component in the upper horizons of the soil profi les.

  10. Geochemistry and Sr-Nd-Pb isotopic characteristics of the Mugouriwang Cenozoic volcanic rocks from Tibetan Plateau: Constraints on mantle source of the underplated basic magma

    LAI ShaoCong; QIN JiangFeng; LI YongFei; LONG Ping


    The Mugouriwang Cenozoic volcanic rocks exposed in the north Qiangtang Block of Tibetan Plateau are mainly composed of basalt and andesitic-basalt, both characterized by the lower SiO2 (51%-54%), high refractory elements (i.e. Mg, Cr, Ni) as well as the moderate enrichment in light rare earth elements (LREE) relative to a slight depleted in Eu and high strength field elements (HFSE, i.e. Nb, Ta, Ti). Besides, the fairly low Sm/Yb value (3.07-4.35) could signify that the rocks should be derived directly from partial melting of the spinel lherzolite at the upper part of the asthenosphere. These rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.705339 to 0.705667; 208Pb/204Pb = 38.8192 to 38.8937; 207Pb/204Pb = 15.6093 to 15.6245; 206Pb/204Pb = 18.6246 to 18.6383), and non-radiogenic Nd (143Nd/144Nd = 0.512604 to 0.512639; εNd = +0.02 to-0.66) in agreement with those values of the BSE mantle reservoir. The DUPAL anomaly of the rocks can be evidently attested by the △8/4Pb = 66.82 to 74.53 , △7/4Pb = 9.88 to 11.42, △Sr>50, implying that the Mugouriwang volcanic rock is likely to be generated by partial melting of a Gondwana-bearing asthenospheric mantle ever matasomatised by the fluid from subduction zone. Depending on the previous study on the high-K calc-alkaline intermediate-felsic volcanics in the study area, this paper proposed that the fluids derived from the subducted Lhasa Block metasomatised the asthenosphere beneath the Qiangtang Block, and induced its partial melting, and then the melt underplated the thickened Qiangtang lithosphere and caused the generation of the Cenozoic adakite-like felsic magmas in the Qiangtang region.

  11. Geochemistry and Sr-Nd-Pb isotopic characteristics of the Mugouriwang Cenozoic volcanic rocks from Tibetan Plateau:Constraints on mantle source of the underplated basic magma


    The Mugouriwang Cenozoic volcanic rocks exposed in the north Qiangtang Block of Tibetan Plateau are mainly composed of basalt and andesitic-basalt,both characterized by the lower SiO2 (51%―54%),high refractory elements (i.e. Mg,Cr,Ni) as well as the moderate enrichment in light rare earth elements (LREE) relative to a slight depleted in Eu and high strength field elements (HFSE,i.e. Nb,Ta,Ti). Be-sides,the fairly low Sm/Yb value (3.07―4.35) could signify that the rocks should be derived directly from partial melting of the spinel lherzolite at the upper part of the asthenosphere. These rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.705339 to 0.705667; 208Pb/204Pb = 38.8192 to 38.8937; 207Pb/204Pb = 15.6093 to 15.6245; 206Pb/204Pb = 18.6246 to 18.6383),and non-radiogenic Nd (143Nd/144Nd = 0.512604 to 0.512639; εNd = +0.02 to -0.66) in agreement with those values of the BSE mantle reservoir. The DUPAL anomaly of the rocks can be evidently attested by the △8/4Pb = 66.82 to 74.53 ,△7/4Pb = 9.88 to 11.42,△Sr>50,implying that the Mugouriwang volcanic rock is likely to be generated by partial melting of a Gondwana-bearing asthenospheric mantle ever matasomatised by the fluid from subduction zone. Depending on the previous study on the high-K calc-alkaline intermediate-felsic volcanics in the study area,this paper proposed that the fluids derived from the subducted Lhasa Block metasomatised the asthenosphere beneath the Qiangtang Block,and induced its partial melting,and then the melt under-plated the thickened Qiangtang lithosphere and caused the generation of the Cenozoic adakite-like felsic magmas in the Qiangtang region.

  12. Anomalous Seismic Velocity Drop in Iron and Biotite Rich Amphibolite to Granulite Facies Transitional Rocks from Deccan Volcanic Covered 1993 Killari Earthquake Region, Maharashtra (India): a Case Study

    Pandey, O. P.; Tripathi, Priyanka; Vedanti, Nimisha; Srinivasa Sarma, D.


    65 Ma Deccan Volcanic Province of western India forms one of the largest flood basaltic eruptions on the surface of the earth. The nature of the concealed crust below this earthquake prone region, which is marked by several low velocity zones at different depths has hardly been understood. These low velocity zones have been invariably interpreted as fluid-filled zones, genetically connected to earthquake nucleation. While carrying out detailed geological and petrophysical studies on the Late Archean basement cores, obtained from a 617 m deep KLR-1 borehole, drilled in the epicentral zone of 1993 Killari earthquake region of the southern Deccan Volcanic Province, we came across several instances where we observed remarkable drop in measured P-wave velocity in a number of high density cores. We provide detailed petrographic and geological data on 11 such anomalous samples which belong to mid-crustal amphibolite to granulite facies transitional rocks. They are associated with a mean P-wave velocity of 6.02 km/s (range 5.82-6.22 km/s) conforming to granitic upper crust, but in contrast have a high mean density of 2.91 g/cm3 (range 2.75-3.08 g/cm3), which characterise mid to lower crust. This velocity drop, which is as much as 15 % in some cores, is primarily attributed to FeOT enrichment (up to about 23 wt%) during the course of mantle-fluid driven retrogressive metasomatic reactions, caused by exhumation of deep-seated mafic rocks. Presence of Iron content (mainly magnetite), widely seen as opaques in thin sections of the rocks, seems to have resulted into sharp increase in density, as well as mean atomic weight. Our study indicates that the measured V p is inversely related to FeOT content as well as mean atomic weight of the rock.

  13. Permeability and continuous gradient temperature monitoring of volcanic rocks: new insights from borehole and laboratory analysis at the Campi Flegrei caldera (Southern Italy).

    Carlino, Stefano; Piochi, Monica; Tramelli, Anna; Troise, Claudia; Mormone, Angela; Montanaro, Cristian; Scheu, Bettina; Klaus, Mayer; Somma, Renato; De Natale, Giuseppe


    The pilot borehole recently drilled in the eastern caldera of Campi Flegrei (Southern Italy), during the Campi Flegrei Deep Drill Project (CFDDP) (in the framework of the International Continental Scientific Drilling Program) allowed (i) estimating on-field permeability and coring the crustal rocks for laboratory experiments, and (ii) determining thermal gradient measurements down to ca. 500 m of depth. We report here a first comparative in situ and laboratory tests to evaluate the rock permeability in the very high volcanic risk caldera of Campi Flegrei, in which ground deformations likely occur as the persistent disturbance effect of fluid circulation in the shallower geothermal system. A large amount of petro-physical information derives from outcropping welded tuffs, cores and geophysical logs from previous AGIP's drillings, which are located in the central and western part of the caldera. We discuss the expected scale dependency of rock permeability results in relation with well-stratigraphy and core lithology, texture and mineralogy. The new acquired data improve the database related to physical property of Campi Flegrei rocks, allowing a better constrain for the various fluid-dynamical models performed in the tentative to understand (and forecast) the caldera behavior. We also present the first data on thermal gradient continuously measured through 0 - to 475 m of depth by a fiber optic sensor installed in the CFDDP pilot hole. As regards, we show that the obtained values of permeability, compared with those inferred from eastern sector of the caldera, can explain the different distribution of temperature at depth, as well as the variable amount of vapor phase in the shallow geothermal system. The measured temperatures are consistent with the distribution of volcanism in the last 15 ka.

  14. Petrogenesis of the Early Permian volcanic rocks in the Chinese South Tianshan: Implications for crustal growth in the Central Asian Orogenic Belt

    Huang, He; Zhang, Zhaochong; Santosh, M.; Zhang, Dongyang; Wang, Tao


    The Paleozoic and Early Mesozoic magmatic suites in the Central Asian Orogenic Belt (CAOB) provide important insights on the crustal growth and reworking process associated with the construction of the largest Phanerozoic orogen on the Earth. Among the tectonic blocks of the CAOB, the South Tianshan Terrane (STT) occupies the southwestern margin and is located adjacent to the Tarim Craton. Here we investigate the Early Permian Xiaotikanlike Formation in the central part of the Chinese STT in Xinjiang in Northwest China. The formation is composed of a series of terrestrial volcanic lava flows and volcanic breccia, interbedded with siltstones, sandstones and sandy conglomerates. Zircon U-Pb and Lu-Hf isotopic analysis, whole-rock major oxide, trace element and Sr-Nd isotopic data are presented for the volcanic lava flows of the Xiaotikanlike Formation exposed in the Boziguo'er, Laohutai and Wensu regions. The new zircon ages from our study, together with those reported in previous investigations on the rhyolitic lava flow from the Wensu region, suggest that the volcanic rocks of the Xiaotikanlike Formation simultaneously erupted at ca. 285 Ma. The lavas of the formation show a wide range of SiO2 (49.88 to 78.56 wt.%). The basaltic rocks show SiO2 from 49.88 to 53.78 wt.%, MgO from 3.73 to 7.01 wt.% and Mg# from 41 to 61. They possess slightly enriched Sr-Nd isotope signature [(87Sr/86Sr)t = 0.70495-0.70624 and εNd(t) = - 0.5 to + 0.6], and have trace and rare earth element patterns similar to those of oceanic island basalts (OIBs). Petrographic and whole-rock chemical characteristics indicate that the basaltic lava flows are dominantly tholeiitic, and were likely derived from a spinel-dominated peridotite asthenospheric mantle source. The felsic lavas of the Xiaotikanlike Formation show SiO2 in the range of 60.71 to 78.56 wt.% and display overall similar immobile element pattern characterized by notable troughs at Nb-Ta, P and Ti and gently sloping REEs. Zircon Lu

  15. Laser fusion 40Ar/39Ar dating on young volcanic rocks.%(极)年轻火山岩激光熔蚀40Ar/39Ar定年

    周晶; 季建清; Alan DEINO; 龚俊峰; 韩宝福; 涂继耀; 桑海清; 徐剑光


    It is extremely difficult to date young volcanic rocks.Accurate 40Ar/39Ar dating research and practice of young volcanic rocks in China put forward several notable and important points during the process of 40Ar/39 Ar dating for young volcanic rocks.Because of the low background level,small sample quantity,and high sensitivity of modem noble gas isotope mass spectrometry device,laser ablation 40Ar/39Ar dating technique has been used in depth application in volcanic rocks dating.With the laser in 40Ar/39Ar dating of young and modern volcanic rocks,the discrepancy of Nier value and initial argon isotope ratio in Quaternary samples may cause a remarkable deviation between K-Ar apparent age and 40 Ar/39 Ar apparent age.The deviation show exponential increase for samples younger than 0.2Ma,but it is not so notable for samples older than 2Ma.There out,the boundary of young and modern volcanic rocks,which is applied to 40 Ar/39 Ar system,is assigned at 0.2Ma:rocks aged 2 ~ 0.2Ma is defined as young volcanic rock,while rocks aged younger than 0.2Ma is defined as modem volcanic rock.The phenocrysts,which are from different argon isotope fractionation system,should be rejected from the sample during 40Ar/39 Ar dating.According to rock texture and grain size of mineral,the matrix should be picked with ideal grain size.In normal cases,about 0.2mm (or 60 ~ 80 mesh) is recommended; Cooling time after irradiation of fast neutron reactor should not be no more than three months,or else it may cause an error on 37 Ar measurement,and consequently on final results.As neutron-flux monitor,standards should be characteristic of high homogeneity during laser 40Ar/39Ar dating.In calibrating the international and domestic standards we have,we find that SB-778-Bi,Bern4M and BT-1 can be ideal standard samples with good homogeneity.We also find that the isochron age calculated from the laser 40Ar/39Ar data of the phenocryst and matrix,which were cooled and crystallized at the same time

  16. Petrogenesis of meta-volcanic rocks from the Maimón Formation (Dominican Republic): Geochemical record of the nascent Greater Antilles paleo-arc

    Torró, Lisard; Proenza, Joaquín A.; Marchesi, Claudio; Garcia-Casco, Antonio; Lewis, John F.


    Metamorphosed basalts, basaltic andesites, andesites and plagiorhyolites of the Early Cretaceous, probably pre-Albian, Maimón Formation, located in the Cordillera Central of the Dominican Republic, are some of the earliest products of the Greater Antilles arc magmatism. In this article, new whole-rock element and Nd-Pb radiogenic isotope data are used to give new insights into the petrogenesis of the Maimón meta-volcanic rocks and constrain the early evolution of the Greater Antilles paleo-arc system. Three different groups of mafic volcanic rocks are recognized on the basis of their immobile element contents. Group 1 comprises basalts with compositions similar to low-Ti island arc tholeiites (IAT), which are depleted in light rare earth elements (LREE) and resemble the forearc basalts (FAB) and transitional FAB-boninitic basalts of the Izu-Bonin-Mariana forearc. Group 2 rocks have boninite-like compositions relatively rich in Cr and poor in TiO2. Group 3 comprises low-Ti island arc tholeiitic basalts with near-flat chondrite-normalized REE patterns. Plagiorhyolites and rare andesites present near-flat to subtly LREE-depleted chondrite normalized patterns typical of tholeiitic affinity. Nd and Pb isotopic ratios of plagiorhyolites, which are similar to those of Groups 1 and 3 basalts, support that these felsic lavas formed by anatexis of the arc lower crust. Geochemical modelling points that the parental basic magmas of the Maimón meta-volcanic rocks formed by hydrous melting of a heterogeneous spinel-facies mantle source, similar to depleted MORB mantle (DMM) or depleted DMM (D-DMM), fluxed by fluids from subducted oceanic crust and Atlantic Cretaceous pelagic sediments. Variations of subduction-sensitive element concentrations and ratios from Group 1 to the younger rocks of Groups 2 and 3 generally match the geochemical progression from FAB-like to boninite and IAT lavas described in subduction-initiation ophiolites. Group 1 basalts likely formed at magmatic

  17. K-Ar age of young volcanic rocks and excess argon--Binary mixing model and quantitative study of excess argon effect


    A binary mixing model for excess argon is suggested in the note. According to this model and the data of excess argon component obtained in our experiment , a quantitative study of the effect of excess argon on real K-Ar age of young volcanic rocks is done. The result indicates that the effect of 5% excess argon component in samples on K-Ar age of the samples more than 2 Ma is less than 7.36% and can lead K-Ar age of 0.5 Ma samples to increase by 32.4%, while 1% excess argon component leads K-Ar age of 0.5 Ma samples to increase by 6.26%. Therefore, when pre-processed excess argon component is ≤1%, K-Ar age of the samples more than 0.5 Ma should be credible. On this basis we suggest a principal opinion for evaluation of previous K-Ar dating results and propose that the matrix is used to determine K-Ar age of young volcanic rocks. For the samples less than 0.2 Ma, in the case of high excess argon content, even if only 1% excess argon component exists in their matrix, it can also greatly affect their K-A age. Thus it must be careful to treat the dating result.

  18. Remagnetization of bulk high-temperature superconductors subjected to crossed and rotating magnetic fields

    Vanderbemden, P [SUPRATECS and Department of Electrical Engineering and Computer Science B28, Sart-Tilman, B-4000 Liege (Belgium); Hong, Z [Centre for Advanced Photonics and Electronics, Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA (United Kingdom); Coombs, T A [Centre for Advanced Photonics and Electronics, Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA (United Kingdom); Ausloos, M [SUPRATECS and Department of Physics B5, Sart-Tilman, B-4000 Liege (Belgium); Babu, N Hari [IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE (United Kingdom); Cardwell, D A [IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE (United Kingdom); Campbell, A M [IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE (United Kingdom)


    Bulk melt-processed Y-Ba-Cu-O (YBCO) has significant potential for a variety of high-field permanent-magnet-like applications, such as the rotor of a brushless motor. When used in rotating devices of this kind, however, the YBCO can be subjected to both transient and alternating magnetic fields that are not parallel to the direction of magnetization and which have a detrimental effect on the trapped field. These effects may lead to long-term decay of the magnetization of the bulk sample. In the present work, we analyze both experimentally and numerically the remagnetization process of a melt-processed YBCO single domain that has been partially demagnetized by a magnetic field applied orthogonal to the initial direction of trapped flux. Magnetic torque measurements are used as a tool to probe changes in the remanent magnetization during various sequences of applied field. The application of a small magnetic field between the transverse cycles parallel to the direction of original magnetization results in partial remagnetization of the sample. Rotating the applied field, however, is found to be much more efficient at remagnetizing the bulk material than applying a magnetizing field pulse of the same amplitude. The principal features of the experimental data can be reproduced qualitatively using a two-dimensional finite-element numerical model based on an E-J power law. Finally, the remagnetization process is shown to result from the complex modification of current distribution within the cross-section of the bulk sample.

  19. Palaeomagnetic results from the southern Sierra Madre Oriental, Mexico: evidence for Early Cretaceous or Laramide remagnetization?

    Böhnel, H.; Gose, W. A.; Testarmata, M. M.; Bocanegra Noriega, G.


    A large suite of samples from the Latest Triassic Huizachal and Early Jurassic Huayacocotla Groups and the Latest Jurassic Taman and Earliest Cretaceous Pimienta Formations was collected in the southern Sierra Madre Oriental for a palaeomagnetic study. Only the samples from three sites belonging to the Huizachal Group and the Las Juntas Formation possibly have retained their primary magnetization. If so, their pole position does not reveal any palaeomagnetically discernable motion relative to cratonic North America. All remaining sites were remagnetized as evidenced by a negative fold test at four sites and the fact that the pole positions cluster better at the 95% significance level if no structural corrections are applied. The tightness of the cluster ( α95 = 4.6 °) and the same polarity, suggest that the samples were remagnetized at some common time. These results permit two interpretations. (1) If the sampling region has not suffered any significant tectonic rotation, then the remagnetization can be dated by comparison with the polar wander path for North America as Early Cretaceous (≈ 130 Ma). This Early Cretaceous phase of deformation is not recognized in the northern Sierra Madre Oriental and clearly pre-dates the Early Tertiary Laramide orogeny. (2) If the southern Sierra Madre Oriental did rotate counterclockwise by ≈ 20 °, then the remagnetization could have originated in any Cretaceous or Early Tertiary time, and may indeed be related to the Laramide deformation. In either case, the data imply that the southern Sierra Madre Oriental constitutes an independent tectonic domain.

  20. [Micro-area characteristics of laminated chert in the volcanic rocks of Xionger Group of Ruyang area and its geological significances].

    Luo, An; Li, Hong-zhong; Zhao, Ming-zhen; Yang, Zhi-jun; Liang, Jin; He, Jun-guo


    The Xionger Group was originated from the volcanic eruption and sedimentation in Precambrian, whose sedimentary strata at the top were named Majiahe Formation. In the Majiahe Formation, there were hydrothermal chert widely distributed, which were exhibited to be interlayers in the volcanic rocks. The polarized microscope, X-ray diffraction (XRD), Raman and electron back scatter diffraction (EBSD) were conducted to study the characteristics in micro area of the jasperite samples, which were from the sedimentary interlayers in the volcanic rocks of Majiahe Formation in Xionger Group. As shown in the microphotographs and EBSD images, the quartz in the chert had small grain size, low degree of crystallinity and close packed structure, which quite agreed with the characteristics of hydrothermal sedimentary chert. In the chert of Xionger Group, there were clear banded (or lamellar) structures which were contributed by the diversities of the grain size and mineral composition. The different bands (or lamellars) had alternative appearance repeatedly, and denoted the diversities and periodic changes in the substance supply during the precipitation. According to the results of the XRD analysis, the majority minerals of the chert was low temperature quartz, whose lattice parameters were a=b=0.4913 nm, c=0.5405 nm and Z=3. As denoted in the EBSD image and result of Raman analysis, several impurity minerals were formed in the chert in different stages, whose geneses and formation time were quite different. The clay minerals and pyrite were scattered in distribution, and should be contributed by the original sedimentation. On contrary, the felsic minerals and mafic silicate minerals were originated from the sedimentation of tuffaceous substance during the volcanic eruption. The minerals of volcanic genesis had relatively larger grain size, and they deposited together with the hydrothermal sediments to form the bands (or lamellars) of coarse minerals. However, the hydrothermal

  1. The Volatile Element Evolution of Intra-plate Alkaline Rocks as Recorded by Apatite: An Example from the Hegau Volcanic Field (Southwest Germany)

    Von Der Handt, A.; Rahn, M. K. W.; Wang, L. X.; Marks, M. A. W.


    The role of volatiles in the petrogenesis of alkaline intra-plate magmas has been the subject of an increasing number of experimental studies. The study of naturally occurring rocks and their volatile contents is often complicated by syn- and post-eruptive degassing and alteration processes. Minerals that incorporate volatiles into their structure such as apatites are often more faithful recorders of the pre-eruptive volatile budget. The Hegau volcanic field in Southwest Germany is part of the Central European Volcanic Province, lies around 60-70 km to the east of the Upper Rhine graben and of Miocene age. Three main lithological units can be distinguished (1) olivine melilites (2) phonolites and (3) the "Deckentuff" series referring to a series of diatreme-filling pipe breccias and lapilli tuff layers. Carbonatites occur subordinately in the Hegau province. Earlier radiometric age dating suggested distinct phases of volcanic activity of Deckentuffs, melilites and phonolites with little overlap, but new apatite fission-track and (U-Th)/He age data suggest a synchronous activity. Apatite is an abundant accessory phase in the Deckentuff and phonolite series and we investigated its major, trace and volatile element composition by EPMA, SIMS and cathodoluminescence imaging. Pronounced core-rim zoning of apatite in places attests that diffusional equilibration was very limited and they likely retained their primary compositions. This allows us to trace the entire magmatic evolution of the Hegau province from its most primitive to most evolved products as well as resolve it in time by combining age dating with compositional analysis. Apatite compositions fall along the OH-F join with low Cl-contents (<0.5 wt%). Volatile contents (Cl, OH, S) are highest in most primitive compositions and decrease with further evolution while F increases. Multiple magmatic cycles can be discerned with a general trend to the more evolved phonolite compositions toward the end of volcanic

  2. Resrarch on Lithology Identification for Volcanic Rock Based on Logging in Santanghu Basin%三塘湖盆地火山岩岩性测井识别方法

    邢贝贝; 马世忠


    火山岩岩性的准确识别是火山岩储层描述与评价的基础.通过对三塘湖盆地火山岩测井相应特征进行的分析归纳,建立了火山岩岩性识别流程.首先利用取芯井段标定准确岩性,然后利用交会图法、模糊聚类分析法以及微电阻率成像测井资料对研究区内储层岩性进行综合识别.将研究区岩性划分为火山熔岩、火山碎屑岩、火山碎屑沉积岩和沉积岩4大类、11小类.通过22口井的资料进行处理验证,岩性识别符合率平均达84.7%.%Accurate identification of volcanic rocks in volcanic reservoir description and evaluation is an important basis. Santanghu basin by volcanic rocks of the corresponding log analysis of the characteristics of induction, the establishment of a volcanic lithology identification process. Firstly, accurate calibration of core lithology well section, then use the intersection graph, fuzzy clustering analysis, and micro-resistivity imaging log lithology of the study area, a comprehensive identification, the study area was divided into the volcanic rock lava, pyroclastic rocks, volcanic rocks and clastic sedimentary rocks 4 categories, 11 categories. 22 wells through the processing of data validation, lithology identification consistent with the average rate of 84.7%.


    徐翠玲; 赵广涛; 何雨旸; 李德平


    Tengchong Cenozoic volcanic rocks, which consist of basaltic trachy-andesite, trachy-andesite, and basaltic andesite, belonging to the high-K and calc-alkaline series, are continental intraplate volcanic rocks erupted after the Tethys Ocean was closed. They are distributed near the collision zone of the Indian plate and the Eurasian plate. The geochemical features of the rocks can be used to define the tectonic properties and magma sources. The systematic geochemical study of the typical rock samples by XRF and ICP MS shows that the rocks are characterized by high K2O,CaO, low TiO2,and high Mg # (averaging about 46). REE shows a LREE-enriched pattern with significantly negative Eu. Large ion lithophilc elements (LILE) and high field strength elements (HFSE) are enriched compared to the primitive mantle, and the LILE have higher enrichment than the HFSE. Th shows a significantly positive anomaly. The geochemical composition of the studied volcanic rocks is similar to the volcanic arc magma, suggesting that the magmat-ic activity was triggered by the subduction of the Indian plate down to the Eurasian plate. Characteristic element ratios show that the magma may be derived from the subduction-related EM I mantle.%腾冲新生代火山岩位于印度板块和欧亚板块碰撞带附近,但是喷发时大洋已经闭合,属于大陆板内火山岩.对其进行地球化学研究,可以用来划分构造属性和推测岩浆来源.采用XRF和ICP-MS对典型岩石样品进行了较系统的岩石地球化学研究,结果表明,岩石类型有玄武质粗面安山岩、粗面安山岩和玄武安山岩,属高钾钙碱性系列;岩石化学显示高K2O、CaO和低TiO2,Mg#较高,平均约为46;稀土元素分布呈右倾,显示明显的Eu负异常;相对于原始地幔富集大离子亲石元素和高场强元素,并具有明显的Th正异常;地球化学组成总体上与岛弧岩浆岩相似,推测其成因与印度板块向欧亚板块俯冲引发的岩浆活

  4. Application of Clinopyroxene Chemistry to Interpret the Physical Conditions of Ascending Magma, a Case Study of Eocene Volcanic Rocks in the Ghohrud Area (North of Isfahan

    Mohammad Sayari


    Full Text Available Introduction Volcanic rocks with a porphyritic texture have experienced two crystallization stages. The first is slow, resulting in phenocrysts, and the second, which took place at, or near the surface, or during intrusion into a cooler body of rock, result in a groundmass of glass, or fine crystals. The pressure and temperature history of a magma during crystallization is recorded in the chemical composition of the phenocrysts during both stages. These phenocrysts provide valuable data about the physicochemical conditions of the parent magma during the process of crystallization. The composition of clinopyroxene (cpx reflects not only the chemical condition and therefore the magmatic series, but also the physical conditions, i.e., temperature and pressure of a magma at the time when clinopyroxene crystallized. The Ghohrud area lies in the middle part of the Urumieh-Dokhtar Magmatic Arc , which is part of a much larger magmatic province extending in a vast region of convergence between Arabia and Eurasia north of the Zagros-Bitlis suture zone (Dilek et al., 2010. In the Ghohrud area, north of Isfahan, exposed Eocene volcanic rocks belong to the first pulse of Cenozoic volcanism of Iran (Sayari, 2015, ranging in composition from andesitic basalt to basalt. The basaltic rocks of the Ghohrud area are composed mainly of plagioclase phenocrysts surrounded by smaller crystals of clinopyroxene in a groundmass of microlites, glass and opaques. In this study, the clinopyroxene and plagioclase of these rocks were analyzed in order to estimate the physicochemical conditions of the parent magmas. Results Clinopyroxene and plagioclase phenocrysts of nineteen samples were analyzed with the electron microprobe. The chemical compositions of the clinopyroxenes were used to estimate both the chemical evolution and temperature and pressure conditions of the magmas during crystallization, using SCG, a specialized software for clinopyroxene thermobarometry (Sayari

  5. Geochemistry and spatial distribution of late-Paleozoic mafic volcanic rocks in the surrounding areas of the Gonghe Basin: Implications for Majixueshan triple-junction and east Paleotethyan archipelagic ocean


    The late-Paleozoic mafic volcanic rocks occurring in the surrounding areas of the Gonghe basin are distributed in the A'nyêmaqên ophiolite zone, Zongwulong tectonic zone and Kuhai-Saishitang volcanic zone. The mafic volcanics in the A'nyêmaqên zone formed an ancient ridge-centered hotspot around the Majixueshan OIB, the Kuhai-Saishitang mafic rocks consist of E-MORB and continental rift basalts and the Zongwulong volcanic rocks are enriched N-MORB. The regionally low Nb/U and Ce/Pb ratios reflect the influence of the OIB material on the mafic magma source. From geochemistry, spatial distribution and tectonic relationship of the mafic rocks, an ancient triple-junction centered at the Majixueshan can be inferred. The existence of the Kuhai-Saishitang aulacogen may have provided a tectonic channel for the Majixueshan OIB materials metasomatizing the magma source for the Zongwulong rocks. The formation of the triple-junction and the rifting of the Zongwulong zone have separated the orogens and massifs in the region.

  6. Sphene (Titanite) as Both Monitor and Driver of Evolution of Felsic Magma: Miocene Volcanic Plutonic and Rocks of the Colorado River Region, NV-AZ, USA

    Miller, C. F.; Colombini, L. L.; Wooden, J. L.; Mazdab, F. K.; Gualda, G. A.; Claiborne, L. E.; Ayers, J. C.


    Sphene is commonly the most abundant accessory mineral in metaluminous to weakly peraluminous igneous rocks. Its relatively large crystals preserve a wide array of zoning patterns and inclusions - notably, abundant other accessories and melt inclusions - and it is a major host for REE, U, Th, and HFSE. Thus it is a valuable repository of information about the history of the magmas from which it forms. Recent development of a Zr-in- sphene thermometer (Hayden et al CMP 155:529 2008) and of sensitive and precise in situ trace element analysis by SHRIMP-RG (Mazdab et al GSA abst 39:6:406 2007) permit more powerful exploitation of this repository. We have initiated a study of sphene in Miocene intrusive and extrusive rocks of the Colorado River extensional corridor for which extensive field, geochemical, and geochronological data provide context. Sphene is present as a late interstitial phase in some gabbros and diorites and common in quartz monzonites and granites. Among extrusive rocks, it occurs as phenocrysts in rhyolite lavas and tuffs that are products of small to giant eruptions (Peach Spring Tuff, >600 km3). Glasses that host sphene in the rhyolites are highly evolved (>76 wt% SiO2). Applying the Zr-in-sphene thermometer (TZr), SHRIMP-RG analyses indicate crystallization T between 730 and 810 C in both plutonic and volcanic rocks. This range is narrower than T estimates for zircon growth (Ti thermometry) for the same suite, which extend to somewhat lower and considerably higher values; zircons also tend to record more events and, evidently, longer histories. Ranges of REE patterns are variable and to some extent sample-specific, but all reveal common characteristics: (1) extremely high concentrations, especially for middle REE (maximum Sm in interiors 10-40x103 x chondrite); (2) deep negative Eu anomalies (Eu/Eu* ca. 0.1-0.2); (3) TZr and REE dropping toward rims - especially pronounced for MREE. Estimated Kds for REE from sphene rims and rhyolite glass or

  7. Using syntectonic remagnetizations for fold geometry and vertical axis rotation: the example of the Cévennes border (France)

    Henry, Bernard; Rouvier, Henri; Le Goff, Maxime


    Classical progressive unfolding makes a distinction between pre-folding, post-folding or synfolding magnetization. It yields the direction of magnetization only in the two first cases. To determine the direction of synfolding magnetization, possible different unfolding percentages during remagnetization have to be assumed according to the site. The small circle approach of Surmont et al. not only leads to this direction, but also to the dip value at each site during remagnetization. In the Cévennes area, extensive palaeomagnetic study reveals syntectonic remagnetization enabling the reconstruction of the shape of a fold for the time of the magnetic overprinting as well as analysis of the rotations undergone by some sites after the remagnetization.

  8. Geochemical Characteristics and Geological Significance of the Kafang Alkaline Volcanic Rocks in Gejiu Area, Yunnan Province%云南个旧卡房碱性火山岩地球化学特征及意义

    张海; 方维萱; 杜玉龙


    Indosinian volcanism in the Gejiu area was divided into three phases: the Anisian Phase, the Early Ladinian Phase, and the Later Ladinian Phase-Norian Phase in the Middle Traissic Epoch. The Anisian alkali volcanic rocks were closely related to the Sn-Cu mineralization in the Kafang copper-tin deposit. The lithological and geochemical studies showed that the volcanic rocks contain 42.22% to 46.61% of SiO2, 8.61% to 18.19% of MgO, 3.52% to 7.52% of K2O+Na2O (K2O/Na2O=0.28 to 25.95) and 2.03% to 3.51% of TiO2. These volcanic rocks belong to alkalic ultrobasic volcanic rocks, and they are rich in high field-strength elements (Nb, Ta) and large-ion lithophile elements (Rb, Ba, Sr, Cs). At the same time, rare earth elements in these volcanic rocks are LREE-enriched and HREE-defected with right-inclined patterns, with obvious Eu anomalies and weak Ce negative anomalies. It may be indicated that diagenetic process of the Kafang alkali volcanic rocks might be mainly influenced by partial melting of mantle, and not obviously influenced by crustal contamination based on their geochemical features. The magmatic source area of these volcanic rocks might have derived from the partial melting of phlogopite-rich (or hornblende-rich) mantle peridotite. Moreover, the Kafang volcanic rocks share geochemical characteristics with the ocean island basalts (OIB). Based on the lithological and geochemical characteristics of the Kafang alkali volcanic rocks, these volcanic rocks might be formed in a geotectonic settings of the back-arc rift basin, and they may provide original source for the Kafang copper-tin deposit in Yunnan province, China.%个旧地区印支期的火山作用主要有三期,即中三叠世安尼期、拉丁尼克早期和拉丁尼克晚期-诺利克期。通过对与成矿关系密切的卡房铜锡矿安尼期碱性火山岩的岩相学、地球化学研究发现: SiO2含量为42.22%~46.61%, MgO含量为8.61%~18.19%, K2O+Na2O值为3.52%~7.52%, K2O/Na2O值为0

  9. Geochemistry and petrogenesis of anorogenic basic volcanic-plutonic rocks of the Kundal area, Malani Igneous Suite, western Rajasthan, India

    A Krishnakanta Singh; G Vallinayagam


    The Kundal area of Malani Igneous Suite consists of volcano-plutonic rocks. Basalt flows and gabbro intrusives are associated with rhyolite. Both the basic rocks consist of similar mineralogy of plagioclase, clinopyroxene as essential and Fe-Ti oxides as accessories. Basalt displays sub-ophitic and glomeroporphyritic textures whereas gabbro exhibits sub-ophitic, porphyritic and intergrannu- lar textures. They show comparable chemistry and are enriched in Fe, Ti and incompatible ele- ments as compared to MORB/CFB. Samples are enriched in LREE and slightly depleted HREE patterns with least significant positive Eu anomalies. Petrographical study and petrogenetic mod- eling of [Mg]-[Fe], trace and REE suggest cogenetic origin of these basic rocks and they probably derived from Fe-enriched source with higher Fe/Mg ratio than primitive mantle source. Thus, it is concluded that the basic volcano-plutonic rocks of Kundal area are the result of a low to moderate degree (> 30%) partial melting of source similar to picrite/komatiitic composition. Within plate, anorogenic setting for the basic rocks of Kundal area is suggested, which is in conformity with the similar setting for Malani Igneous Suite.

  10. Specific activities of natural rocks and soils at quaternary intraplate volcanism north of Sana’a, Yemen

    Harb, Shaban; El-Kamel, Abd El-Hadi; Abbady, Abd El-Bast; Saleh, Imran Issa; El-Mageed, Abdallah Ibrahim Abd


    The level of natural radioactivity in rocks and soil of 32 samples collected from locations at North Sana′a in Yemen was measured. Concentrations of radionuclides in rocks and soils samples were determined by gamma-ray spectrometer using high purity germanium (HPGe) detector with specially designed shield. The average radioactivity concentrations of 226Ra, 232Th, 40K were determined and expressed in Bq/kg. The results showed that these radionuclides were present in concentrations of 21.79 ± 3.1, 19.5 ± 2.6 and 399.3 ± 16 Bq/kg, respectively, for rocks. For soil, the corresponding values were 48.2 ± 4.4, 41.7 ± 4.5 and 939.1 ± 36 Bq/kg, respectively. Also, the radiological hazard of the natural radionuclide content, radium equivalent activity, total dose rates, external hazard index and gamma activity concentration index of the (rocks/soils) samples in the area under consideration were calculated. The dose rates at 1 m above the ground from terrestrial sources were 38.39 and 86.89 nGy/h for rocks and surface soil, respectively, which present no significant health hazards to humans. PMID:22363113

  11. Discriminating four tectonic settings: Five new geochemical diagrams for basic and ultrabasic volcanic rocks based on log–ratio transformation of major-element data

    Surendra P Verma; Mirna Guevara; Salil Agrawal


    We present five new discriminant function diagrams based on an extensive database representative of basic and ultrabasic rocks from four tectonic settings of island arc, continental rift, ocean-island, and mid-ocean ridge. These diagrams were obtained after loge-transformation of concentration ratios of major-elements – a technique recommended for a correct statistical treatment of compositional data. Higher % success rates (overall values from ∼83 to 97%) were obtained for proposing these new diagrams as compared to those (∼82 to 94%) obtained from the discriminant analysis of the raw major-element concentration data (i.e., without the loge-transformation and without taking ratios of the compositional data, but using exactly the same database to provide an unbiased comparison), suggesting that such a data transformation constitutes a statistically correct and recommended technique. The new diagrams also resulted in less mis-classification of basic and ultrabasic rocks from known tectonic settings than the diagrams obtained from the raw data. The use of these highly successful new discriminant function diagrams is illustrated using Miocene to Recent basic and ultrabasic rocks from three areas of Mexico with complex or controversial tectonic settings (Mexican Volcanic Belt, Los Tuxtlas volcanic field, and Eastern Alkaline Province), as well as older rocks from three areas (Deccan, Malani, and Bastar) of India. Additionally, the major-element data from two ‘known’ continental arc settings are used to show that they are similar to those from the island arc setting. Continental rift setting is inferred for all Mexican cases and for one cratonic area of India (Malani) and an IAB setting for the Bastar craton. The Deccan flood basalt province of India is used to warn against an indiscriminate use of those discrimination diagrams that do not explicitly include the likely setting of the area under evaluation. An Excel template is also provided for an easy

  12. Diachronous folding and cleavage in an intraplate setting (Central High Atlas, Morocco) determined through the study of remagnetizations

    Calvín, P.; Casas-Sainz, A. M.; Villalaín, J. J.; Moussaid, B.


    Remagnetizations are common in intraplate basins. When remagnetizations occur at an intermediate stage between different tectonic processes, they can be used for paleo-geometrical reconstructions and relative dating of different structures. This has a particular interest in geological frameworks where other geological time markers are absent. In order to apply this methodology, it is necessary to calculate the regional remagnetization direction and subsequently to use this reference direction to restore the attitude of the beds at the moment of remagnetization acquisition. In this work, we use this methodology for dating a pervasive cleavage (whose time of formation is controversial) and the associated structures in the Central High Atlas (Morocco). Paleomagnetic directions from 64 sites were used to calculate the remagnetization direction (D = 330.9°, I = 35.1°, A/n = 6.107) which is coincident with the Albian-Cenomanian (ca. 100 M.a.) expected direction for NW Africa. This direction was used to restore the Mesozoic paleo-geometry of beds allowing us to analyze bedding orientation, cleavage and folding relationships between the present day and the Cretaceous geometry. After restoration we conclude that the development of cleavage post-dates remagnetization, being in relation with Cenozoic basin inversion. However, the paleo-geometry shows incipient folds at Cretaceous times, which can be related to an intra-Mesozoic compressional event.

  13. /sup 87/Sr//sup 86/Sr ratios and their source materials of the volcanic rocks from the Pliocene Misasa Group in San-in district

    Nagao, T.; Nishikawa, J. (Hokkaido Univ., Sapporo (Japan). Faculty of Science)


    /sup 87/Sr//sup 86/Sr ratios of the volcanic rocks from the Pliocene Misasa Group and the andesite from the Quaternary Daisen volcano in San'in district were determined. The alkaline basalt and the calc-alkaline andesite from the Misasa Group have a restricted range of /sup 87/Sr//sup 86/Sr ratios (basalt 0.7051 -- 0.7052; andesite 0.7052) and Rb/Sr ratios (basalt 0.03 -- 0.04; andesite 0.04) with low Rb (basalt 19.4 -- 34.5 ppm; andesite 29.2 ppm) and Sr (basalt 631 -- 851 ppm; andesite 729 ppm) contents. These results may indicate that the calc-alkaline andesite may not have been derived from the alkaline basaltic magma by fractional crystallization or assimilation of older sialic materials. An analysis of the Daisen andesite yields a /sup 87/Sr//sup 86/Sr ratio 0f 0.7050, which is almost identical with that of the Misasa andesite. In /sup 87/Sr//sup 86/Sr ratios, the Misasa alkaline basalt, calc-alkaline andesite and the Daisen calc-alkaline andesite are very similar to the Oki-Dogo alkaline basalt (0.7048) which carries ultramafic inclusions and is possibly of the upper mantle origin (Kaneoka et al., 1978). This similarity in Sr isotopic ratios suggests that both the Misasa and Daisen volcanic rocks may have been derived from the upper mantle similar to the source material of the Oki-Dogo alkaline basalt.

  14. 40Ar-39Ar dating and tectonic implications of volcanic rocks recovered at IODP Hole U1342A and D on Bowers Ridge, Bering Sea

    Sato, Keiko; Kawabata, Hiroshi; W. Scholl, David; Hyodo, Hironobu; Takahashi, Kozo; Suzuki, Katsuhiko; Kumagai, Hidenori


    During the Integrated Ocean Drilling Program (IODP), a total of 41.54 m of basement rock, consolidated volcaniclastic sediment, was recovered beneath a thin sediment cover. The drilled site is at the eastern end of the crestal area of Bowers Ridge, a north and westward sweeping offshoot of the Aleutian Arc into the Bering Sea. The volcanic sequence recovered from Holes U1342A and U1342D was divided into six major lithologic units. We used the single grain 40Ar-39Ar dating method performed by step-wise heated laser fusion technique to date andesites of Unit 1. Thereby two ages of Oligocene volcanism (34-32 Ma, 28-26 Ma) were distinguished each other according to our 40Ar-39Ar data. These ages refute a hypothesized Cretaceous origin in the North Pacific as an exotic arc massif or sector of the Hawaiian-Emperor chain and indicate that the Bowers Ridge is a Bering-Sea formed arc or remnant arc that ceased forming in the latest Oligocene to the earliest Miocene time.

  15. Evolution of fluid-rock interactions: fluid inclusion, isotopic, and major/minor element chemistry of hydrothermally altered volcanic rock in core RN-17B, Reykjanes, Iceland

    Fowler, A. P.; Zierenberg, R. A.; Schiffman, P.; Marks, N. E.; Fridleifsson, G. O.


    The Reykjanes Peninsula, Iceland, hosts a seawater-dominated geothermal system. Previous studies indicate an evolution of the system from meteoric to seawater. The inclined 4-inch diameter RN-17B drill core was collected from 2798.5 m to 2808.5 m (~2555 m below surface) at in situ temperature of approximately 330°C. Samples for this study were obtained from the Iceland Deep Drilling Project (IDDP). The core contains hydrothermally altered rocks of basaltic composition. Hydrothermal alteration ranges from upper greenschist to lower amphibolite grade, dependent on protolith size and composition. Veins in the core grade inward from radial epidote + acicular hornblende + titanite + pyrite, to clearer equant and compositionally zoned epidote vein centers. Felted amphibole replaces hyaloclastite and smaller crystalline clasts within the core, but is absent from the centers of crystalline pillow basalt fragments. Amphibole in vein selvages and vesicle fillings is green and acicular. Electron microprobe analyses of amphibole indicate it spans a compositional range of ferrohornblende through paragasite. The pistacite component (Xps) of vein epidote ranges from 16.5 to 36.7. The Xps component shows both normal and reverse zoning within single epidote crystals across this range, and follows no distinct pattern. Vein epidote adjacent to the wall rock has a higher aluminum concentration than vein centers. This may be due to mobilization of aluminum from plagioclase in the wall rock during albitization. Solutions flowing through open fractures may have lower Al-content and thus precipitate more Fe-rich epidote than those next to the fracture walls. Primary fluid inclusions in epidote range in size from <1 to 10 μm in diameter. Secondary fluid inclusions are <1 μm in diameter and not measurable. Calculated fluid inclusion salinities range from 0.5 to 7.6 weight percent NaCl, with lower salinities adjacent to the wall rock and higher salinities in the vein centers

  16. Geochemical Evidence for the Formation of Soils by Interaction Between Guano and Volcanic Rocks, Rata Island, Fernando de Noronha (Pernambuco

    Sonia Maria Barros de Oliveira


    Full Text Available The phosphatic soils found in the northern part of the Rata island, in the Fernando de Noronha Archipelago, wereformed by reaction of bird guano with weathered mafi c rocks of the Quixaba Formation. Phosphate minerals identifi edinclude crandallite as a major constituent. The unique guano signature preserved in soil is characterized by high levelsof Cu, Pb, Zn, As, U, and Sr. On the other hand, the inheritance of the geochemical signature of the nepheline-basalts isdemonstrated by the anomalous concentrations of Ba, Nb, Ta, Cr, Hf, V and Zr in soils, and by the remarkable similaritybetween REE patterns in rock and soils.

  17. Geochemistry and petrogenesis of anorogenic basic volcanic-plutonic rocks of the Kundal area, Malani Igneous Suite, western Rajasthan, India

    Krishnakanta Singh, A.; Vallinayagam, G.


    The Kundal area of Malani Igneous Suite consists of volcano-plutonic rocks. Basalt flows and gabbro intrusives are associated with rhyolite. Both the basic rocks consist of similar mineralogy of plagioclase, clinopyroxene as essential and Fe-Ti oxides as accessories. Basalt displays sub-ophitic and glomeroporphyritic textures whereas gabbro exhibits sub-ophitic, porphyritic and intergrannular textures. They show comparable chemistry and are enriched in Fe, Ti and incompatible elements as compared to MORB/CFB. Samples are enriched in LREE and slightly depleted HREE patterns with least significant positive Eu anomalies. Petrographical study and petrogenetic modeling of [Mg]-[Fe], trace and REE suggest cogenetic origin of these basic rocks and they probably derived from Fe-enriched source with higher Fe/Mg ratio than primitive mantle source. Thus, it is concluded that the basic volcano-plutonic rocks of Kundal area are the result of a low to moderate degree (conformity with the similar setting for Malani Igneous Suite.

  18. Database for the geologic map of upper Eocene to Holocene volcanic and related rocks in the Cascade Range, Washington

    Barron, Andrew D.; Ramsey, David W.; Smith, James G.


    This geospatial database for a geologic map of the Cascades Range in Washington state is one of a series of maps that shows Cascade Range geology by fitting published and unpublished mapping into a province-wide scheme of lithostratigraphic units. Geologic maps of the Eocene to Holocene Cascade Range in California and Oregon complete the series, providing a comprehensive geologic map of the entire Cascade Range that incorporates modern field studies and that has a unified and internally consistent explanantion. The complete series will be useful for regional studies of volcanic hazards, volcanology, and tectonics.

  19. Late Miocene remagnetization within the internal sector of the Northern Apennines, Italy

    Aiello, I.W.; Hagstrum, J.T.; Principi, G.


    Paleomagnetic and geologic evidence indicates that Upper Jurassic radiolarian cherts of both the Tuscan Cherts Formation (continental margin, Tuscan Units) and the Monte Alpe Cherts Formation (oceanic crust, Ligurian Units) were remagnetized during Miocene orogenesis of the Northern Apennines of Italy. Characteristic overprint magnetizations with reversed polarities have been found over a large area within the internal sector of the Northern Apennines, including eastern Liguria, Elba Island and the Thyrrenian margin, and west of the Middle Tuscan Ridge. The reversed-polarity overprint (average direction: D=177??, I=-52??, ??95=15??) was most likely acquired during Late Miocene uplift and denudation of the orogenic chain, and thermochemical remagnetization was a probable consequence of increased circulation of orogenic fluids. Similarly, mostly reversed-polarity directions of magnetization have been found by other workers in overlying post-orogenic Messinian sediments (D=177??, I=-57??, ??95=3??), which show little counterclockwise (CCW) vertical-axis rotation with respect to stable Europe (-8??5??). The Monte Alpe Cherts sampled at sites in the external sector of the Northern Apennines, close to major tectonic features, have normal- polarity overprint directions with in situ W-SW declinations. Since the overlying post-orogenic Messinian sediments have not been substantially rotated about vertical axes, the evidence points to an earlier,pre-Late Miocene remagnetization in the external parts of the orogenic chain. ?? 2004 Elsevier B.V. All rights reserved.

  20. [Combination of phosphorus solubilizing and mobilizing fungi with phosphate rocks and volcanic materials to promote plant growth of lettuce (Lactuca sativa L.)].

    Velázquez, María S; Cabello, Marta N; Elíades, Lorena A; Russo, María L; Allegrucci, Natalia; Schalamuk, Santiago


    Arbuscular mycorrhizal fungi (AMF) increase the uptake of soluble phosphates, while phosphorus solubilizing fungi (S) promote solubilization of insoluble phosphates complexes, favoring plant nutrition. Another alternative to maintaining crop productivity is to combine minerals and rocks that provide nutrients and other desirable properties. The aim of this work was to combine AMF and S with pyroclastic materials (ashes and pumices) from Puyehue volcano and phosphate rocks (PR) from Rio Chico Group (Chubut) - to formulate a substrate for the production of potted Lactuca sativa. A mixture of Terrafertil®:ashes was used as substrate. Penicillium thomii was the solubilizing fungus and Rhizophagus intraradices spores (AMF) was the P mobilizer (AEGIS® Irriga). The treatments were: 1) Substrate; 2) Substrate+AMF; 3) Substrate+S; 4) Substrate+AMF+S; 5) Substrate: PR; 6) Substrate: PR+AMF; 7) Substrate: PR+S and 8) Substrate: PR+AMF+S. Three replicates were performed per treatment. All parameters evaluated (total and assimilable P content in substrate, P in plant tissue and plant dry biomass) were significantly higher in plants grown in substrate containing PR and inoculas with S and AMF. This work confirms that the combination of S/AMF with Puyehue volcanic ashes, PR from the Río Chico Group and a commercial substrate promote the growth of L. sativa, thus increasing the added value of national geomaterials. Copyright © 2017 Asociación Argentina de Microbiología. Publicado por Elsevier España, S.L.U. All rights reserved.


    张国全; 王勤生; 俞跃平; 杨超; 肖娟


    通过对浙东地区地层特征与划分、同位素年龄等综合分析,结合近年来的新资料,认为该区前寒武纪地层由太古宙地壳物质再循环重熔结晶形成;芝溪头变质杂岩原岩可能由古元古界八都(岩)群与石炭系-二叠系沉积地层组成,二者共同于晚二叠世-中三叠世发生变质作用;中生代大规模火山岩浆活动主要发生在早白垩世晚期:磨石山群形成于早白垩世晚期,以大规模喷发酸性火山岩为特征;永康群、天台群中的众多地层也主要形成于早白垩世晚期,是不同火山构造盆地(洼地)喷发-沉积的同期异相堆积物或同期同物异名地层,以双峰式火山岩、“红层”为特征。%A series of comprehensive studies, including stratigraphic investigation and isotopic dating, have been completed to constrain the age and origin of volcanic rocks in eastern Zhejiang. Our data indicate that Precambrian strata in this region were derived from Archean crust which had experienced recycling and remelting. The protolith of the Zhixitou metamorphic complex is probably composed of the Paleoproterozoic Badu Group and Carboniferous-Permian sedimentary strata, both of which had suffered metamorphism during the P3-T2. Large-scale Mesozoic volcanic-magmatic activities mostly took place in K1^2 because there are few J1 or K1^1- K1^2 strata and J2-L strata are absent. Most of the Moshishan Group was formed in K1^3, during the large-scale eruption of acidic magmas. Most strata of the Yongkang and Tiantai Groups were formed in K1^3, too. They represent different facies of volcanic eruption and sedimentation, and they are characterized by bimodal volcanism and red sediments.

  2. The pre-Kos Plateau Tuff Volcanic Rocks on Kefalos Peninsula (Kos Island, Dodecanese, Greece): Crescendo to the Largest Eruption of the Modern Aegean Arc

    Bachmann, O.; Schnyder, C.


    Young volcanic rocks (K-Ar ages of 3 to 0.5 Ma) from the Kefalos Peninsula (Kos Island, Dodecanese, Greece) erupted prior to the voluminous (>60 km\\ 3) Kos Plateau Tuff (KPT; Ar-Ar age of 0.16 Ma) were studied in order to better define the conditions that led to the paroxysmal eruption of the modern Aegean Arc. Two different whole-rock compositions were sampled on Kefalos peninsula; dacites (63-65 wt% SiO2) and rhyolites (75-77 wt% SiO2). Kefalos dacites are crystal-rich (>40% crystals), show high Sr-Ba contents compared to other continental arcs, and have "adakitic" Sr/Y ratios (>40). Kefalos rhyolites are typical high- SiO2 arc magmas, similar in composition and mineralogy to the KPT, but displaying lower crystallinities (30% in most of the KPT). The high Sr/Y ratios of the dacites is surprising in an area where the subducting slab is not particularly hot and the continental crust relatively thin (~30 km). If the low Y and high Sr-Ba contents result from the fact that magma formed deep enough to supress plagioclase and have garnet present, dacite magma generation must have occurred in the mantle. There is geochemical and mineralogical evidence for the Kefalos and KPT rhyolites being generated by fractional crystallization from magmas similar to the Kefalos dacites. However, the few distinctions between KPT and Kefalos rhyolites (KPT is more voluminous, contains more crystals, has lower whole-rock U and Th contents, and lower MgO-SiO2, but higher Al2O3-FeOtot in biotite) suggest slightly different conditions in the magma chambers. These observations, together with increasing explosivity of the volcanic products from ~3 Ma to 0.16 Ma, may indicate that the build-up to the large KPT eruption could be the result of an increase in magmatic water input in the system through time.

  3. Trace elements and REE fractionation in subsoils developed on sedimentary and volcanic rocks: case study of the Mt. Vulture area, southern Italy

    Mongelli, Giovanni; Paternoster, Michele; Rizzo, Giovanna; Sinisi, Rosa


    There is an increasing interest in the distribution of rare earth elements (REEs) within soils, primarily as these elements can be used to identify pedogenetic processes and because soils may be future sources for REE extraction, despite much attention should be paid to the protection and preservation of present soils. Here, we evaluate the processes that control the distribution of REEs in subsoil horizons developed over differing lithologies in an area of low anthropogenic contamination, allowing estimates of the importance of source rocks and weathering. Specifically, this study presents new data on the distribution of REEs and other trace elements, including transition and high-field-strength elements, in subsoils developed on both Quaternary silica-undersaturated volcanic rocks and Pliocene siliciclastic sedimentary rocks within the Mt. Vulture area of the southern Apennines in Italy. The subsoils in the Mt. Vulture area formed during moderate weathering (as classified using the chemical index of alteration) and contain an assemblage of secondary minerals that is dominated by trioctahedral illite with minor vermiculite. The REEs, high-field-strength elements, and transition metals have higher abundances in subsoils that developed from volcanic rocks, and pedogenesis caused the Mt. Vulture subsoils to have REE concentrations that are an order of magnitude higher than typical values for the upper continental crust. This result indicates that the distribution of REEs in soils is a valuable tool for mineral exploration. A statistical analysis of inter-elemental relationships indicates that REEs are concentrated in clay-rich fractions that also contain significant amounts of low-solubility elements such as Zr and Th, regardless of the parent rock. This suggests that low-solubility refractory minerals, such as zircon, play a significant role in controlling the distribution of REEs in soils. The values of (La/Yb)N and (Gd/Yb)N fractionation indices are dependent on

  4. The Diagenesis of Volcanic Rocks and Its Effects on the Reservior Quality in Xujiaweizi Fault Depression, Soliao Basin%徐家围子断陷火山岩成岩作用与储层质量

    胡文婷; 柳成志; 赵辉; 栾慷; 周振东; 姚洪鹏


    The Xjiaweizi fault depression is the largest volcanic reserviors ever discovered in China which is located in the northern Songliao Basin.Study of the petrological characteristics, pore types and the diagenesis of volcanic rocks, shows that volcanic rocks types can be divided into two kinds,including lavas and pyroclastic rocks, the major volcanic reservior rocks are rhyolite and ignimbrite.The reservior spaces include primary pore, primary facture,secondary pore and secondary fracture.While the favorable volcanic rock reserviors in this area are mainly composed of initial bubbles, micropores resulted from devitrification and secondary dissolution pores.The diagenesis of volcanic rocks can be devided into three stages which include syndiagenetic phase, hypergenesis phase and burialdiagenesis phase.There are 12 volcanic diagenesis types, and the reservior guality are greatly effected by the diagenesis events including packing action,devitrify and dissolution.%徐家围子断陷位于松辽盆地北部古中央隆起带东侧,断陷期火山活动强烈,营城组火山岩是本区深层天然气重要的储集空间.对徐家围子断陷营城组火山岩的岩石学特征、储集空间类型以及火山岩的成岩作用研究表明,本区火山岩岩石类型有熔岩和火山碎屑岩两大类,以流纹岩、熔结凝灰岩为主.孔隙类型有4种,不同的岩石类型具有不同的孔隙类型组合,最主要的孔隙类型为原生气孔、脱玻化孔和次生溶孔.火山岩的成岩演化可以分为三个阶段,目前本区火山岩主要出于埋藏成岩阶段II期.本区火山岩一共有12种成岩作用类型,不同成岩阶段的成岩作用类型不相同,对储层质量影响最重要的成岩作用是脱玻化作用、充填作用和溶解作用.

  5. Pan-African adakitic rocks of the north Arabian-Nubian Shield: petrological and geochemical constraints on the evolution of the Dokhan volcanics in the north Eastern Desert of Egypt

    Obeid, Mohamed A.; Azer, Mokhles K.


    The Precambrian basement of Egypt is part of the Red Sea Mountains and represents the north-western part of the Arabian-Nubian Shield (ANS). Five volcanic sections are exposed in the Egyptian basement complex, namely El Kharaza, Monqul, Abu Had, Mellaha and Abu Marwa. They are located in the north Eastern Desert (ED) of Egypt and were selected for petrological and geochemical studies as they represent the Dokhan volcanics. The volcanics divide into two main pulses, and each pulse was frequently accompanied by deposition of immature molasse type sediments, which represent a thick sequence of the Hammamat group in the north ED. Compositionally, the rocks form a continuum from basaltic andesite, andesite, dacite (lower succession) to rhyodacite and rhyolite (upper succession), with no apparent compositional gaps. These high-K calc-alkaline rocks have strong affinities to subduction-related rocks with enriched LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and negative Nb anomalies relative to NMORB. The lower succession displays geochemical characteristics of adakitic rocks with SiO2 >53 wt%, Al2O3 >15 wt%, MgO >2.5 wt%, Mg# >49, Sr >650 ppm, Y 25 ppm, Cr >50 ppm and Sr/Y >42.4. They also have low Nb, Rb and Zr compared to the coexisting calc-alkaline rhyodacites and rhyolites. The highly fractionated rhyolitic rocks have strong negative Eu anomalies and possess the geochemical characteristics of A-type suites. Trace element geochemical signatures indicate a magma source consistent with post-collisional suites that retain destructive plate signatures associated with subduction zones. The adakitic rocks in the northern ANS are generated through partial melting of delaminated mafic lower crust interacting with overlying mantle-derived magma. The Dokhan volcanics were likely generated by a combination of processes, including partial melting, crystal fractionation and assimilation.

  6. 用作混凝土掺合料的火山岩的组成与火山灰活性%Constitution and Pozzolanic Activity of Volcanic Rocks Used as Mineral Admixture in Concrete

    喻乐华; 周双喜; 欧辉; 邓文武


    采集江西火山岩区代表性硅质火山岩样品,用化学分析、岩相学和X射线衍射(XRD)方法研究其化学组成、矿物类型和结构构造,采取火山岩石粉水泥砂浆强度比测定其火山灰活性指数,通过统计方法讨论并揭示硅质火山岩主要性能与火山灰活性的关系。结果表明:火山岩样品(除个别外)28 d火山灰活性指数≥65%,可作为水泥砂浆和混凝土用天然火山灰质材料;珍珠岩火山灰活性最高,这是其玻璃质比例甚高和富硅铝质高温型矿物所致;角砾状熔结凝灰岩含较多大砾状岩屑和低温型矿物致使火山灰活性较低。火山岩岩相学分析的基质含量与XRD分析的非晶态相含量在岩石性能及对火山灰活性影响方面一致。火山岩活性指数与非晶态含量、化学成分SiO2含量或化学成分SiO2+Al2O3含量的复相关性表现良好,可应用活性指数与这些岩石主要参数的拟合方程预测火山岩的火山灰活性指数,从而判断其是否具备用作混凝土掺合料的潜质。%Representative specimens of siliceous volcanic rocks were firstly collected from main volcanic areas in Jiangxi Province, and then their chemical composition, mineral type, structure, texture and vitreous proportion were analyzed by chemical composition analysis, petrographic analysis and X-ray diffraction. The pozzolanic activity index for these volcanic rocks was determined via com-pression strength ratio analysis of cement mortar and cement mixing mortar with powder of volcanic rock. The relations between rock performance and pozzolanic activity were further discussed based on the statistical significance. The results indicate that the volcanic rocks possess a pozzolanic activity index of ≥65%so to satisfy a demand of natural pozzolanic materials used as cement mortar and concrete according to the relevant standards. Perlite has the highest pozzolanic activity among these volcanic rocks

  7. Rock magnetic studies on sediments from Erlongwan maar lake, Long Gang Volcanic Field, Jilin province, NE China

    Frank, Ute


    Detailed rock magnetic investigations were carried out on two 23-m-long sediment cores from Erlongwan maar lake, NE China. The completely laminated sediment sequence of the lake is interrupted by 410 graded layers with thicknesses between 0.1 and 150 cm. Magnetite of PSD-size was identified as the main magnetic carrier mineral by temperature-dependent measurements of the saturation magnetization and determination of hysteresis parameters. The minerogenic components in the laminated sediments and the graded layers are nearly identical, and their rock magnetic characteristics reflect the prevailing conditions, anoxic or oxic, during deposition. The most reliable criteria for estimating the availability of oxygen is whether an increase in minerogenic influx is linked to a shift in the magnetic grain size spectrum to coarser (oxic) or finer (anoxic) grains. Comparison of different rock magnetic parameters indicative for magnetic grain size and coercitivity revealed, that the S-ratio which is known to reflect the presence of high coercive minerals, is grain size indicative in sediments with a monomineralic magnetic composition.

  8. Magnetic fabrics and rock magnetism of the Xiong'er volcanic rocks and their implications for tectonic correlation of the North China Craton with other crustal blocks in the Nuna/Columbia supercontinent

    Xu, Huiru; Yang, Zhenyu; Peng, Peng; Ge, Kunpeng; Jin, Zhenmin; Zhu, Rixiang


    The tectonic background of the Paleoproterozoic Xiong'er volcanic rocks (XVR) is important for understanding the tectonic evolution of the North China Craton (NCC), as well as its paleogeographic position during the assembly of the Nuna/Columbia supercontinent. Here we report the results of the first anisotropy of magnetic susceptibility (AMS) study of the XVR, and use the interpreted flow directions to constrain the emplacement mechanism and to assess its geological significance for the reconstruction of the Nuna/Columbia supercontinent. Thirty lavas were sampled from three sections in western Henan Province. Detailed rock magnetic analyses, including measurements of hysteresis loops, magnetization versus temperature curves and first order reverse curves, were performed to identify the main magnetic phases and grain sizes. The inferred directions from the AMS results reveal a radial flow pattern with an eruption center probably located near Xiong'er Mountain. Our data suggest that the XVR may have been emplaced in a triple-conjugated continental rift on the south margin of the NCC, probably initiated from a paleoplume. Based on this interpretation, a comparison of geological and paleomagnetic results among the proposed crustal blocks in the Nuna/Columbia supercontinent suggests a close linkage of the NCC with São Francisco-Congo, Rio de la Plate and Siberia.

  9. Remagnetization of Lower Carboniferous Carbonates, Northeastern Ireland: Preliminary Paleomagnetic Results

    Pannalal, S. J.; Symons, D. T.; Stanley, G.; Sangster, D. F.


    Conodont color alteration index (CAI) values provide indirect paleotemperature estimates for rocks that have been subjected to a thermal event either by burial metamorphism or by hot hydrothermal fluids. Extensive CAI studies have been conducted on the Lower Carboniferous carbonates of Ireland that are host to the major sedimentary lead and zinc deposits in the Irish ore field. The CAI values show that these rocks have been affected by a regional thermal event with a decreasing trend from south (CAI=7.0) to north Ireland (CAI=1). In addition, the relative timing of the regional metamorphic event and ore mineralization in the Irish ore field, critical to the much debated genetic theories for ore genesis, has remained elusive. A paleomagnetic study, that includes thermal and alternating-field demagnetization and isothermal remanence procedures, has been conducted on 235 specimens representing 18 sites from Lower Carboniferous carbonates (CAI age than the A component and may indicate a secondary magnetization, likely the result of weathering of the carbonates by oxidizing fluids. In addition, this regional hydrothermal event may also be related to the observed CAI values in Lower Carboniferous carbonates of northeastern Ireland.

  10. Attempts of whole-rock K/Ar dating of mesozoic volcanic and hypabissal igneous rocks from the Central Subbetic (Southern Spain: A case of differential Argon loss related to very low-grade metamorphism

    Sanz de Galdeano, C.


    Full Text Available 12 samples of basic intrusives within Triassic rocks «ophites» and 11 samples of volcanic and associated intrusives within Jurassic to Early Cretaceous sequences of the Subbetic Zone were subjected to whole-rock K/Ar dating in combination with chemical/petrological analysis. Satisfactory results were obtained only from a number of samples of volcanic rocks, however, analytical ages commonly agree, within about 10 relative percent, with those deduced from stratigraphic location. «Ophite» samples, on the other hand, may reveal considerably lower analytic ages than the volcanics and show much stronger scattering, even among samples collected within a small area. It is argued that the inferred loss of Ar results from very-low-grade alpine metamorphic alteration, which affected the «ophites» more intensely than the higher volcanic rocks. Other post-emplacement chemical changes, such as the degree of secondary oxidation of Fe, are also distintive among the two groups of samples, and are to some extent consistent with the above view in that the alteration environment of the ophites should have produced conditions for more penetrative fluid-rock interactions and homogeneous recrystallization. Overall, the magmatic activity from which the ophitic rocks originated might have started in the Late Triassic and continued in the Lower Jurassic. 80th, the «ophites» and the volcanics are though to be the result of magmatic events Collowing tensional to transtensive crustal movements affecting the external basins of the Betic Cordilleras Crom Late Triassic to Early Cretaceous times.Doce muestras de cuerpos básicos intrusivos en rocas triásicas («ofitas» y 11 muestras de volcanitas y rocas intrusivas asociadas en secuencias jurásico-cretáceas de la zona Subbética han sido objeto de datación radiométrica K/Ar (roca total en combinación con análisis químico-petrográfico. Las edades analíticas obtenidas son 's

  11. Silicic volcanism on Mars evidenced by tridymite in high-SiO2 sedimentary rock at Gale crater

    Morris, Richard V.; Vaniman, David T.; Blake, David F.; Gellert, Ralf; Chipera, Steve J.; Rampe, Elizabeth B.; Ming, Douglas W.; Morrison, Shaunna M.; Downs, Robert T.; Treiman, Allan H.; Yen, Albert S.; Grotzinger, John P.; Achilles, Cherie N.; Bristow, Thomas F.; Crisp, Joy A.; Des Marais, David J.; Farmer, Jack D.; Fendrich, Kim V.; Frydenvang, Jens; Graff, Trevor G.; Morookian, John-Michael; Stolper, Edward M.; Schwenzer, Susanne P.


    Tridymite, a low-pressure, high-temperature (>870 °C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ˜40 wt.% crystalline and ˜60 wt.% X-ray amorphous material and a bulk composition with ˜74 wt.% SiO2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (˜17 wt.% of bulk sample), tridymite (˜14 wt.%), sanidine (˜3 wt.%), cation-deficient magnetite (˜3 wt.%), cristobalite (˜2 wt.%), and anhydrite (˜1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (˜39 wt.% opal-A and/or high-SiO2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides-perchlorates-chlorates), and has minor TiO2 and Fe2O3T oxides (˜5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a “Lake Gale” catchment environment can account for Buckskin's tridymite, cristobalite, feldspar, and any residual high-SiO2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.

  12. Silicic volcanism on Mars evidenced by tridymite in high-SiO2 sedimentary rock at Gale crater.

    Morris, Richard V; Vaniman, David T; Blake, David F; Gellert, Ralf; Chipera, Steve J; Rampe, Elizabeth B; Ming, Douglas W; Morrison, Shaunna M; Downs, Robert T; Treiman, Allan H; Yen, Albert S; Grotzinger, John P; Achilles, Cherie N; Bristow, Thomas F; Crisp, Joy A; Des Marais, David J; Farmer, Jack D; Fendrich, Kim V; Frydenvang, Jens; Graff, Trevor G; Morookian, John-Michael; Stolper, Edward M; Schwenzer, Susanne P


    Tridymite, a low-pressure, high-temperature (>870 °C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.% X-ray amorphous material and a bulk composition with ∼74 wt.% SiO2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (∼17 wt.% of bulk sample), tridymite (∼14 wt.%), sanidine (∼3 wt.%), cation-deficient magnetite (∼3 wt.%), cristobalite (∼2 wt.%), and anhydrite (∼1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (∼39 wt.% opal-A and/or high-SiO2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides-perchlorates-chlorates), and has minor TiO2 and Fe2O3T oxides (∼5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a "Lake Gale" catchment environment can account for Buckskin's tridymite, cristobalite, feldspar, and any residual high-SiO2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.

  13. Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing

    Chang, J.M.; Feeley, T.C.; Deraps, M.R.


    The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of volcanic rocks from St. Paul (0??54-0??003 Ma) and St. George (2??8-1??4 Ma) Islands, the two largest Pribilof Islands. Rocks from St. George can be divided into three groups: group 1 is a high-MgO, low-SiO. 2 suite composed primarily of basanites; group 2 is a high-MgO, high-SiO 2 suite consisting predominantly of alkali basalts; group 3 is an intermediate- to low-MgO suite that includes plagioclase-phyric subalkali basalts and hawaiites. Major and trace element geochemistry suggests that groups 1 and 2 formed by small-degree partial melting of amphibole-bearing to amphibole-free garnet peridotite. Group 1 rocks were the earliest melts produced from the most hydrous parts of the mantle, as they show the strongest geochemical signature of amphibole in their source. The suite of rocks from St. Paul ranges from 14??4 to 4??2 wt % MgO at relatively constant SiO 2 contents (43??1-47??3 wt %). The most primitive St. Paul rocks are modeled as mixtures between magmas with compositions similar to groups 1 and 2 from St. George Island, which subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved rocks. Plagioclase-phyric group 3 rocks from St. George are modeled as mixtures between an evolved melt similar to the evolved magmas on St. Paul and a fractionated group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts are ???1400??C and are similar to those of mid-ocean ridge basalts (MORB). Similarly, 87Sr/. 86Sr and 143Nd/. 144Nd values for all rocks are MORB-like, in the range of 0??702704-0??703035 and 0??513026-0??513109, respectively. 208Pb/. 204Pb vs 206Pb/. 204Pb values lie near the MORB end-member but show a linear trend towards HIMU (high time-integrated 238U/. 204Pb). Despite isotopic similarities to MORB, many of the major and

  14. Chlorine isotope composition of volcanic rocks and gases at Stromboli volcano (Aeolian Islands, Italy): Inferences on magmatic degassing prior to 2014 eruption

    Liotta, Marcello; Rizzo, Andrea L.; Barnes, Jaime D.; D'Auria, Luca; Martelli, Mauro; Bobrowski, Nicole; Wittmer, Julian


    Among the magmatic volatiles, chlorine (Cl) is degassed at shallow depths offering the opportunity to investigate the behavior of magmatic degassing close to the surface, and the possible occurrence of chemical and isotopic fractionation related to gas/melt partitioning. However, it is still unclear if the isotopic composition of Cl (δ37Cl) can be used as a proxy of magmatic degassing. In this work, we investigate the concentrations of chlorine and sulfur, and the Cl isotope composition of rocks and plume gases collected at Stromboli volcano, Aeolian Islands, Italy. This volcano was chosen because it is characterized by persistent eruptive activity (i.e., Strombolian explosions) and by the presence of magma at very shallow levels in the conduits. Rocks belonging to the different magmatic series erupted throughout the formation of the volcano have δ37Cl values ranging between - 1.0 and + 0.7‰. The isotopic composition seems independent of the Cl concentration of the rocks, but shows a negative correlation with SiO2 content. Plume gases have a greater isotopic compositional variability than the rocks (- 2.2‰ ≤ δ37Cl ≤ + 1.5‰) and the composition seems related to the level of volcanic activity at Stromboli. Gases collected in 2011-2013 during days of ordinary eruptive activity are characterized by δ37Cl values ranging from + 0.3 to + 1.5‰ and S/Cl molar ratios between 1.4 and 2.2, similar to previous S/Cl measurements performed at Stromboli with other techniques. Plume gases collected in July 2014, in days of high-level eruptive activity preceding the onset of the 2014 effusive eruption, have negative δ37Cl values (- 2.2‰ ≤ δ37Cl ≤ - 0.1‰) and S/Cl between 0.9 and 1.2, which are among the lowest S/Cl values measured at this volcano. The amplitude of the volcanic tremor and the variation in the inclination of very long period (VLP) seismic signal polarization clearly indicate that in July 2014 the intensity and frequency of Strombolian

  15. Geochemical and zircon isotopic evidence for extensive high level crustal contamination in Miocene to mid-Pleistocene intra-plate volcanic rocks from the Tengchong field, western Yunnan, China

    Li, Linlin; Shi, Yuruo; Williams, Ian S.; Anderson, J. Lawford; Wu, Zhonghai; Wang, Shubing


    SHRIMP zircon Pb/U dating of Cenozoic volcanic rocks in the Tengchong area, western Yunnan Province, China, shows that the dacite and andesitic breccia lavas from Qushi village were intruded at 480 ± 10 ka and 800 ± 40 ka, respectively. Moreover, Pb/U dating of trachyandesite from Tuantian village and olivine basalt from Wuhe village give weighted mean 206Pb/238U ages of 2.82 ± 0.08 Ma and 12.28 ± 0.30 Ma. Corrections for initial 230Th disequilibrium of zircon were used for the former two younger ages. The Tengchong volcanic rocks have a large range of SiO2 (48.6-66.9 wt.%) and mostly belong to a high-K calc-alkaline series. The lavas originated from heterogeneous sources and were modified by subsequent fractional crystallization. The REE and other trace element patterns of the Tengchong volcanic rocks resemble magmas having a large component of continental crust. All have similar degrees of LREE and HREE fractionation and are enriched in LILE, La, Ce and Pb, with depletions in Nb, Ta, Ti, Sr and P relative to primitive mantle. Zircon δ18O values of 6.96 ± 0.17 and 7.01 ± 0.24‰ and highly varied negative εHf(t) values of - 1.5 to - 11.0 and - 10.3 to - 13.7, as well as the presence of inherited zircon grains in the studied samples, indicate that the magmas contain crustal material on a large scale. The Tengchong volcanic rocks have HFSE ratios (e.g., Nb/Ta, La/Nb, Zr/Y) similar to continental flood basalts, indicative of an intra-plate extensional tectonic setting. Widespread distributed faults might have facilitated upwelling of mantle-derived melts and eruptions from shallow crustal magma chambers to form the large volcanic field.

  16. 松辽盆地北部徐家围子断陷火山岩储层成因机制%Volcanic rock reservoirs mechanism in North Songliao Basin Xujiaweizi Fault Depression

    韩刚; 张文婧; 任延广; 黄清华; 梁江平; 包丽


    松辽盆地北部徐家围子断陷徐深气田是我国迄今为止发现的最大的火山岩气田.营城组火山岩具有分布广、厚度大、岩性复杂等特点,既有酸性的流纹岩、流纹质熔结凝灰岩,也有中性的安山岩、安山质集块岩和基性的玄武岩.研究表明,酸性流纹岩、流纹质熔结凝灰岩普遍发育较好的原生气孔,而火山角砾岩、火山集块岩一般发育火山岩原生裂缝,这些为优质火山岩储层形成奠定了物质基础,成岩作用过程中深部流体活动和选择性溶蚀作用,不仅增大了火山岩的有效储集空间,同时也改善了火山岩储集层空间结构,而构造运动所形成的构造裂缝,不仅为深部流体活动和油气运移提供了主要通道,而且有效地增大了火山岩的油气储集空间.%The Xushen gas field locates in Xujiaweizi Fault Depression of North Songliao Basin. It is so far the greatest volcanic rock gas field in China. The Xushen gas field develops typical Yingcheng Formation volcanic rocks. The Yingcheng Formation volcanic rocks have such characteristic as extensive distribution range, great thickness, and complex lithology. By using methods such as core and scanning electron microscope observation, body of casting slice, analysis and assay, we found the lithology of Yingcheng Formation volcanic rocks consist of not only acidic rhyolite, rhyolite-cementing tuff, but also neutral andesite, andesite-agglomerate and basic basalt. We conducted subtly researchs. The purpose is to see into the cause-mechanism of volcanic rocks reservoirs, and then provide reliable evidences in conducting oil-gas exploration, seeking favorable reservoirs and improving the success rate of prospecting well. Researches have made clearly that acidic rhyolite, rhyolite-cementing tuffs develop generally preferable original pore, but volcanic breccias and volcanic agglomerates commonly develop volcanic primary fracture. These establish foundation for

  17. Hydraulic characterization of volcanic rocks in Pahute Mesa using an integrated analysis of 16 multiple-well aquifer tests, Nevada National Security Site, 2009–14

    Garcia, C. Amanda; Jackson, Tracie R.; Halford, Keith J.; Sweetkind, Donald S.; Damar, Nancy A.; Fenelon, Joseph M.; Reiner, Steven R.


    An improved understanding of groundwater flow and radionuclide migration downgradient from underground nuclear-testing areas at Pahute Mesa, Nevada National Security Site, requires accurate subsurface hydraulic characterization. To improve conceptual models of flow and transport in the complex hydrogeologic system beneath Pahute Mesa, the U.S. Geological Survey characterized bulk hydraulic properties of volcanic rocks using an integrated analysis of 16 multiple-well aquifer tests. Single-well aquifer-test analyses provided transmissivity estimates at pumped wells. Transmissivity estimates ranged from less than 1 to about 100,000 square feet per day in Pahute Mesa and the vicinity. Drawdown from multiple-well aquifer testing was estimated and distinguished from natural fluctuations in more than 200 pumping and observation wells using analytical water-level models. Drawdown was detected at distances greater than 3 miles from pumping wells and propagated across hydrostratigraphic units and major structures, indicating that neither faults nor structural blocks noticeably impede or divert groundwater flow in the study area.Consistent hydraulic properties were estimated by simultaneously interpreting drawdown from the 16 multiple-well aquifer tests with an integrated groundwater-flow model composed of 11 well-site models—1 for each aquifer test site. Hydraulic properties were distributed across volcanic rocks with the Phase II Pahute Mesa-Oasis Valley Hydrostratigraphic Framework Model. Estimated hydraulic-conductivity distributions spanned more than two orders of magnitude in hydrostratigraphic units. Overlapping hydraulic conductivity ranges among units indicated that most Phase II Hydrostratigraphic Framework Model units were not hydraulically distinct. Simulated total transmissivity ranged from 1,600 to 68,000 square feet per day for all pumping wells analyzed. High-transmissivity zones exceeding 10,000 square feet per day exist near caldera margins and extend

  18. LA-ICP-MS dating of Zhangjiakou Formation volcanic rocks in the Zhangjiakou region and its geological significance


    The geological cross section of the Zhangjiakou Formation is originally established in the Zhangjiakou region. From the dating of four samples from typical cross sections of the Zhangjiakou Formation in the Zhangjiakou region, the age range is found to be mainly from 143.0±3.7 Myr to 136.1±1.4 Myr, which almost equals the sum of the age range of the Zhangjiakou Formation (136-135 Myr) and that of the Tuchengzi Formation (143-136 Myr) in the Luanping region, and the Zhangjiakou and Tuchengzi Formations in Zhang- jiakou-Luanping of North Hebei are found to be conformable. The evidence above indicates that the Zhangjiakou and Tuchengzi For- mations, which are distributed in Zhangjiakou-Luanping, were formed under the same geological setting. Volcanic activity was earlier and longer (143-136 Myr) in the western region (Zhangjiakou) than that in the eastern region (Luanping). The "Zhangjiakou Forma- tion" in Chengde and Pingquan formed in the period in which the Yixian Formation in West Liaoning formed, so should be renamed the Yixian Formation.

  19. A study of cathodoluminescence and trace element compositional zoning in natural quartz from volcanic rocks: mapping titanium content in quartz.

    Leeman, William P; MacRae, Colin M; Wilson, Nick C; Torpy, Aaron; Lee, Cin-Ty A; Student, James J; Thomas, Jay B; Vicenzi, Edward P


    This article concerns application of cathodoluminescence (CL) spectroscopy to volcanic quartz and its utility in assessing variation in trace quantities of Ti within individual crystals. CL spectroscopy provides useful details of intragrain compositional variability and structure but generally limited quantitative information on element abundances. Microbeam analysis can provide such information but is time-consuming and costly, particularly if large numbers of analyses are required. To maximize advantages of both approaches, natural and synthetic quartz crystals were studied using high-resolution hyperspectral CL imaging (1.2-5.0 eV range) combined with analysis via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Spectral intensities can be deconvolved into three principal contributions (1.93, 2.19, and 2.72 eV), for which intensity of the latter peak was found to correlate directly with Ti concentration. Quantitative maps of Ti variation can be produced by calibration of the CL spectral data against relatively few analytical points. Such maps provide useful information concerning intragrain zoning or heterogeneity of Ti contents with the sensitivity of LA-ICPMS analysis and spatial resolution of electron microprobe analysis.

  20. Upper Paleozoic mafic and intermediate volcanic rocks of the Mount Pleasant caldera associated with the Sn-W deposit in southwestern New Brunswick (Canada): Petrogenesis and metallogenic implications

    Dostal, Jaroslav; Jutras, Pierre


    Upper Paleozoic ( 365 Ma) mafic and intermediate volcanic rocks of the Piskahegan Group constitute a subordinate part of the Mount Pleasant caldera, which is associated with a significant polymetallic deposit (tungsten-molybdenum-bismuth zones 33 Mt ore with 0.21% W, 0.1% Mo and 0.08% Bi and tin-indium zones 4.8 Mt with 0.82% Sn and 129 g/t In) in southwestern New Brunswick (Canada). The epicontinental caldera complex formed during the opening of the late Paleozoic Maritimes Basin in the northern Appalachians. The mafic and intermediate rocks make up two compositionally distinct associations. The first association includes evolved rift-related continental tholeiitic basalts, and the second association comprises calc-alkaline andesites, although both associations were emplaced penecontemporaneously. The basalts have low Mg# 0.34-0.40, smooth chondrite-normalized REE patterns with (La/Yb)n 5-6, primitive mantle-normalized trace element patterns without noticeable negative Nb-Ta anomalies, and their ɛNd(T) ranges from + 2.5 to + 2.2. The basalts were generated by partial melting of a transition zone between spinel and garnet mantle peridotite at a depth of 70-90 km. The calc-alkaline andesites of the second association have chondrite-normalized REE patterns that are more fractionated, with (La/Yb)n 7-8.5, but without significant negative Eu anomalies. Compared to the basaltic rocks, they have lower ɛNd(T) values, ranging from + 0.5 to + 1.9, and their mantle-normalized trace element plots show negative Nb-Ta anomalies. The ɛNd(T) values display negative correlations with indicators of crustal contamination, such as Th/La, Th/Nb and SiO2. The andesitic rocks are interpreted to have formed by assimilation-fractional crystallization processes, which resulted in the contamination of a precursor basaltic magma with crustal material. The parent basaltic magma for both suites underwent a different evolution. The tholeiitic basalts experienced shallow-seated fractional

  1. Mineralogy, Geochemistry and Fluid Inclusion Data from the Tumanpınarı Volcanic Rock-Hosted Fe-Mn-Ba Deposit, Balıkesir-Dursunbey, Turkey

    Ali Haydar Gultekin


    Full Text Available The Tumanpınarı mineralization is a volcanic rock-hosted epithermal Fe-Mn-Ba deposit located in the southwestern part of Dursunbey, Balıkesir, Turkey. The deposit constitutes one of the most important deposits of the Havran-Dursunbey metallogenic sub-province in which numerous Early Miocene Fe-Mn-Ba deposits are distributed. The ore occurs as open-space fillings in faults, fractures, and breccias in the andesite. Early hydrothermal activity was responsible for four types of hypogene alteration in decreasing intensity: silicification, sericitization, hematization and argillic alteration. The mineral assemblage includes pyrolusite, psilomelane, hematite, and barite as well as minor magnetite, manganite, poliannite, limonite, braunite, bixbyite, galena, pyrite, and goethite. Mineralogically, three ore types are recognized as pyrolusite + psilomelane + hematite + barite ore, pyrolusite + psilomelane + poliannite ore and barite + pyrolusite + psilomelane + hematite ore (barite-dominant ore. In addition to Fe, Mn and Ba, the ore contains substantial quantities of Pb, Zn, As. Chemically, the transition from fresh to altered rocks has little effect on the elemental levels for Si, Al, Fe, Ca, Mg, K, Rb, Sr and H2O. The homogenization temperature of fluid inclusions hosted in the main stage quartz and barite ranged from 113 to 410 °C with salinities ranging from 0.4 to 14.9 eq. wt % NaCl, respectively. Overall, the available data suggest that the deposits formed as the result of the interaction of two aqueous fluids: a higher-salinity fluid (probably magmatic and a dilute meteoric fluid.

  2. Constraints on the upper boundary age of the Tiaojishan Formation volcanic rocks in West Liaoning-North Hebei by LA-ICP-MS dating

    ZHANG Hong; WANG MingXin; LIU XiaoMing


    In West Liaoning-North Hebei, the Tiaojishan Formation volcanic rocks are developed and many iso-topic ages with a very wide age range were obtained in them as well. These different ages have direct influence on confirming and interpreting the related geological issues in West Liaoning-North Hebei and have restricted the study of geological problems related. In the field, the Tiaojishan Formation and the overlying Tuchengzi Formation are in parallel unconformable contact. By systematical sampling and dating of LA-ICP-MS in the Tiaojishan Formation (especially in the upper part) in Luanping, Chengde basins of Northern Hebei Province and in Lingyuan, Jinlingsi-Yangshan basins of Western Liaoning Province, combined with the dating data obtained by previous researchers, results indicatethat the upper boundary age of the Tiaojishan Formation is from 156 to 153 Ma, and the lower boundary age of the Tiaojishan Formation is about 165 Ma, the Tiaojishan Formation was formed from the late of middle Jurassic to late Jurassic Epoch.

  3. 塔河地区火山岩类型、分布与主控因素%Types, distribution and main controlling factors of volcanic rocks in Tahe area

    唐跃; 崔泽宏; 王靓靓


    通过岩心观察、地球化学方法、测井地震资料综合分析,认为塔河地区火山岩主要发育英安岩和玄武岩两大类.火山喷发主要以裂隙式喷发、侵出和溢流为主,形成塔河地区大面积火山熔岩流.火山岩的分布由西南向东北逐渐减薄,其分布主要受控于以下几个因素:①盐体覆盖区及其周缘附近,晚海西运动盐体的活动明显控制上覆火山岩岩体形态展布和分布特征,受盐体活动影响,火山岩主要残留于盐边凹陷地带和盐体覆盖区南翼,盐间凹陷火山岩厚度相对较大,盐隆地带残缺较大;②北部地区火山岩分布主要受基底喷发裂隙控制;③盐体南部斜坡地带玄武岩可能从塔里木盆地塔中地区溢流而来.%Based on core observation, geochemical exploration and a comprehensive analysis of logging and seismic data, the authors hold that that two types of volcanic rocks, I.e., dacite and basalt, occur in Tahe area of Tarim basin, that fissure eruption, extrusion and overflow made up the main forms of volcanic eruption, and that lava flow resulted in the formation of extensive volcanic rocks in Tahe area. The distribution of volcanic rocks is characterized by gradual thinning from southwest to northeast,controlled by the following factors: ①in salt body covered area and its vicinity, the activity of salt body during late Hercynian movement significandy controlled the shape and distribution of the overlying volcanic rock, as evidenced by the facts that the residual volcanic rocks are mainly found in salt-edge sags and the south wing of the area covered with salt body and that these rocks are thickest in sags between salt bodies and thinnest in the salt bulge area which experienced uplifting and erosion;② the distribution of the volcanic rocks in the north of the study area is mainly controlled by the basement fractures; ③the volcanic rocks in the southern slope area of the salt body might have been derived

  4. Geological characteristics and eruption cycle partition of jurassic volcanic rocks from Northern Abagaqi, Inner Mongolia%内蒙古阿巴嘎旗北部侏罗纪火山岩地质特征与喷发旋回

    何雨栗; 李小强; 杨国林; 王浩飞; 王文佳; 孙柏年


    内蒙古阿巴嘎旗北部位于西伯利亚板块东南大陆边缘晚古生代陆缘增生带的二连-贺根山板块对接带的西北侧。晚古生代至中新生代岩浆及火山活动频繁,火山岩分布广泛。通过对该区1:5万区域地质填图和实测地质剖面工作,以及室内的详细研究发现出露于本区的侏罗纪火山岩(白音高老组J3 b)以中酸性熔岩和火山碎屑岩为主,岩性特征自下而上具有喷发韵律性。依据岩石组合、岩石特征反映其为陆相火山岩建造。将本区白音高老组火山岩定为中酸性火山喷发旋回,依据岩相划分出5个喷发韵律,每一韵律岩浆成分总体上以流纹质为主,底部常为英安质或安山质,以爆发相为主,喷溢相较少,说明该时期火山活动较强,属较强盛时期。总体上白音高老组反映了喷溢-爆发相、爆发相相间的周期,具有由弱到强的反韵律特征。%The Northern Abagaqi is located in the northwest of the suture belt between Erlian and Hegenshan Plate that is part of the accretion zone of late paleozoic continental margin on the southeast continental margin of the Siberian Plate. The volcanic magma activity is frequent and volcanic rocks are widely distributed from the late paleozoic to the middle cenozoic. Based on 1:50 000 regional geological mapping of the actual geological section and a detailed research on Northern Abagaqi, the Baiyingaolao Formation (J3b) of jurassic rock, was given a intermediate-acid lava and pyroclastic rock priority and its lithologic structure was of bottom-up eruption rhythmicity. According to the rock assemblage and characteristics, the rock reflected its continental volcanic building. The Baiyingaolao Formation volcanic rocks had an acidic volcanic eruption cycle, on account of the facies that could be divided into five eruption rhythms, and each rhythm magma composition was of mainly rhyolitic volcanic rock, and its bottom was often of

  5. Geochronology, Nd isotopes and reconnaissance geochemistry of volcanic and metavolcanic rocks of the São Luís Craton, northern Brazil: Implications for tectonic setting and crustal evolution

    Klein, Evandro L.; Luzardo, Renê; Moura, Candido A. V.; Lobato, Denise C.; Brito, Reinaldo S. C.; Armstrong, Richard


    New field work, in addition to zircon geochronology, Nd isotopes and reconnaissance geochemical data allow the recognition of Paleoproterozoic volcanic and metavolcanic sequences in the São Luís Craton of northern Brazil. These sequences record at least five volcanic pulses occurring probably in three distinct epochs and in different tectonic settings. (1) The Pirocaua Formation of the Aurizona Group comprises early arc-related calc-alkaline metapyroclastic rocks of 2240 ± 5 Ma formed from juvenile protoliths in addition to minor older crustal components. (2) The Matará Formation of the Aurizona Group holds mafic tholeiitic and ultramafic metavolcanic rocks of back arc and/or island arc setting, which are likely coeval to the Pirocaua Formation. (3) The Serra do Jacaré volcanic unit is composed of tholeiitic basalts and predominantly metaluminous, normal- to high- K calc-alkaline andesites of 2164 ± 3 Ma formed in mature arc or active continental margin from juvenile protoliths along with subordinate older (Paleoproterozoic) materials and associated to the main calc-alkaline orogenic stage. (4) The Rio Diamante Formation consists of late-orogenic metaluminous, medium- K, calc-alkaline rhyolite to dacite and tuffs of 2160 ± 8 Ma formed in continental margin setting from reworked Paleoproterozoic crust (island arc) with incipient Archean contribution. (5) The Rosilha volcanic unit is composed of weakly peraluminous, medium- K, calc-alkaline dacite and tuff formed probably at about 2068 Ma from reworked crustal protoliths. As a whole the volcanic and metavolcanic rocks record and characterized better the previously proposed orogenic evolution of the São Luís Craton.

  6. Revealing magma degassing below closed-conduit active volcanoes: Geochemical features of volcanic rocks versus fumarolic fluids at Vulcano (Aeolian Islands, Italy)

    Mandarano, Michela; Paonita, Antonio; Martelli, Mauro; Viccaro, Marco; Nicotra, Eugenio; Millar, Ian L.


    The elemental and isotopic compositions of noble gases (He, Ne, and Ar) in olivine- and clinopyroxene-hosted fluid inclusions have been measured for rocks at various degrees of evolution and belonging to high-K calcalkaline-shoshonitic and shoshonitic-potassic series in order to cover the entire volcanological history of Vulcano Island (Italy). The major- and trace-element concentrations and the Sr- and Pb-isotope compositions for whole rocks were integrated with data obtained from the fluid inclusions. 3He/4He in fluid inclusions is within the range of 3.30 and 5.94 R/Ra, being lower than the theoretical value for the deep magmatic source expected for Vulcano Island (6.0-6.2 R/Ra). 3He/4He of the magmatic source is almost constant throughout the volcanic history of Vulcano. Integration of the He- and Sr-isotope systematics leads to the conclusion that a decrease in the He-isotope ratio of the rocks is mainly due to the assimilation of 10-25% of a crustal component similar to the Calabrian basement. 3He/4He shows a negative correlation with Sr isotopes except for the last-erupted Vulcanello latites (Punta del Roveto), which have anomalously high He isotope ratios. This anomaly has been attributed to a flushing process by fluids coming from the deepest reservoirs, since an input of deep magmatic volatiles with high 3He/4He values increases the He-isotope ratio without changing 87Sr/86Sr. A comparison of the He-isotope ratios between fluid inclusions and fumarolic gases shows that only the basalts of La Sommata and the latites of Vulcanello have comparable values. Taking into account that the latites of Vulcanello relate to one of the most-recent eruptions at Vulcano (in the 17th century), we infer that the most probable magma which actually feeds the fumarolic emissions is a latitic body that ponded at about 3-3.5 km of depth and is flushed by fluids coming from a deeper and basic magma.

  7. Paleomagnetic study on orogenic belt:An example from Early Cretaceous volcanic rocks,Inner Mongolia,China

    REN; Shoumai; ZHU; Rixiang; HUANG; Baochun; ZHANG; Fuqin


    We report paleomagnetic results for Early Cretaceous lava flows collected from the Suhongtu area of Inner Mongolia, the middle part of the Tianshan-Mongolia Fold Belt (TMFB).Rock-magnetic experiments for different lava flows indicate that the main magnetic mineral is pseudo-single-domain (PSD) magnetite. The characteristic high-temperature remanence component is isolated by thermal demagnetization temperature steps between 300℃ and 585℃,which yields a mean direction of D= 23.6°, /= 56.0° with α95 = 2.3°. We interpret this high-temperature remanence component as primary magnetization based mainly upon the petrographic analysis, which shows that the shape of magnetite crystal is relatively rounded square or polygon without internal reflection and deuterogenous phenomenon. The corresponding pole of the high-temperature remanence component is at 71.1°N, 200.5°E with A95 = 2.7°.This Early Cretaceous pole is in good agreement with those for Siberia, North China, and Inner Mongolia, suggesting that these continental blocks had already sutured together in the Early Cretaceous, which would further provide constraints on better understanding of the formation and evolution of the TMFB.

  8. Stratigraphy, structure, and some petrographic features of Tertiary volcanic rocks in the USW G-2 drill hole, Yucca Mountain, Nye County, Nevada

    Maldonado, Florian; Koether, S.L.


    A continuously cored drill hole designated as USW G-2, located at Yucca Mountain in southwestern Nevada, penetrated 1830.6 m of Tertiary volcanic strata composed of abundant silicic ash-flow tuffs, minor lava and flow breccias, and subordinate volcaniclastic rocks. The volcanic strata penetrated are comprised of the following in descending order: Paintbrush Tuff (Tiva Canyon Member, Yucca Mountain Member, bedded tuff, Pah Canyon Member, and Topopah Spring Member), tuffaceous beds of Calico Hills, Crater Flat Tuff (Prow Pass Member, Bullfrog Member, and Tram unit), lava and flow breccia (rhyodacitic), tuff of Lithic Ridge, bedded and ash-flow tuff, lava and flow breccia (rhyolitic, quartz latitic, and dacitic), bedded tuff, conglomerate and ash-flow tuff, and older tuffs of USW G-2. Comparison of unit thicknesses at USW G-2 to unit thicknesses at previously drilled holes at Yucca Mountain indicate the following: (1) thickening of the Paintbrush Tuff members and tuffaceous beds of Calico Hills toward the northern part of Yucca Mountain; (2) thickening of the Prow Pass Member but thinning of the Bullfrog Member and Tram unit; (3) thinning of the tuff of Lithic Ridge; (4) presence of approximately 280 m of lava and flow breccia not previously penetrated by any drill hole; and (5) presence of an ash-flow tuff unit at the bottom of the drill hole not previously intersected, apparently the oldest unit penetrated at Yucca Mountain to date. Petrographic features of some of the units include: (1) decrease in quartz and K-feldspar and increases in biotite and plagioclase with depth in the tuffaceous beds of Calico Hills; (2) an increase in quartz phenocrysts from the top to the bottom members of the Crater Flat Tuff; (3) a low quartz content in the tuff of Lithic Ridge, suggesting tapping of the magma chamber at quartz-poor levels; (4) a change in zeolitic alteration from heulandite to clinoptilolite to mordenite with increasing depth; (5) lavas characterized by a rhyolitic

  9. Geochemistry and zircon U-Pb-Hf isotopes of Early Paleozoic arc-related volcanic rocks in Sonid Zuoqi, Inner Mongolia: Implications for the tectonic evolution of the southeastern Central Asian Orogenic Belt

    Chen, Yan; Zhang, Zhicheng; Li, Ke; Yu, Haifei; Wu, Tairan


    An Early Paleozoic acid volcanic sequence has been recently detected southeast of Sonid Zuoqi in central Inner Mongolia to constrain the tectonic evolution of the Central Asian Orogenic Belt in this area. First, the volcanic rocks have zircon U-Pb ages of 439-445 Ma. They are characterized by (a) a high silica content, moderate alkali content and low iron content; (b) enrichment in light rare earth elements, depletion of heavy rare earth elements, and negative Eu anomalies; and (c) negative Nb, Ta, and Ti anomalies. Finally, the volcanic samples yield εHf(t) values of - 4.7 to + 9.2 with TDM2 ages of 835-1724 Ma. For petrogenesis, they were possibly arc derived, from predominant juvenile materials with subordinate ancient continental crust. Combined with previous studies, the Early Paleozoic Sonid Zuoqi arc magmatism can be divided into three stages: a primitive arc stage represented by 464-490 Ma low-K, calcic granitoids; a normal continental arc stage represented by 439-445 Ma medium-K, calcic to calcic-alkalic plutons and volcanic rocks and a syn-collisional stage represented by 423-424 Ma high-K granites. Furthermore, the timing and tectonic settings of the above magmatic rocks show similarities to those in Xilinhot and other areas of the northern Early to Mid-Paleozoic orogenic belt (NOB), although the rock assemblies and their proportions vary more or less in different areas. Accordingly, the NOB that formed on this arc was probably attributed to the northward subduction of the Paleo-Asian Ocean beginning at 500 Ma, which experienced this type of arc development and was terminated by a soft collision before the Late Devonian.

  10. Volcanic gas

    McGee, Kenneth A.; Gerlach, Terrance M.


    In Roman mythology, Vulcan, the god of fire, was said to have made tools and weapons for the other gods in his workshop at Olympus. Throughout history, volcanoes have frequently been identified with Vulcan and other mythological figures. Scientists now know that the “smoke" from volcanoes, once attributed by poets to be from Vulcan’s forge, is actually volcanic gas naturally released from both active and many inactive volcanoes. The molten rock, or magma, that lies beneath volcanoes and fuels eruptions, contains abundant gases that are released to the surface before, during, and after eruptions. These gases range from relatively benign low-temperature steam to thick hot clouds of choking sulfurous fume jetting from the earth. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other volcanic gases are hydrogen sulfide, hydrochloric acid, hydrogen, carbon monoxide, hydrofluoric acid, and other trace gases and volatile metals. The concentrations of these gas species can vary considerably from one volcano to the next.

  11. Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere

    Tatsumoto, M.; Basu, A.R.; Wankang, H.; Junwen, W.; Guanghong, X.


    The UThPb, SmNd, and RbSr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000-1100??C and possible depths of 70-80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the 206Pb 204Pb ratios of the minerals. On a NdSr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher 143Nd 144Nd and a much broader range of 87Sr 86Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in NdSr space. The garnets consistently show lower ?? and ?? values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highest ?? and ?? values among all the minerals and rocks studied. In a plot of ??207 and ??208, the host basalts for all three areas show lower ??207 and higher ??208 values than do the xenoliths, indicating derivation of basalts from Th-rich (relative to U) sources and xenoliths from U-rich sources. The xenolith data trends toward the enriched mantle components, EMI and EMII-like, characterized by high 87Sr 86Sr and ??207 values but with slightly higher 143Nd 144Nd. The EMI trend is shown more distinctly by the host

  12. Relationship between Volcanic Rocks and Hydrocarbon Accumulation during Dominant Period of Basin Formation in Liaohe Depression%主成盆期火山岩与油气成藏关系探讨

    陈振岩; 仇劲涛; 王璞珺; 李湃; 张培先; 刘鑫; 郝涛; 聂桂民


    The dominant period of basin formation is defined as the period with the strongest tectonic movements,the largest extent of subsidence and the best development of source rocks.The sedimentary period of the 3rd member(Shasan,E2s3) and 4th member(Shasi,E2s4) of Shahejie Formation is the dominant period for basin formation.There were many episodes of volcanic movements during Cenozoic in Liaohe Depression,forming distribution of volcanic rocks with many series and types,which changes with the tectonic center and has the less strong activity in the earlier stages.The volcanic movements in the dominant period of Liaohe depression are abnormally strong and the appearances of volcanic rocks are frequent.The reservoirs which are altered by structural fractures and the corrosion and dissolution by formation water are favorable for oil and gas accumulation.The superposition in plane and the alternation in profile provide enough provision conditions of oil and gas sources for volcanic rock reservoirs.Furthermore,the volcanic rocks from dominant period of basin are of large thickness,widespread distribution and various traps and accumulation types,owning many advantages for oil and/or gas accumulation in many respects.The volcanic rock reservoirs for oil and/or gas in the areas of Huangshatuo and Oulituozi are typical representations among the above favorable reservoirs.The volcanic rocks from the dominant period of basin formation with favorable combination of oil and/or gas accumulation,enriching the content of oil and /or gas accumulation and the types of oil and /or gas exploration,are the important field for hydrocarbon discovery and exploration.%裂陷盆地的主成盆期是构造活动性最强、沉降幅度最大、烃源岩发育最好的时期,辽河坳陷的主成盆期是E2s4~E2s3时期。辽河坳陷新生代发生了多期火山活动,形成了多套、多类型的火山岩分布,火山活动总体上具有早强晚弱、平面上具有随沉降中

  13. Petrogenesis of Indosinian volcanic rocks in Songpan-Garze fold belt of the northeastern Tibetan Plateau:New evidence for lithospheric delamination


    In the Songpan-Garze fold belt of the northeastern Tibetan Plateau, an Indosinian lithospheric delamination model has been proposed, based on previous investigation of widespread granitoids. However, this model lacks comparable information from volcanism in the area. During the Indosinian delamination in the Songpan-Garze fold belt, whether partial melting of litho- spheric mantle taken place is debated. This paper reports U-Pb zircon LA-ICP-MS ages, geochemical and Sr-Nd-Hf isotopic compositions from the Aba and Wasai calc-alkaline volcanic rocks in the central Songpan-Garze fold belt. Obtained magma crystallization ages are 210±3 Ma for the Aba andesite and 205±1 Ma for the Wasai andesite. These are consistent with magma crystallization ages of the late Indosinian granitoids in the Songpan-Garze fold belt that formed in a post-collisional tectonic setting. The Aba and Wasai andesites have distinct geochemical singnatures. The former has higher Al2O3, K2O, Rb but lower Na2O, Ba and Sr contents, suggesting differences in their magmatic evolution. The Aba andesites have ISr values of 0.7070-0.7076 and εNd(t) values of -3.9 to -5.3, and the Wasai andesites have ISr values of 0.7075-0.7077 and εNd(t) values of -3.6 to -3.9. Zircons show εHf(t) values of -3.7 to 0.3 for the Aba andesites and -2.7 to 5.5 for the Wasai andesites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that fractional crystallization and crustal assimilation processes are not key roles for their magma evolution, implying that their chemical compositions are those of primary melts. We suggest that the magma of the Aba andesites originated predominantly from a crustal source, with a minor mantle-derived component. The source region of the magma was likely at the crust-mantle boundary. The magma of the Wasai andesites resulted from partial melting of lithospheric mantle, which was probably metasomatized by fluids so that it was amphibole bearing. The petrogenesis of the Aba and Wasai

  14. 火山岩砖的制备及性能研究%Study on Preparation of Bricks with Volcanic Rock and Its Performances

    邱文达; 邓湘云; 李建保


    The volcanic brick was prepared with some solid wastes such as volcanic rock, fly ash, bottom slag and so on by using the semi-dry pressing technology. Some additives were used to improve the properties. The phase compositions and microstructures were characterized by XRD and SEM. The water absorption, porosity, bulk density and compressive strength were measured. The results showed that the main crystalline phases are CaAl2Si2O8, Al2SiO5, Al2Si2O5(OH)4, Na(Si3Al)O8, Fe2O3, CaSiO3, and they interlace as granular, small rod like crystals and strips. The pores disperse uniformly in the samples. Analysis shows that the porosity is about 24% -42% , water absorption is about 16% -27% , bulk density is about 1.55-1.57 g/cm3, compressive strength is about 25-103 Mpa.%以火山岩、粉煤灰、炉底渣等工业固体废弃物为原料,通过添加一些添加剂,采用压制成型方法,制备了高性能的墙砖.采用XRD、SEM测试技术分别对样品的相组成和显微结构进行了分析,并测量了样品的吸水率、气孔率、体积密度及抗压强度.结果表明,经合理烧结后的样品的主晶相为钙长石、蓝晶石、高岭土、钠长石、氧化铁、硅酸钙.主晶相晶体呈粒状、针棒状、板条状交织排列,气孔均布于样品中.分析表明样品的气孔率达24%~ 42%,吸水率达16%~27%,体积密度达1.55~1.57 g/cm3,抗压强度达25~ 103 MPa,符合国标GB13544-2000标准.

  15. Effect of Pozzolanic Reaction of Volcanic Rocks in Cement-based Material%水泥基材料中火山岩火山灰反应的效益

    喻乐华; 李琳国; 管亮亮; 何兵兵; 廖妙星


    Based on the concept that strength of cement-based material results from hydration of cement and pozzolanic reaction of pozzolans (secondary-hydration reaction), the mechanical strength of cement mortar blend-ed with volcanic rock powder was tested, and effect of pozzolanic reaction of volcanic rock was calculated by us-ing the strength values. The results show that the effect of pozzolanic reaction for volcanic rocks in cement-base material decreases as the dosage of volcanic rock increases and the hydration time prolongs. The relationships a-mong them are expressed quantitatively by using fitting regression equation. It finds out that from the effect val-ues of pozzolanic reaction, in most cases, replacement of cement by volcanic rocks results in reducing impacts on strength of cement-based material, and the higher the replacement volume is, the greater the strength decreases.%基于水泥基材料强度由水泥的水化反应和火山灰质材料的火山灰反应(二次水化反应)共同构成之概念,测试掺火山岩粉末的水泥砂浆力学强度计算,表征单位火山岩的火山灰反应之效益。结果表明火山岩火山灰反应的效益随着水泥基材料中掺量增大而减小,同掺量情况下随着水化时间延长而减小。这些关系可用拟合回归方程量化表达。研究的火山岩火山灰反应效益可知大多数情况下火山岩替代水泥会对水泥基材料强度有降低影响,掺量越大强度降低幅度也越大。研究成果有助于工程上优化选择火山灰质材料在水泥基材料中的应用。

  16. Magnetic properties of the remagnetized Middle-Ordovician limestones of the Ponón Trehué Formation (San Rafael Block, central-western Argentina): Insights into the Permian widespread Sanrafaelic overprint

    Fazzito, Sabrina Y.; Rapalini, Augusto E.


    The widespread Sanrafaelic remagnetization reset most of the early Cambrian to mid-Ordovician carbonate platform of the Argentine Precordillera and the calcareous units of the San Rafael Block. We conducted a detailed rock-magnetic study on the Middle-Ordovician limestones of the Ponón Trehué Formation at both limbs of a tight anticline exposed in the San Rafael Block (Mendoza province, central-western Argentina) that are carriers of a syntectonic magnetization of Permian age. We found that the magnetic overprint in the Ponón Trehué Formation is carried by both pyrrhotite and magnetite, with goethite and subordinate haematite likely related to weathering. Hysteresis parameters, frequency dependence of magnetic susceptibility, Cisowski and modified Lowrie-Fuller tests suggest the presence of ultrafine particles of chemical origin. Demagnetization of natural remanent magnetization and of three-axis isothermal remanence confirm pyrrhotite and magnetite as important contributors to the remanence. Both minerals carry the same magnetic syntectonic component suggesting a coeval or nearly coeval remanence acquisition and therefore mineral formation. This and the results of the magnetic fabric analyses indicate an authigenic origin of the magnetic minerals during folding associated with the Sanrafaelic tectonic phase (ca. 280 Ma). Although the chemically active (oxidizing?) fluids expelled from the orogen as it developed in the early Permian is a viable explanation for the Sanrafaelic remagnetization, the role of the nearly coeval magmatism in Precordillera and the San Rafael Block remains to be properly evaluated.

  17. A new genetic interpretation for the Caotaobei uranium deposit associated with the shoshonitic volcanic rocks in the Hecaokeng ore field, southern Jiangxi, China

    Dong-Sheng Yang


    ± 5.7 Ma in the shoshonitic volcanic rock is broadly coeval with main-stage U mineralization, which is probably attributable to a tectonothermal event related to the intrusion of the granite porphyries and further supports our genetic reinterpretation. It is thus concluded that the granite porphyry intrusions and associated magma may provide the fluids, ore components, and the thermal energy for U mineralization. However, some other types of fluids and metal sources (e.g., meteoric-derived fluids, which are yet to be identified could have been substantially involved in the mineralization process. Our new genetic explanation may point to significant potential for mid-Cretaceous granite-related hydrothermal U deposits in Jiangxi and other parts of Southeast China.

  18. Revealing the magmas degassing below closed-conduit active volcanoes: noble gases in volcanic rocks versus fumarolic fluids at Vulcano (Aeolian Islands, Italy)

    Mandarano, Michela; Paonita, Antonio; Martelli, Mauro; Viccaro, Marco; Nicotra, Eugenio; Millar, Ian L.


    With the aim to constrain the nature of magma currently feeding the fumarolic field of Vulcano, we measured the elemental and isotopic compositions of noble gases (He, Ne, and Ar) in olivine- and clinopyroxene-hosted fluid inclusions in high-K calcalcaline-shoshonitic and shoshonitic-potassic series so as to cover the entire volcanological history of Vulcano Island (Italy). The major and trace-element concentrations and the Sr- and Pb-isotope compositions for whole rocks were integrated with data obtained from the fluid inclusions. 3He/4He in fluid inclusions is within the range of 3.30 and 5.94 R/Ra, being lower than the value for the deep magmatic source expected for Vulcano Island (6.0-6.2 R/Ra). 3He/4He of the magmatic source is almost constant throughout the volcanic record of Vulcano. Integration of the He- and Sr-isotope systematics leads to the conclusion that a decrease in the He-isotope ratio of the rocks is mainly due to the assimilation of 10-25% of a crustal component similar to the Calabrian basement. 3He/4He shows a negative correlation with Sr isotopes except for the last-emitted Vulcanello latites (Punta del Roveto), which have high He- and Sr-isotope ratios. This anomaly has been attributed to a flushing process by fluids coming from the deepest reservoirs. Indeed, an input of deep magmatic volatiles with high 3He/4He values increases the He-isotope ratio without changing 87Sr/86Sr. A comparison of the He isotope ratios between fluid inclusions and fumarolic gases showed that only the basalts of La Sommata and the latites of Vulcanello have comparable values. Taking into account that the latites of Vulcanello relate to one of the most-recent eruptions at Vulcano (in the 17th century), we infer that that the most probable magma which actually feeds the fumarolic emissions is a latitic body ponding at about 3-3.5 km of depth and flushed by fluids coming from a deeper and basic magma.

  19. From olivine nephelinite, basanite and basalt to peralkaline trachyphonolite and comendite in the Ankaratra volcanic complex, Madagascar: 40Ar/39Ar ages, phase compositions and bulk-rock geochemical and isotopic evolution

    Cucciniello, Ciro; Melluso, Leone; le Roex, Anton P.; Jourdan, Fred; Morra, Vincenzo; de'Gennaro, Roberto; Grifa, Celestino


    The Ankaratra volcanic field covers an area of 3800 km2 in central Madagascar and comprises of lava flows, lava domes, scoria cones, tuff rings and maars emplaced at different ages (Miocene to Recent). The volcanic products include ultramafic-mafic (olivine-leucite nephelinite, basanite, alkali basalt, hawaiite and tholeiitic basalt), intermediate (mugearite and benmoreite) and felsic rocks (trachyphonolite, quartz trachyte and rhyolite), the latter often peralkaline. The 40Ar/39Ar determinations for mafic lavas yield ages of 17.45 ± 0.12 Ma, 16.63 ± 0.08 Ma and 8.62 ± 0.09 Ma, indicating a prolonged magmatic activity. The mineralogical and geochemical variations suggest that the magmatic evolution of the alkali basalt-hawaiite-mugearite-benmoreite-trachyte series can be accounted for by removal of olivine, feldspars, clinopyroxene, Fe-Ti oxides and accessory phases, producing residual trachytic and trachyphonolitic compositions mineralogically very similar to those of other volcanic areas and tectonic settings. The Ankaratra olivine leucite nephelinites, basanites and tholeiitic basalts do not seem to be associated with significant amounts of evolved comagmatic rocks. The 87Sr/86Sr (0.70504-0.71012), 143Nd/144Nd (0.51259-0.51244) and 206Pb/204Pb (17.705-18.563) isotopic ratios of trachytes and comendite are consistent with open-system processes. However, other trachyphonolites have 143Nd/144Nd (0.51280), 206Pb/204Pb (18.648), 207Pb/204Pb (15.582) and 208Pb/204Pb (38.795) similar to those of mafic rocks, suggesting differentiation processes without appreciable interaction with crustal materials. The Ankaratra volcanism is to be directly linked to a broadly E-W-trending intracontinental extension. A large-scale thermal anomaly, associated with an anomalously hot source region, is not required to explain the Cenozoic magmatism of Madagascar.

  20. Igneous Rocks of the East Pacific Rise: The alkali volcanic suite appear to be differentiated from a tholeiitic basalt extruded from the mantle.

    Engel, A E; Engel, C G


    The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte). In contrast, the more submerged parts of the Rise are largely composed of a tholeiitic basalt which has low concentrations of K, P, U, Th, Pb, and Ti. This tholeiitic basalt is either the predominant or the only magma generated in the earth's mantle under oceanic ridges and rises. It is at least 1000-fold more abundant than the alkali suite, which is probably derived from tholeiitic basalt by magmatic differentiation in and immediately below the larger volcanoes. Distinction of oceanic tholeiites from almost all continental tholeiites is possible on the simple basis of total potassium content, with the discontinuity at 0.3 to 0.5 percent K(2)O by weight. Oceanic tholeiites also are readily distinguished from some 19 out of 20 basalts of oceanic islands and seamount cappings by having less than 0.3 percent K(2)O by weight and more than 48 percent SiO(2). Deep drilling into oceanic volcanoes should, however, core basalts transitional between the oceanic tholeiites and the presumed derivative alkali basalts. The composition of the oceanic tholeiites suggests that the mantle under the East Pacific Rise contains less than 0.10 percent potassium oxide by weight; 0.1 part per million of uranium and 0.4 part of thorium; a potassium:rubidium ratio of about 1200 and a potassium: uranium ratio of about 10(4).

  1. Re - Os isotopic constraints on the origin of volcanic rocks, Gorgona Island, Colombia: Os isotopic evidence for ancient heterogeneities in the mantle

    Walker, R.J.; Echeverria, L.M.; Shirey, S.B.; Horan, M.F.


    The Re - Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155??43 Ma. Subsequent episodes of volcanism produced basalts at 88.1??3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts. Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (??Nd+9??1). Initial 187Os/186Os normalized to carbonaceous chondrites at 155 Ma (??Os) ranged from 0 to +22, and model-initial ?? values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (<20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re - Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions. ?? 1991 Springer-Verlag.

  2. Magnetic anisotropy in rhyolitic ignimbrite, Snake River Plain: Implications for using remanent magnetism of volcanic rocks for correlation, paleomagnetic studies, and geological reconstructions

    Finn, David R.; Coe, Robert S.; Kelly, Henry; Branney, Michael; Knott, Thomas; Reichow, Marc


    Individual ignimbrite cooling units in southern Idaho display significant variation of magnetic remanence directions and other magnetic properties. This complicates paleomagnetic correlation. The ignimbrites are intensely welded and exhibit mylonite-like flow banding produced by rheomorphic ductile shear during emplacement, prior to cooling below magnetic blocking temperatures. Glassy vitrophyric lithologies commonly have discrepantly shallow remanence directions rotated closer to the orientation of the subhorizontal shear fabric when compared to the microcrystalline center of the same cooling unit. To investigate this problem, we conducted a detailed paleomagnetic and rock magnetic study of a vertical profile through a single ignimbrite cooling unit and its underlying baked soil. The results demonstrate that large anisotropy of thermal remanent magnetization correlates with large (up to 38°) deflections of the stable remanence direction. Anisotropy of magnetic susceptibility revealed no strong anisotropy. A strong lineation and deflection of the remanence declination suggest that rheomorphic shear above magnetic blocking temperatures is the dominant mechanism controlling the formation of the magnetic fabric, with compaction contributing to a lesser extent. Nucleation and growth of anisotropic fine-grained magnetite in volcanic glass at high temperatures after, and perhaps also during, emplacement is indicated by systematic variation of magnetic properties from the quickly chilled ignimbrite base to the interior. These properties include remanence directions, anisotropy, coercivity, susceptibility, strength of natural remanent magnetization, and dominant unblocking temperature. The microcrystalline ignimbrite center has a magnetic direction that is the same as the underlying baked soil and, therefore, is a more reliable recorder of the paleofield direction than the glassy margins of highly welded ignimbrites.

  3. Reassessment of petrogenesis of Carboniferous–Early Permian rift-related volcanic rocks in the Chinese Tianshan and its neighboring areas

    Linqi Xia


    Full Text Available The Carboniferous−Early Permian rift-related volcanic successions, covering large areas in the Chinese Tianshan and its adjacent areas, make up a newly recognized important Phanerozoic large igneous province in the world, which can be further divided into two sub-provinces: Tianshan and Tarim. The regional unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks, the ages (360–351 Ma of the youngest ophiolite and the peak of subduction metamorphism of high pressure–low temperature metamorphic belt and the occurrence of Ni-Cu-bearing mafic-ultramafic intrusion with age of ∼352 Ma and A-type granite with age of ∼358 Ma reveal that the final closure of the Paleo-Asian Ocean might take place in the Early Mississippian. Our summation shows that at least four criteria, being normally used to identify ancient asthenosphere upwelling (or mantle plumes, are met for this large igneous province: (1 surface uplift prior to magmatism; (2 being associated with continental rifting and breakup events; (3 chemical characteristics of asthenosphere (or plume derived basalts; (4 close links to large-scale mineralization and the uncontaminated basalts, being analogous to those of many “ore-bearing” large igneous provinces, display Sr-Nd isotopic variations between plume and EM1 geochemical signatures. These suggest that a Carboniferous asthenosphere upwelling and an Early Permian plume played the central role in the generation of the Tianshan–Tarim (central Asia large igneous province.

  4. The discovery of Middle Jurassic volcanic rocks in north-ern Fujian Province and its geological significance%闽北地区中侏罗世火山岩的发现及其地质意义

    李亚楠; 邢光福; 邢新龙; 陈世忠; 段政; 杨东; 胡凌云


    There are only very few Middle Jurassic volcanic rocks exposed along the southeast coast of China, but they are very im-portant for the study of the regional Mesozoic tectonic magmatic activiy and tectonic evolution. The authors discovered for the first time the Middle Jurassic volcanic rocks in the Tieshan area of Zhenghe during the survey of large area of Cretaceous volcanic rocks in northern Fujian Province. The zircon U-Pb age obtained by LA-ICP-MS technique is 173.63 ± 0.80Ma. The volcanic rocks are welded tuff, belonging to the calc-alkaline series. Geochemical data show that they are peraluminous rocks rich in potassium and large ion lithophile elements but poor in high field strength elements. Therefore, these rocks must have been formed in a continental margin arc environment, related to the subduction of the paleo-Pacific plate.%中侏罗世火山岩在东南沿海分布极少,但对区域中生代构造-岩浆活动过程及大地构造演化的研究具有重要意义.在闽北地区大面积白垩纪火山岩区调查中,在政和铁山地区发现中侏罗世火山岩,用LA-ICP-MS同位素测试技术测得的锆石U-Pb年龄为173.63±0.80Ma.火山岩属钙碱性系列流纹质熔结凝灰岩,显示过铝质、富钾、富集大离子亲石元素,亏损高场强元素等地球化学特征,推断其形成于陆缘弧环境,与古太平洋板块早期俯冲作用有关.

  5. Characterization of the volcanic and hypabissal rocks of the Paleoproterozoic Iricoumé Group in the Pitinga region and Balbina Lake area, Amazonian Craton, Brazil: Petrographic distinguishing features and emplacement conditions

    Simões, Matheus Silva; Almeida, Marcelo Esteves; de Souza, Antonio Gilmar Honorato; da Silva, Desaix Paulo Balieiro; Rocha, Paloma Gabriela


    The Iricoumé Group (1.897 to 1.875 Ma) is a widespread volcanic sequence in the Amazonian Craton, South American Platform. In the Pitinga region, it consists of acidic to intermediate ignimbrites with cogenetic surge and ash-fall deposits associated to hypabissal and effusive acidic rocks. In the Balbina Lake area it consists of acidic to intermediate effusive rocks, crystal-rich ignimbrites and pumice-rich ignimbrites. Detailed petrographic studies of phenocrysts and crystal fragments provided characterization and distinctive features of pyroclastic, effusive and hypabissal rocks. The phenocrysts of hypabissal rocks were affected by high temperature resorption and flow-related physical fragmentation, the effusive rocks can have a considerable content of crystal fragments due to intense dissolution and fragmentation of the phenocrysts and the pyroclastic rocks crystal fragments are generated mainly by decompression mechanic fragmentation. Dissolution and resorption of quartz and feldspar crystals may have acted at pressure conditions between 500 MPa and 100 MPa, in response to rapid decompression in the magma ascent. The viscosity values of 7,5-10 log η (Pa s) for anhydrous conditions decrease exponentially with estimated water addition. The amount of phenocrysts and their intratelluric character in the volcanic units of Iricoumé Group can be related to a magma chamber with high content of crystals. We suggest that even with the addition of water in the magma, the viscosities had a drastic increase with progressive crystal growth due to the viscosity dependence on the solid fraction and the deposits were generated by the extrusion of viscous lava and pyroclastic flows.

  6. 云南腾冲全新世火山岩地球化学特征及其成因%A study on the geochemical characteristics and petrogenesis of Holocene volcanic rocks in the Tengchong volcanic eruption field, Yunnan Province, SW China

    李欣; 刘嘉麒


    Studies of major element, trace element and Sr-Nd-Pb isotopic compositions of the Hotocene volcanic rocks in the Tengchong volcanic eruption field, including Heikongshan, Dayingshan and Maanshan volcanic rocks, indicate they are high potassium cale-alkaline series rocks including trachybasalt, basaltic trachyandesite, trachyandensite, and dacite. These rocks are enriched in LILE and LREE, and depleted in Nb-Ta-Ti in mantle-normalized incompatible trace element patterns. They have relatively high "Sr/ "Sr (0.705862-0.710614), low l43Nd/144Nd (0.511941 -0.512526) and relatively high radiogenic Pb isotopes (208Pb/204Pb = 38.962-39. 155, 207Pb/204Pb = 15.620~15.659 and, 206Pb/204Pb = 17. 872-18.269). The geochemical characteristics of the Holocene volcanic rocks in the Tengchong volcanic eruptive field indicate that the magma sources of Heikongshan, Dayingshan, Maanshan are from enriched mantle which possibly resulted from palaeo-oceanic crust subduction. The magma sources of these three volcanoes are same but with different evolution stage. The magma evolution of Dayingshan is higher than that of Heikongshan and Maanshan. The comparison with geochemical characteristics of post - collisional K-rich rocks in South Tibet shows that the magma sources of Holocene volcanic rocks in the Tengchong volcanic eruption field and post-collisional K-rich rocks in South Tibet are different%通过对腾冲火山群的黑空山、打莺山和马鞍山火山岩主量元素、微量元素、Sr-Nd-Pb同位素地球化学的研究表明,腾冲全新世火山岩为高钾钙碱性系列,包括粗面玄武岩、玄武粗安岩、粗面安山岩和英安岩.该套火山岩富集大离子亲石元素和轻稀土元素,亏损Nb-Ta-Ti不相容元素,具有高的87Sr/86Sr比值(0.705862 ~0.710614),低的143Nd/144Nd比值(0.511941~0.512526)和较高的放射性成因Pb同位素组成(208Pb/204Pb =38.962~ 39.155;207Pb/204Pb=15.620 ~ 15.659;206Pb/204Pb=17.872~18.269).主量、微量和同

  7. Petrology and U-Pb zircon geochronology of bimodal volcanic rocks from the Maierze Group, northern Tibet: Constraints on the timing of closure of the Banggong-Nujiang Ocean

    Fan, Jian-Jun; Li, Cai; Xie, Chao-Ming; Wang, Ming; Chen, Jing-Wen


    We present new zircon U-Pb dates, major and trace element chemistry, and Hf isotopic compositions for bimodal volcanic rocks of the Maierze Group (MG) in the Maierze area of the Southern Qiangtang-Baoshan block, northern Tibet. We discuss the implications of these data for the evolution of this region. The MG bimodal volcanic rocks consist of basalts and dacites that yield LA-ICP-MS zircon U-Pb ages of 122 and 120 Ma, respectively. The MG basalts have light rare earth element (LREE)-enriched chondrite-normalized REE patterns (LaN/YbN = 13-14), high Ti/V ratios (45-64), high Zr (190-270 ppm) and Nb (22-41 ppm) concentrations, and Zr/Y ratios (7-9) that are similar to those of within-plate basalts. The MG basalts also have low MgO and total Fe2O3 (TFe2O3) concentrations, significant enrichments in the LREE and the light ion lithophile elements (LILEs; Rb, Ba, Th, U and Pb), and weak depletions in the high field strength elements (HFSE; Nb, Ta, and Ti), all of which are clearly evident in primitive-mantle-normalized multi-element variation diagrams. The MG dacites are more enriched in the LILE (e.g., Rb, Ba, Th, U, K, and Pb) and more depleted in the HFSE (e.g., Nb, Ta, and Ti) than the MG basalts. Moreover, the dacites have variable zircon εHf(t) values (- 6.3 to + 6.3). These features indicate that the parental magma for the MG basalts was likely derived from an enriched lithospheric mantle source that was contaminated by subduction-related fluids or melts. In contrast, the MG dacites were derived from mixing of the MG basaltic magma with a second magma derived from partial melting of the continental crust. The geochemical and Hf isotopic characteristics of the MG bimodal volcanic rocks suggest that they formed during the initial stages of development of a back-arc basin. From south to north, the Bangong-Nujiang suture zone, the Duolong gold-rich porphyry copper deposit, and the Maierze bimodal rocks are interpreted to represent a remnant of a complete volcanic arc

  8. Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust-mantle mixing and metamorphism in the deep crust

    Wang, Rui; Collins, William J.; Weinberg, Roberto F.; Li, Jin-xiang; Li, Qiu-yun; He, Wen-yan; Richards, Jeremy P.; Hou, Zengqian; Zhou, Li-min; Stern, Richard A.


    Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 °C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (ɛHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73-8.97 ‰) values, and extremely low ɛHf i (-12.7 to -9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low

  9. Features of Minerogenic Series Related to Continental Volcanic Rocks in the Southeastern Coastal Area of China-A Case Study of the Daiyunshan-Shiniushan Volcanic Depression in Fujian


    The Daiyunshan-Shiniushan volcanic depression in Fujian Province is situated in a volcanic belt of the southeast coastal area in China along the west Pacific Ocean. A new breakthrough has been made in the exploration of gold and silver ore deposits in recent years. The minerogenic series of the Daiyunshan-Shiniushan volcanic depression is discussed in this paper based on the analysis of major metallogenic types and factors. According to the study of enormous ore deposits and occurrences in the study area, two minerogenic series have been recognized: 1. Late Jurassic Au-Ag-Pb-Zn minerogenic series related to intermediate-acid, acid volcanic formations; 2. nonmetallic minerogenic series (pyrophyllite, alunite pearlite, andalusite, zeolite, corundum and so on) related to intermediate and acid volcanic formations. The division and study of the minerogenic series have revealed metallogenic and time-space distribution characteristics of the ore deposits in the volcanic belt of the southeast coastal area in China , which are of economic importance for metallogenic prognosis .

  10. Influences of diagenetic effects on volcanic rock reservoirs under two different sedimentary environments%两种不同沉积环境下火山岩储层成岩作用研究

    王盛鹏; 林潼; 孙平; 梁浩; 王东良; 苟红光


    According to the studies of rock types and reservoir space features of Carboniferous volcanic rocks in the Santanghu Basin, there were two kinds of environments for volcanic rock generation including subaerial and subaqueous in the study area. Based on core description, casting chip, scanning electron microscopy and element analysis, the generation features of volcanic rocks under the two different environments and the corresponding influences on reservoirs were discussed from the aspects of diagenetic effects. It was concluded that different diagenetic effects influenced volcanic reservoirs in different extents. The volcanic rocks which erupted and deposited on land in the study area were severely filled by zeolites and chlorites, and then improved obviously due to weathering and leaching. The extent of weathering and leaching was closely related to the improvement of reservoir. The volcanic reservoirs which deposited under water were improved owing to organic acid erosion. Primary pores were seldom found and a large amount of matrix dissolved holes generated. The generation mechanisms of subaqueous volcanic reservoirs pointed out new areas for explorations in the future.%基于三塘湖盆地石炭系火山岩储层的岩石类型、储集空间特征研究,识别出该区存在陆上喷发沉积和水下沉积2种环境.并通过岩心描述、铸体薄片和扫描电镜微观显示以及元素分析等手段,重点从成岩作用的角度对这2种环境下火山岩储层的发育特征及影响作用开展研究.研究认为,各种成岩作用对火山岩储层起到了不同程度的影响作用.该区陆上喷发沉积的火山岩受沸石和绿泥石填充较强烈,后期又遭受风化淋滤作用,使得储层发生明显的改善;风化强度与储层改善有着密切关系.水下沉积的火山岩储层改善主要受有机酸溶蚀作用影响,其原生孔隙不发育,但发育大量的基质溶孔.对水下沉积火山岩储层发育机理的认

  11. Strong Tectonic Rotation Evidenced by the Study of a Widespread Remagnetization in the Corbières Thrust Sheet (France)

    Rouvier, H.; Henry, B.; Le Goff, M.


    This study was carried out in the eastern part of the French Pyrénées where the tectonic structures, E-W oriented, bend northward at the Corbières virgation. This change affects the North-Pyrenean zone that overlains the foreland at the North Pyrenean thrust and its eastern extension, the Corbières thrust sheet. In this area, the Corbières thrust sheet is formed by the Mesozoic cover overlying down flat the Cenozoic formations of the Aude basin. The studied sites belongs to the North Pyrenean zone and to the Corbières thrust sheet and correspond to ages from Lias to Lower Cretaceous. Rock-magnetism analyses indicate that magnetite is the main magnetization carrier. Natural Remanent Magnetization mostly corresponds to a viscous component A and a component B determined during treatments from 300 to 400-500°C. Some samples also present a higher temperature component C, which cannot be isolated because of strong mineralogical alteration during thermal demagnetization at these temperatures or of too weak magnetization intensity. Dip variation in neighboring sites allows fold tests indicating synfolding remagnetization (i.e. acquired between or during different tectonic phases) for the component B. Direction of the latter has been then determined using small circles method for groups of neighboring sites. Previous study presented last year and made in the large Saint-Paul de Fenouillet syncline showed that western E-W part of the syncline is only locally affected by weak rotations while a large counterclockwise rotation affected all the eastern part of the syncline at the Corbières virgation during the Tertiary tectonic phase. New data have been acquired in numerous sites from the Corbières thrust sheet south from the Aude River. They point out that the different units of the Corbières thrust sheet also show large counterclockwise rotation. The emplacement of the the Corbières thrust sheet then appears as the result of a large movement of rotation of the North

  12. Geochronology and Petrogenesis of Intermediate-Basic Volcanic Rocks from Changling Depression in the Songliao Basin%松辽盆地长岭断陷中基性火山岩的时代与其成因

    温升福; 刘曼丽; 刘玮; 李瑞磊; 苟军; 武鹏飞; 王天豪; 柳小明; 孙德有


    锆石 U Pb 定年结果显示,松辽盆地长岭断陷松南180井中基性火山岩形成于101~116 Ma 的早白垩世晚期,属于营城组,非火石岭组火山岩。岩相学观察主要由安山岩和橄榄玄武岩组成,化学成分显示为玄武岩、粗面玄武岩和玄武质粗面安山岩,属碱性系列,镁质量分数较低,镁值较小(Mg#=0.27~0.53)。稀土元素总量较高(w (∑ REE)=(164.98~257.27)×10-6),轻重稀土分馏明显((La/Yb)N =6.60~10.96),铕异常微弱(δEu=0.85~1.02)。富集大离子亲石元素和轻稀土元素,Rb,K 相对亏损,相容元素(Cr、Co、Ni)质量分数低,高场强元素 Nb、Ta 弱富集,整体表现出与 OIB(洋岛玄武岩)一致的稀土图谱和微量元素特征。岩浆源区为软流圈地幔,经历了深部地幔流体的交代富集作用,岩浆未遭受地壳物质的混染。%Zircon U Pb dating results show that the intermediate-basic volcanic rocks from Changling depression in the Songliao basin formed at Early Cretaceous,rangingd from 101 Ma to 116 Ma,which belong to Yingcheng Formation rather than Huoshiling Formation.Petrologically,the volcanic rocks are composed dominantly of olivine basalt and andesite,but geochemical data show that the volcanic rocks belong to alkaline series and are composed mainly of basalt,trachybasalt and basaltic trachyandesite.The intermediate-basic volcanic rocks have low contents of MgO and small Mg#(0.27 0.53);The chondrite-normalized rare earth element (REE)patterns indicate that the volcanic rocks show significant fractionation of HREE and LREE [(La/Yb)N = 6.60 10.96]and weak Eu anoalies (δEu=0.85 1.02);The trace element geochemistry are characterized by enrichment in large ionic lithophile elements and LREEs,weak positive anomalies in Nb,Ta,depletion in compatible elements (Cr,Co,Ni)and Rb,K.All of the samples display highly consistent REE patterns and

  13. Mapping argillic and advanced argillic alteration in volcanic rocks, quartzites, and quartz arenites in the western Richfield 1° x 2 ° quadrangle, southwestern Utah, using ASTER satellite data

    Rockwell, Barnaby W.; Hofstra, Albert H.


    The Richfield quadrangle in southwestern Utah is known to contain a variety of porphyry Mo, skarn, polymetallic replacement and vein, alunite, and kaolin resources associated with 27-32 Ma calc-alkaline or 12-23 Ma bimodal volcano-plutonic centers in Neoproterozoic to Mesozoic carbonate and siliciclastic rocks. Four scenes of visible to shortwave-infrared image data acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor were analyzed to generate maps of exposed clay, sulfate, mica, and carbonate minerals, and ASTER thermal infrared data were analyzed to identify quartz and carbonate minerals. Argillic and advanced argillic alteration minerals including alunite, pyrophyllite, dickite, and kaolinite were identified in both undocumented (U) and known (K) areas, including in the southern Paradise Mtns. (U); in calc-alkaline volcanic rocks in the Wah Wah Mtns. between Broken Ridge and the NG area (U/K); at Wah Wah Summit in a small zone adjacent to 33.1 Ma diorite and marble (U); in fractures cutting quartzites surrounding the 20-22 Ma Pine Grove Mo deposit (U); in volcanic rocks in the Shauntie Hills (U/K); in quartzites in the west-central San Francisco Mtns. (U); in volcanic rocks in the Black Mtns. (K); and in mainly 12-13 Ma rhyolitic rocks along a 20 km E-W belt that includes the Bible Spring fault zone west of Broken Ridge, with several small centers in the Escalante Desert to the south (U/K). Argillized Navajo Sandstone with kaolinite and (or) dickite ± alunite was mapped adjacent to calc-alkaline intrusions in the Star Range (U). Intense quartz-sericite alteration (K) with local kaolinite was identified in andesite adjacent to calc-alkaline intrusions in the Beaver Lake Mountains. Mo-bearing phyllic alteration was identified in 22.2 Ma rhyolite plugs at the center of the NG alunite area. Limestones, dolomites, and marbles were differentiated, and quartz and sericite were identified in most unaltered quartzites. Halos of

  14. 再生火山岩骨料混凝土抗压强度及干缩性能试验%Experimental study on compressive strength and dry-shrinkage property of recycled volcanic rock coarse aggregate concrete


    A recycled volcanic rock coarse aggregate concrete is developed,which the aggregate replacement rate is 100%. The compressive strength and dry-shrinkage performance of recycled volcanic rock aggregate concrete are studied. The results show that the compressive strength of recycled concrete after curing 28 days than that of normal concrete is reduced by about 13%,and the dry-shrinkage strain is about 1. 25 times of the reference concrete.%配置了骨料取代率为100%的再生火山岩粗骨料混凝土,并对其抗压强度和干缩性能进行试验研究,结果表明:再生火山岩混凝土28 d抗压强度比基准混凝土抗压强度降低约13%;干燥收缩应变约为基准混凝土的1.25倍。

  15. Remagnetizations used to unravel large-scale fold kinematics: A case study in the Cameros Basin (Northern Spain)

    García-Lasanta, C.; Casas-Sainz, A.; Villalaín, J. J.; Oliva-Urcia, B.; Mochales, T.; Speranza, F.


    The occurrence of a generalized remagnetization at 100 Ma makes the Cameros Basin, an 8 km thick Cretaceous extensional trough inverted during the Cenozoic, a perfect natural frame to apply paleomagnetic vectors to geometrical reconstructions. The widespread remagnetization that occurred between the extensional and compressional episodes, linked to low-grade metamorphism, provides a tool to reconstruct the attitude of beds at the remagnetization time, thus giving a picture of basin geometry during the Cretaceous. This snapshot is compared with the present-day geometry to constrain the large-scale kinematic evolution of folds between these two stages. According to this methodology, a syncline, preinversion geometry of the sedimentary basin was determined and the position of its main axial surface was accurately located. Comparing with the present-day, postinversion geometry, a northward hinge migration of around 5 km is inferred. This migration is the result of the southward back thrusting in the southern basin border, favored by the detachment level at the base of the Mesozoic cover. Conversely, the main northward directed thrust, which involved both the Paleozoic basement and the Mesozoic cover, did not significantly affect the internal structure of the basin in spite of its overall displacement of more than 20 km.



    A comparative study of major elements, trace elements, and isotopes in high- and moderate-Mg volcanic sequences of 16–14 and 14–13 Ma, respectively, has been performed in the Bereya volcanic center. In the former (small volume) sequence, contaminated by crustal material basalts and trachybasalts of K–Na series were followed by uncontaminated basanites and basalts of transitional (K–Na–K) compositions and afterwards by picrobasalts and ba­salts of K series. From pressure estimates using equati...

  17. U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: constraints on the age of volcanism and metamorphism

    Christopher L. Kirkland


    Full Text Available Zircons from a felsic volcanic rock in the Toija Formation and a synvolcanic gabbro intrusion in the Salittu Formation within the Orijärvi area were dated by U-Th-Pb SIMS in order to provide depositional constraints on these formations. Zircon crystals from the felsic rock preserve a two-stage crystallisation history with zoned core domains and homogeneous rim domains. Inner domains yield a 1878±4 Ma concordia age, interpreted to determine the crystallisation of this rock. Rims yield a 1815±3 Ma concordia age interpretedto determine the regional metamorphism. Small rounded zircon grains from the Salittu gabbro, located within the Jyly shear zone, yield a concordia age of 1792±5 Ma. We interpret the grain textures to suggest that they recrystallised from inherited zircon seeds during the heat and fluid flow into the shear zone. Although no direct ages for the Salittu Formation have been recovered, field relationships imply that it was deposited between 1878−1875 Ma.

  18. Catastrophic volcanism

    Lipman, Peter W.


    Since primitive times, catastrophes due to volcanic activity have been vivid in the mind of man, who knew that his activities in many parts of the world were threatened by lava flows, mudflows, and ash falls. Within the present century, increasingly complex interactions between volcanism and the environment, on scales not previously experienced historically, have been detected or suspected from geologic observations. These include enormous hot pyroclastic flows associated with collapse at source calderas and fed by eruption columns that reached the stratosphere, relations between huge flood basalt eruptions at hotspots and the rifting of continents, devastating laterally-directed volcanic blasts and pyroclastic surges, great volcanic-generated tsunamis, climate modification from volcanic release of ash and sulfur aerosols into the upper atmosphere, modification of ocean circulation by volcanic constructs and attendent climatic implications, global pulsations in intensity of volcanic activity, and perhaps triggering of some intense terrestrial volcanism by planetary impacts. Complex feedback between volcanic activity and additional seemingly unrelated terrestrial processes likely remains unrecognized. Only recently has it become possible to begin to evaluate the degree to which such large-scale volcanic processes may have been important in triggering or modulating the tempo of faunal extinctions and other evolutionary events. In this overview, such processes are examined from the viewpoint of a field volcanologist, rather than as a previous participant in controversies concerning the interrelations between extinctions, impacts, and volcanism.

  19. 赣东北晚白垩世橄榄玄粗岩(Shoshonite)系列火山岩厘定的地质证据%Shoshonite Series Volcanic Rocks in Northern Jiangxi Province

    项媛馨; 巫建华; 余达淦; 刘帅


    赣东北广泛发育晚白垩世红色沉积盆地,并常伴有镁铁质火山活动.位于东乡-广丰断裂带南侧的广丰盆地产有上、下两套镁铁质火山岩,而位于该断裂带北侧的玉山盆地产有一套镁铁质火山岩.研究表明,广丰盆地上部的镁铁质火山岩和玉山盆地的镁铁质火山岩均呈块状构造,多气孔,气孔内常充填硅质、碳酸盐、绿泥石;斑晶矿物含量为15%左右,以斜长石和单斜辉石为主,并有少量橄榄石和斜方辉石,斜长石斑晶常见钾长石环边;基质为似粗面结构,斜长石定向排列,也具有钾长石环边;岩石SiO2含量为49.8% ~53.6%(平均51.7%),富碱(Na2O+ K2O为5.00% ~ 6.94%,平均6.35%)、富钾(K2O为2.40% ~ 3.99%,平均2.98%)、K2O/Na2O(0.68 ~ 1.08,平均0.79)比值高,在SiO2 -( Na2O+K2O)图解上落入碱性系列范围,在SiO2 - K2O和SiO2 - K2O/Na2O图解上均落入橄榄玄粗岩范围;全铁含量(7.10%~9.99%,平均8.26%)低,但具有高的Fe2 O3/FeO比值,在AFM图解中具有钙碱性系列特征,在MgO -∑Fe图解上落入钙碱性系列趋势范围内,且大多数位于Lesser Antilles( CA-1)和Cascades( CA-2)两钙碱性趋势线之间,表现出亲钙碱性特征;其地球化学总体特征与典型橄榄玄粗岩系列一致.本文通过对矿物学和地球化学这两方面特征的总结,厘定其为橄榄玄粗岩系列火山岩.%The red sedimentary basin was appeared extensively with large scare of mafic volcanic rocks in Late Cretaceous. Guangfeng basin has two sets of mafic volcanic rocks, which located in the south of Dongxiang-Guangfeng fault line. Yushan basin has one set of mafic volcanic rocks, which located in the north of Dongxiang-Gua.ngfeng fault line. Results show that the mafic volcanic rocks are lump structure, and filled silicious, carbonate and chlorite in many blowholes in Guangfeng and Yushan basins. The content of its phenocrysts is about 15% , the main

  20. Miocene rifting in the Los Angeles basin: Evidence from the Puente Hills half-graben, volcanic rocks, and P-wave tomography

    Bjorklund, Tom; Burke, Kevin; Zhou, Hua-Wei; Yeats, Robert S.


    Formation of the Puente Hills half-graben in the northeastern Los Angeles basin and eruption of the Glendora and El Modeno Volcanics (16 14 Ma) help to define the timing of extension in the basin. Normal faulting on the proto-Whittier fault ca. 14 Ma established the Puente Hills half-graben, in which sedimentary strata accumulated between ca. 14 and 10 Ma and into which diabase sills intruded. North-South contraction began to invert the Puente Hills half-graben ca. 7 Ma, leading to formation of the Puente Hills anticline and the Whittier fault. Our high-resolution three-dimensional P-wave velocity model shows two anomalous higher velocity (6.63 km/s) bodies at depths between 9 and 18 km, which we attribute to dioritic plutons named here for Whittier Narrows and El Modeno. The stocklike Whittier Narrows pluton could have been a source for the Glendora Volcanics and the diabase sills in the Puente Hills half-graben. The sill-shaped El Modeno pluton was a likely source for the El Modeno Volcanics. The northwesterly alignment of the plutons may mark the location of the northeastern Los Angeles basin rift boundary, which is associated with the clockwise rotation of the western Transverse Ranges. Three active faults, the Elysian Park blind thrust, the Puente Hills blind thrust, and the Whittier fault, converge on the Whittier Narrows pluton, which may have played a role in their location and segmentation.

  1. Natural factors and mining activity bearings on the water quality of the Choapa basin, North Central Chile: insights on the role of mafic volcanic rocks in the buffering of the acid drainage process.

    Parra, Amparo; Oyarzún, Jorge; Maturana, Hugo; Kretschmer, Nicole; Meza, Francisco; Oyarzún, Ricardo


    This contribution analyzes water chemical data for the Choapa basin, North Central Chile, for the period 1980-2004. The parameters considered are As, Cu Fe, pH, EC, SO₄⁻², Cl⁻¹, and HCO[Formula: see text], from samples taken in nine monitoring stations throughout the basin. Results show rather moderate contents of As, Cu, and Fe, with the exception of the Cuncumén River and the Aucó creek, explained by the influence of the huge porphyry copper deposit of Los Pelambres and by the presence of mining operations, respectively. When compared against results obtained in previous researches at the neighboring Elqui river basin, which host the El Indio Au-Cu-As district, a much reduced grade of pollution is recognized for the Choapa basin. Considering the effect of acid rock drainage (ARD)-related Cu contents on the fine fraction of the sediments of both river basins, the differences recorded are even more striking. Although the Los Pelambres porphyry copper deposit, on the headwaters of the Choapa river basin, is between one and two orders of magnitude bigger than El Indio, stream water and sediments of the former exhibit significantly lower copper contents than those of the latter. A main factor which may explain these results is the smaller degree of H( + )-metasomatism on the host rocks of the Los Pelambres deposit, where mafic andesitic volcanic rocks presenting propylitic hydrothermal alteration are dominant. This fact contrast with the highly altered host rocks of El Indio district, where most of them have lost their potential to neutralize ARD.

  2. Quaternary basaltic volcanism in the Payenia volcanic province, Argentina

    Søager, Nina

    The extensive Quaternary volcanism in the Payenia volcanic province, Mendoza, Argentina, is investigated in this study by major and trace element analyses, Sr, Nd, Hf and Pb-isotopic analyses and Zr-Hf isotope dilution data on samples from almost the entire province. The samples are mainly...... in basalts from all the studied volcanic fields in Payenia is signs of lower crustal contamination indicating assimilation of, in some cases, large amounts of trace element depleted, mafic, plagioclase-bearing rocks. The northern Payenia is dominated by backarc basalts erupted between late Pliocene to late...

  3. 满洲里南部塔木兰沟组火山岩年代学与地球化学%Chronology and Geochemistry of Volcanic Rocks in Tamulangou Formation from Southern Manchuria, Inner-Mongolia

    赵忠华; 孙德有; 苟军; 任云生; 付长亮; 张学元; 王晰; 柳小明


    The volcanic rocks of Tamulangou Formation in southern Manchuria are composed mainly of olivine basalt, pyroxene basalt and basaltic andesite. LA -ICP -MS zircon U -Pb dating results show that the volcanic rocks formed at the end of Middle Jurassic, ranged from 164 Ma to 161 Ma.Geochemical data show that the volcanic rocks belong to high-K calc-alkaline series, have low contents of MgO(l. 64%- 9. 59%) and small Mg#(0. 26 - 0. 66)5 The chondrite-normalized rare earth element (REE) patterns indicate that the volcanic rocks are enriched in light rare earth elements(LREE) with significant fractionation of HREE and LREE ((La/Yb)N = 6. 86 - 22. 29) and weak Eu anoalies(δEu = 0.70-0.93); The trace element geochemistry are characterized by enrichment in large ionic lithophile elements(e. G. Rb, Ba, K) and LREEs, depletion in high field strength elements(e. G. Nb, Ta, P, Ti) and compatible elements (Cr, Co, Ni); The initial 176 Hf/177 Hf ratios of the magmatic zircons from volcanic rocks vary from 0. 282 691 to 0. 282 740, and the corresponding eHf(t) values range from 0. 67 to 2. 30. The geochemical data suggest that the magma could be derived from partial melting of lithosphere mantle which had been metasomatized by subduction fluids, and probably experienced fractional crystallization of olivine and clinopyroxene,and insignificant crustal contamination. Combined with the regional research results, we suggest that the metasomatic fluids could be originated from the oceanic crust of Mongol - Okhotsk Ocean, and these volcanic rocks formed under a lithospheric extension after the closure of Mongol - Okhotsk Ocean.%满洲里南部塔木兰沟组火山岩主要由橄榄玄武岩、辉石玄武岩和玄武安山岩组成.LA - ICP -MS锆石U- Pb定年结果显示,火山岩形成于164~161 Ma的中侏罗世末期.岩石主体属高钾钙碱性系列,镁含量较低[w( MgO)=1.64%~9.59%],镁指数较小(Mg#=0.26~0.66);稀土元素配分模式呈轻稀土富集右倾型,

  4. Petrology and geochemistry of volcanic rocks from the island of Panarea: implications for mantle evolution beneath the Aeolian island arc (southern Tyrrhenian sea)

    Calanchi, N.; Peccerillo, A.; Tranne, C. A.; Lucchini, F.; Rossi, P. L.; Kempton, P.; Barbieri, M.; Wu, T. W.


    Major, trace element and radiogenic isotope (Sr, Nd, Pb) data are reported for a suite of rocks from the Panarea volcano, a large structure that is largely hidden below sea level and outcrops only as a group of small islands between Lipari-Vulcano and Stromboli in the eastern Aeolian arc. The exposed rocks mostly consist of high-potassium calc-alkaline (HKCA) andesites, dacites and some rhyolites; shoshonitic basalts have been collected from submarine centres; mafic calc-alkaline (CA) rocks occur as thin layers of late-erupted strombolian scoriae. Major and trace element data are scattered, but define generally linear trends on inter-element diagrams; Sr-isotope ratios do not display significant increase with evolution, although rough positive trends of 87Sr/ 86Sr versus SiO 2 and Rb/Sr can be recognised within some units. The mafic rocks display varying enrichment in potassium, from CA to shoshonitic compositions, and are characterised by variable abundances of incompatible trace elements, which increase with potassium. There is an increase of 87Sr/ 86Sr ratios and a decrease of 143Nd/ 144Nd and 206Pb/ 204Pb ratios from CA to HKCA and shoshonitic mafic rocks. The scattered and incomplete nature of the outcrops make it difficult to constrain magmatic evolution at Panarea; geochemical and isotopic data suggest that AFC and mixing were important evolutionary processes. However, geochemical modelling does not support the possibility that the first-order compositional variations observed in the mafic rocks are the result of these processes, and suggests a genesis in a heterogeneous mantle source. Recent studies have highlighted strong differences in terms of incompatible trace element ratios and isotopic signatures, between the western-central and the eastern Aeolian arc. Rocks from the western islands (Alicudi, Filicudi, Salina, Vulcano) have typical magmatic arc geochemical signatures and relatively unradiogenic Sr-isotope compositions. By contrast, the eastern

  5. Geochemistry and Tectonic Setting of Late Triassic Volcanic Rocks in Reshui Area,Qinghai%青海热水地区晚三叠世火山岩地球化学特征及构造环境

    李运冬; 刘小玉


    青海热水地区位于秦祁昆三大造山带的结合部位,该区发育晚三叠世鄂拉山组火山岩。岩石类型以英安岩及英安质火山碎屑岩为主,其次为中性的安山质火山岩和酸性的流纹质火山岩,为一套高 K、低 Ti、准铝质的钙碱性系列火山岩。SiO2含量为59.62%~76.46%,总碱含量 ALK 为6.59%~8.04%,ANK 在1.14~1.80,里特曼系数σ在1.72~2.41,岩石矿物结晶分离程度高,但固结程度一般。稀土总量 REE在126.76×10-6~240.07×10-6,LREE/HREE均值为10.19,属轻稀土富集型,且轻稀土富集程度高于重稀土富集程度,δEu 值多在0.37~0.56亏损较为明显,稀土元素标准化配分曲线均呈右倾较缓的平滑曲线。大离子半径亲石元素 Rb、Ba、U、K、Th较为富集,高场强元素 Nb、Ta、Ti、P等较为亏损,原始地幔标准化配分曲线呈明显左高右低的“W”型,具有明显的弧火山岩特点。Mg#大部分在16~36,206 Pb/204 Pb=18.181~18.218,207 Pb/204 Pb=15.570~15.588,208 Pb/204 Pb=38.368~38.529,显示岩浆来源于下地壳。该区火山岩与 A型俯冲构造活动有关,利用 Hf/3-Th-Nb/16等构造环境判别图解分析,其形成于陆缘弧环境。%Reshui area is located in the binding site of Qinling,Qilianshan and Kunlunshan orogenic belt, and the late Triassic volcanic rock developed in the area is part of Elashan formation.As a set of volcano rocks with high K content,low Ti content and metaluminous calc-alkaline characteristics,the late Triassic rock are mainly consisted of dacite and dacitic pyroclastic rock,followed by neutral andesitic volcano rock and acidic rhyolitic volcano rock.The content of SiO2 is between 59.62% and 76.46%,the content of ALK ranges from 6.59% to 8.04%,the content of ANK is from 1.14 to 1.80,and the value of Rittmann coefficient varies between 1.72 and 2.41,the degree of rock mineral fractionation crystallization is high,but the degree of consolidation is common.The total of REE is between 126.76

  6. Geochemistry of Volcanic Rocks from International Ocean Discovery Program (IODP) Site 1438, Amami Sankaku Basin: Implications for Izu-Bonin-Mariana (IBM) Arc Initiation

    Hickey-Vargas, R.; Ishizuka, O.; Yogodzinski, G. M.; Bizimis, M.; Savov, I. P.; McCarthy, A. J.; Arculus, R. J.; Bogus, K.


    IODP Expedition 351 drilled 150 m of volcanic basement overlain by 1461 m of sedimentary material at Site 1438 in the Amami Sankaku basin, just west of the Kyushu Palau Ridge, the locus of IBM arc initiation. Age interpretations based on biostratigraphy (Arculus et al., Nat. Geosci., in-press) determined that the age of the basement section is between 64 and 51 Ma, encompassing the age of the earliest volcanic products of the IBM arc. The Site 1438 volcanic basement consists of multiple flows of aphyric microcrystalline to finely crystalline basalts containing plagioclase and clinopyroxene with rare olivine pseudomorphs. New XRF major and ICPMS trace element data confirm findings of shipboard analysis that the basalts are moderately differentiated (6-14 % MgO; Mg# = 51-83; 73-490 ppm Cr and 58-350 ppm Ni) with downcore variations related to flow units. Ti/V and Ti/Sc ratios are 16-27 and 75-152, respectively, with lowest values at the base of the core. One prominent characteristic of the basalts is their depletion of immobile highly incompatible elements compared with MORB. Basalts have MORB-normalized La/Nd of 0.5 to 0.9, and most have Th/La andesites from three sills in the lowermost sedimentary unit have arc-like trace element patterns with La/Nb > 3 and primitive mantle normalized La/Yb > 1. Our results suggest that mantle melting at the onset of subduction involved exceptionally depleted sources. Enrichment over time may be related to increasing subduction inputs and/or other processes, such as entrainment of fertile asthenosphere during extension of the overriding plate.

  7. Volcanism on Mars. Chapter 41

    Zimbelman, J. R.; Garry, W. B.; Bleacher, J. E.; Crown, D. A.


    Spacecraft exploration has revealed abundant evidence that Mars possesses some of the most dramatic volcanic landforms found anywhere within the solar system. How did a planet half the size of Earth produce volcanoes like Olympus Mons, which is several times the size of the largest volcanoes on Earth? This question is an example of the kinds of issues currently being investigated as part of the space-age scientific endeavor called "comparative planetology." This chapter summarizes the basic information currently known about volcanism on Mars. The volcanoes on Mars appear to be broadly similar in overall morphology (although, often quite different in scale) to volcanic features on Earth, which suggests that Martian eruptive processes are not significantly different from the volcanic styles and processes on Earth. Martian volcanoes are found on terrains of different age, and Martian volcanic rocks are estimated to comprise more than 50% of the Martian surface. This is in contrast to volcanism on smaller bodies such as Earth's Moon, where volcanic activity was mainly confined to the first half of lunar history (see "Volcanism on the Moon"). Comparative planetology supports the concept that volcanism is the primary mechanism for a planetary body to get rid of its internal heat; smaller bodies tend to lose their internal heat more rapidly than larger bodies (although, Jupiter's moon Io appears to contradict this trend; Io's intense volcanic activity is powered by unique gravitational tidal forces within the Jovian system; see "Volcanism on Io"), so that volcanic activity on Mars would be expected to differ considerably from that found on Earth and the Moon.


    I. S. Chuvashova


    Full Text Available A comparative study of major elements, trace elements, and isotopes in high- and moderate-Mg volcanic sequences of 16–14 and 14–13 Ma, respectively, has been performed in the Bereya volcanic center. In the former (small volume sequence, contaminated by crustal material basalts and trachybasalts of K–Na series were followed by uncontaminated basanites and basalts of transitional (K–Na–K compositions and afterwards by picrobasalts and ba­salts of K series. From pressure estimates using equation [Scarrow, Cox, 1995], high-Mg magma originated at the deep range of 115–150 km. In the latter (high-volume sequence, basalts and basaltic andesites of transitional (Na–K–Na compositions and basalts of Na series were overlain by basalts and trachybasalts of K–Na series. First, there was a strong melting of its shallow garnet-free part with coeval weak melting of more deep garnet-bearing portion, then only a deep garnet-bearing portion of the lithospheric mantle melted. It is suggested that the sequential formation of high- and moderate-Mg melts reflected the mid-Miocene thermal impact of the lithosphere by hot material from the Transbaikalian low-velocity domain, which had the potential temperature Tp as high as 1510 °С. This thermal impact triggered the rifting in the lithosphere of the Baikal Rift System.

  9. Geochemistry and petrogenesis of Early Cretaceous volcanic rocks from Mengyin basin%蒙阴盆地早白垩世火山岩地球化学特征及其岩石成因

    牛漫兰; 傅朋远; 吴齐; 朱光; 夏文静


    蒙阴盆地早白垩世火山岩主要出露于青山群八亩地组和方戈庄组中.火山岩SiO2含量为53.71% ~ 69.08%,主要为安山质岩石和流纹岩,以钙碱性系列为主(σ平均3.42);富集Rb、Ba等大离子亲石元素和轻稀土元素,相对亏损Nb、Ta、P、Ti等高场强元素和重稀土元素;具有富集的Sr-Nd-Pb同位素组成(87Sr/86Sr(t)=0.7051~ 0.7110,εNd(t)=-5.16~-16.4).根据MgO含量和Mg#值,可将安山质火山岩划分为高镁(Mg #>60)和低镁(Mg#<60)两个系列.高镁安山质岩石的MgO、Cr、Ni及不相容元素的含量明显高于低镁安山质岩石,且具有相对高的87Sr/86Sr(t)和较低的εNd(t)值,Sr-Nd同位素组成趋向于EMⅡ型富集地慢端元.低镁安山质岩石地球化学特征类似于埃达克岩,其同位素组成介于EM Ⅰ型富集地幔端元与华北麻粒岩相下地壳之间.流纹岩具有较低的MgO、Cr、Ni含量和明显的负Eu、Sr异常,同位素组成类似于扬子中、上地壳.研究表明,蒙阴盆地高镁安山质岩石由曾遭受俯冲扬子陆壳熔体改造的华北岩石圈地幔部分熔融形成,且岩浆在上升过程中经历了华北下地壳物质一定程度的混染或混合作用;低镁安山质岩石主要由华北下地壳部分熔融形成;流纹岩为岩石圈伸展减薄背景下仰冲至华北克拉通之上的扬子中、上地壳部分熔融的产物.地质现象显示,郯庐断裂带的伸展活动控制着蒙阴断陷盆地的发育,与蒙阴盆地早白垩世火山岩的成因有着密切的联系,在华北克拉通破坏中可能起到了重要的作用.%Early Cretaceous volcanic rocks in Mengyin basin are exposed in Bamudi and Fanggezhuang formations. These rocks are mainly andesitic rocks and rhyolites with SiO2 contents varied between 53.71% ~69. 08% , belong to calc-alkaline to alkaline series ( with a average of σ = 3. 42). These volcanic rocks are characterized by large ion lithophile element ( LILE) , light

  10. Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo-Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

    Khalaf, E. A.; Abdel Motelib, A.; Hammed, M. S.; El Manawi, A. H.


    This paper describes the Neogene lava-sediment mingling from the Abu Treifiya Basin, Cairo-Suez district, Egypt. The lava-sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area. Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava-sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma

  11. Chemo-probe into the mantle origin of the NW Anatolia Eocene to Miocene volcanic rocks: Implications for the role of, crustal accretion, subduction, slab roll-back and slab break-off processes in genesis of post-collisional magmatism

    Ersoy, E. Yalçın; Palmer, Martin R.; Genç, Ş. Can; Prelević, Dejan; Akal, Cüneyt; Uysal, İbrahim


    Post-collisional Cenozoic magmatic activity in NW Anatolia produced widespread volcanism across the region. In the Biga Peninsula, in the west, medium-K calc-alkaline to ultra-K rocks with orogenic geochemical signature were emplaced at 43-15 Ma (Biga orogenic volcanic rocks; BOVR). Volcanic activity in the Central Sakarya region, to the east, is mainly restricted to 53-38 Ma, but also continued during the Early Miocene with small basaltic extrusives (Sakarya orogenic volcanic rocks; SOVR). This study presents a new set of geochemical data (whole rock major and trace elements and Sr-Nd-Pb isotopic compositions), obtained from the Cenozoic calc-alkaline volcanic rocks from these two regions. While there is considerable overlap in the emplacement time of volcanism in the two areas, the post-collisional volcanic rocks of these two regions differ in terms of their geochemical compositions: (1) the BOVR show an age-dependent increase in K and other large-ion lithophile elements (LILE), coupled with an increase in radiogenic Sr and Pb compositions from the Eocene to Miocene; whereas (2) the SOVR are characterized by more sodic compositions with lower K and less radiogenic Sr contents with respect to the BOVR, which were unchanged in Eocene and Miocene. We conclude that these geochemical features were principally related to the distinct modes of subduction-related mantle enrichment processes. We suggest that the Eocene to Miocene progressive enrichment in the BOVR mantle was related to successive subduction of oceanic and crustal materials in the western Aegean, while the SOVR mantle was dominantly enriched during the pre-collisional events. Magma generation in the western region was related to subduction roll-back processes associated with post-collisional extension. In the east, thermal perturbation of the mantle in response to asthenospheric upwelling due to slab break-off process was responsible for the magma generation. The time-dependent increase of K (and other

  12. Effect of petrophysical properties and deformation on vertical zoning of metasomatic rocks in U-bearing volcanic structures: A case of the Strel'tsovka caldera, Transbaikal region

    Petrov, V. A.; Andreeva, O. V.; Poluektov, V. V.


    The development of vertical zoning of wall-rock metasomatic alteration is considered with the Mesozoic Strel'tsovka caldera as an example. This caldera hosts Russia's largest uranium ore field. Metasomatic rocks with the participation of various phyllosilicates, carbonates, albite, and zeolites are widespread in the ore field. In the eastern block of the caldera, where the main uranium reserves are accommodated, hydromica metasomatic alteration gives way to beresitization with depth. Argillic alteration, which is typical of the western block, is replaced with hydromica and beresite alteration only at a significant depth. Postore argillic alteration is superposed on beresitized rocks in the lower part of the section. Two styles of vertical metasomatic zoning are caused by different modes of deformation in the western and eastern parts of the caldera. Variations of the most important petrophysical properties of host rocks—density, apparent porosity, velocities of P- and S-waves, dynamic Young's modulus, and Poisson coefficient—have been determined by sonic testing of samples taken from different depths. It is suggested that downward migration of the brittle-ductile transition zone could have been a factor controlling facies diversity of metasomatic rocks. Such a migration was caused by a new phase of tectonothermal impact accompanied by an increase in the strain rate or by emplacement of a new portion of heated fluid. Transient subsidence of the brittle-ductile boundary increases the depth of the hydrodynamically open zone related to the Earth's surface and accelerates percolation of cold meteoric water to a greater depth. As a result, the temperature of the hydrothermal solution falls down, increasing the vertical extent of argillic alteration. High-grade uranium mineralization is also localized more deeply than elsewhere.

  13. Neogene felsic volcanic rocks in the Hoggar province: Volcanology, geochemistry and age of the Azrou trachyte-phonolite association (Algerian Sahara)

    Ben El Khaznadji, Riad; Azzouni-Sekkal, Abla; Benhallou, Amel; Liégeois, Jean-Paul; Bonin, Bernard


    The Azrou volcanic district, located to the south-east to the Atakor district in the Hoggar, has a landscape is governed by a number of felsic volcanic highs and dissected mafic plateau lavas. Our new Rb-Sr age (i.e. 23.1 ± 1.6 Ma) indicates that the Azrou felsic lavas are contemporaneous with the Achkal ring complexes (Anahef region). The Azrou felsic lavas (mainly trachyte and phonolite) show remarkably homogeneous compositions both in major elements (57.5 ≤ SiO2≤ 63.1 wt%; 10.8 ≤(Na2O + K2O)≤12.4 wt%), trace elements (33.2 ≤ Th ≤ 107 ppm; 170 ≤ La≤472 ppm; 8.7<(La/Yb)N < 27.3) and radiogenic isotopes (0.703359 < 87Sr/86Sr < 0.706539; 0.512727 <143Nd/144Nd < 0.512925; 2<εNd <5.84. These data indicate that the lavas have been only very weakly contaminated by the Precambrian basement. Geodynamically, this genesis coupled with the low volume of both trachytic and phonolitic trends implies the reworking of pre-existing shear-zones allowing the rapid ascent of these small batches of magmas. This is in agreement with the general model of linear delamination along these shear zones due to the Africa-Europe convergence developed by Liégeois et al. (2005) and recently imaged by the magneto-telluric investigation of Bouzid et al. (2015).

  14. Biogeochemistry of REE elements and tetrad effect in the soil-plant system: a study on volcanic rock covers in southernmost Brazil

    Maria do Carmo Lima e Cunha


    Full Text Available This paper deals with the distribution of REE in rock, soil and plant in an area of monzonitic rocks from southernmost Brazil. The REE patterns in Schinus lensticifolius show a negative-Ce anomaly and a prominent tetrad effect, characterized as W-type that are not present in rock and soil samples. The REE patterns in the soils and rocks sampled are very similar and there is no fractionation of REE during the processes of soil formation. The W-type patterns are interpreted as indicating that REE were absorved by S. lentiscifolius as simple ions rather than as complex ions, or, alternatively, that the transport of REE in the plant metabolic processes was as free ions. The recognition of tetrads, either, M- or W-type patterns, is an additional tool for understanding the biogeochemistry of REE and can contribute to the study of monitoring processes of contaminated environment or to mineral prospecting.Este trabalho trata da distribuição dos ETR na rocha, solo e planta em área de ocorrência de rochas monzoníticas do extremo sul do Brasil. O padrão dos ETR em Schinus lentiscifolius apresenta anomalia negativa de Ce e significativo efeito tétrade, do tipo W, ausente no padrão da rocha e do solo. A configuração das curvas da rocha e do solo é similar e sem fracionamento das ETR durante a pedogênese. O padrão em W é interpretado como decorrente da absorção dos ETR pela planta na forma de íons livres e não complexados, ou, alternativamente, que o transporte das ETR nos processos metabólicos foi na forma de íons livres. O reconhecimento de tétrades, seja do tipo W ou M, é uma ferramenta adicional na compreensão da biogeoquímica dos ETR e pode contribuir para o estudo de processos de monitoramento de ambientes contaminados ou para pesquisas em prospecção mineral.

  15. 基于三相孔隙弹性理论的火山岩气层岩石物理响应特征分析%Analysis of rock physics response of gas-bearing volcanic reservoir based on three-phase poroelastic theory

    吴清岭; 赵海波; 李来林; 范兴才


    Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated porous media. Considering reservoir reference pressure and coupling drag of two fluids in pores, the effects of frequency, porosity, and gas saturation on the phase velocities of the P- and S-waves are discussed in detail under field conditions. The effects of porosity and gas saturation on Vp/Vs are also provided. The data for our numerical experiments are from a sample of deep volcanic rock from Daqing. The numerical results show that the frequency dispersion effect can be ignored for deep volcanic rocks with low porosity and low permeability. It is concluded that for deep volcanic rocks the effect of gas content in pores on Vp/Vs is negligible but the effect of porosity is significant when there is a certain amount of water contained in the pores. The accurate estimate of lithology and porosity in this case is relatively more important.

  16. Recognition method for volcanic rock facies by well logging%利用测井方法识别火山岩岩相方法

    张程恩; 潘保芝; 王飞; 边会媛; 韩雪; 陈刚


    利用实际岩石薄片数据及测井数据,通过GR-TH及GR-AC交会图方法将松南气田营城组岩石 结构划分为熔岩结构、熔结结构和凝灰结构3种.凝灰结构和熔结结构对应爆发相,熔岩结构对应喷溢相.结合次生孔隙度的大小及岩心、录井信息对出现的6种亚相:热碎屑流亚相、热基浪亚相、空落亚相、上部亚相、中部亚相和下部亚相进一步划分,其结果与地质上的岩相亚相基本一致.%Using actual rock slices and logging data, the rock structures of Yingcheng Formation in Songnan gas field was divided into lava, welding and tuff structures by the cross plots of GR-TH and GR-AC. Tuff and welding structures correspond to explosive facies, while lava structure corresponds to extrusive facies. The six different subfacies (thermal debris flow, thermal base surge, airfall, upper, middle and lower subfacies) recognized by secondary porosity, core and logging data show a high accuracy compared with the actual geological results.

  17. Monazite geochronology and geothermobarometry in polymetamorphic host rocks of volcanic-hosted massive sulphide mineralizations in the Mesoproterozoic Areachap Terrane, South Africa

    Bachmann, Kai; Schulz, Bernhard; Bailie, Russell; Gutzmer, Jens


    The Areachap Terrane represents the medium-to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1240-1300 Ma) volcanic arc bound to the margin of the Archean Kaapvaal Craton in the east, and the polydeformed and metamorphosed Proterozoic Namaqua Province in the west. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of this terrane, adjacent to an important Mesoproterozoic crustal suture between the Archean Kaapvaal Craton and the Namaqua Province, which developed during the ∼1200-1000 Ma Namaquan Orogeny. The Areachap Terrane comprises highly deformed bimodal volcanic and volcano sedimentary successions that host a number of massive sulphide base metal orebodies. Samples from three of these orebodies at different locations were analysed to determine the age and P-T conditions of metamorphism along the Areachap Terrane. Metamorphic ages were determined by electron microprobe chemical dating of monazite. Garnet- and amphibole-bearing mineral assemblages were used for geothermobarometry at the Areachap Mine, located in the northern sector of the Areachap Terrane, monazite geochronology yields evidence for two populations of Th-U-Pb-ages at 1432 ± 30 Ma - a possible protolith age - and a metamorphic age of 1153 ± 21 Ma. Kantienpan and Copperton, representing the central and southern sector of the Areachap Terrane respectively, yield monazite ages for a younger metamorphic event with U-Th-Pb-ages of 1108 ± 19 Ma and 1104 ± 17 Ma, respectively. Geothermobarometric data give a differentiated view on the metamorphic evolution of the Areachap Terrane. An arc consistent clockwise P-T evolution path and upper amphibolite-facies peak metamorphic conditions are consistent at the three locations. The Areachap site shows a short prograde development with 8.0 kbar maximum pressure at circa 700 °C maximum temperature and a subsequent retrograde metamorphism. At Kantienpan, on the other hand, maximum metamorphic

  18. Implications of volcanic erratics in Quaternary deposits of North Greenland

    Funder, Svend Visby; Larsen, Ole


    Erratic boulders, petrographically similar to the volcanics exposed around Kap Washington, are found on islands and along the coast much further to the east. Isotopic measurements on two such boulders show that these volcanic rocks are of the same age as the Kap Washington volcanics. The regional...

  19. 松辽盆地火山岩岩性识别中测井数据的选择及判别方法%Selection and identification of logging data for lithology recognition of volcanic rocks in Songliao Basin

    张莹; 潘保芝


    依照松辽盆地深层火山岩岩性分类方案中的二级分类类型,对32口有准确岩心薄片定名资料的火山岩井段测井响应特征进行了总结,根据各测井响应区间值建立一个假设随机样本,应用逐步判别分析方法,在多个测井参数中按其对岩性分类判别能力的大小进行筛选,最终选取钾、铀、光电吸收截面指数、中子及自然伽马5个具有显著判别能力的参数,并在Bayers准则下建立判别函数,选择分类效果明显的两个判别函数建立岩性识别图版,克服了测井参数引入过多对岩性识别模型的不利影响.利用上述方法对松辽盆地徐家围子断陷火山岩井段的50个资料点的实测数据进行了处理,其回判数点正确率为88%.%According to the secondary taxonomic type in the lithology classification scheme for deep volcanic rocks in Songliao Basin, we summarized logging response characteristics of volcanic intervals in 32 wells that have exact nomination materials of core thin sections. A hypothetical random sample based on the interval value of logging response of each well was determined, and various logging parameters were screened according to their distinguishability in lithologic classification by using the stepwise discriminant analysis method. Five parameters with significant distinguishability, including potassium, uranium, photoelectric absorption coefficient, neutron and natural gamma ray, were finally chosen and discriminant functions were established with the Bayers criterion. In addition, two discriminant functions with obvious classification effect were adopted to establish a plate for lithology identification in order to diminish the negative impact of employing overmany logging parameters on lithology identification models. Moreover, the above-mentioned approach was applied to process measured data from 50 material points of faulted volcanic intervals in Xujiaweizi of Songliao Basin, and the rate to verify

  20. Origin of silicic volcanic rocks in Central Costa Rica: a study of a chemically variable ash-flow sheet in the Tiribí Tuff

    Hannah, Rachel; Vogel, Thomas; Patino, Lina; Alvarado, Guillermo; Pérez, Wendy; Smith, Diane


    Chemical heterogeneities of pumice clasts in an ash-flow sheet can be used to determine processes that occur in the magma chamber because they represent samples of magma that were erupted at the same time. The dominant ash-flow sheet in the Tiribí Tuff contains pumice clasts that range in composition from 55.1 to 69.2 wt% SiO2. It covers about 820 km2 and has a volume of about 25 km3 dense-rock equivalent (DRE). Based on pumice clast compositions, the sheet can be divided into three distinct chemical groupings: a low-silica group (55.1-65.6 wt% SiO2), a silicic group (66.2-69.2 wt% SiO2), and a mingled group (58.6-67.7 wt% SiO2; all compositions calculated 100% anhydrous). Major and trace element modeling indicates that the low-silica magma represents a mantle melt that has undergone fractional crystallization, creating a continuous range of silica content from 55.1-65.6 wt% SiO2. Eu/Eu*, MREE, and HREE differences between the two groups are not consistent with crystal fractionation of the low-silica magma to produce the silicic magma. The low-silica group and the silicic group represent two distinct magmas, which did not evolve in the same magma chamber. We suggest that the silicic melts resulted from partial melting of relatively hot, evolved calc-alkaline rocks that were previously emplaced and ponded at the base of an over-thickened basaltic crust. The mingled group represents mingling of the two magmas shortly before eruption. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at

  1. Petrogenesis and tectonic settings of volcanic rocks of the Ashele Cu-Zn deposit in southern Altay, Xinjiang, Northwest China: Insights from zircon U-Pb geochronology, geochemistry and Sr-Nd isotopes

    Wu, Yufeng; Yang, Fuquan; Liu, Feng; Geng, Xinxia; Li, Qiang; Zheng, Jiahao


    The Early-Mid-Devonian Ashele Formation of the southern margin of the Chinese Altay hosts the Ashele Cu-Zn volcanogenic massive sulfide (VMS) deposit and consists of intercalated volcanic and sedimentary rocks that have experienced regional greenschist-facies metamorphism. We studied the petrography, zircon U-Pb geochronology, geochemistry, and Sr-Nd isotopes of dacites and basalts in order to understand the petrogenesis of these rocks and the regional tectonic evolution. Two dacites yielded LA-MC-ICP-MS zircon U-Pb ages of 402 ± 6 Ma and 403 ± 2 Ma. The dacites are calc-alkaline, and characterized by high Na2O/K2O ratios (3.6-9.3), and high Mg# values (47-63), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb, Ta, Ti, and P, and relatively positive εNd(t) values (+3.6 to +7.5), collectively suggesting a sanukitic magma affinity. The variations in the major and trace elements of the dacites indicate that Fe-Ti oxide, plagioclase, and apatite were fractionated during their petrogenesis. The basalts are tholeiitic, and are characterized by high Mg# values (66-73), and negative Nb and Ta anomalies. The geochemical characteristics of the basalts are similar to those of N-MORB. Those characteristics together with the positive εNd(t) values (+6.8 to +9.2) of the basalts, indicate that the precursor magma was derived mainly from an N-MORB-type depleted asthenospheric mantle in an island arc setting. The geochemical similarities between the basalts and dacites indicate that they both originated from a similar depleted mantle source via partial melting under different magmatic conditions in each case, possibly related to ridge subduction.

  2. Topaz magmatic crystallization in rhyolites of the Central Andes (Chivinar volcanic complex, NW Argentina): Constraints from texture, mineralogy and rock chemistry

    Gioncada, Anna; Orlandi, Paolo; Vezzoli, Luigina; Omarini, Ricardo H.; Mazzuoli, Roberto; Lopez-Azarevich, Vanina; Sureda, Ricardo; Azarevich, Miguel; Acocella, Valerio; Ruch, Joel


    Topaz-bearing rhyolite lavas were erupted as domes and cryptodomes during the early history of the Late Miocene Chivinar volcano, in Central Andes. These are the only topaz rhyolite lavas recognized in Central Andes. Textural, mineralogical and geochemical data on the Chivinar rhyolites suggest that topaz crystallized from strongly residual, fluorine-rich, peraluminous silicate melts of topazite composition before the complete solidification of the lava domes. Crystallization of the rhyolitic magma began with sodic plagioclase and alkali feldspar phenocrysts in the magma chamber, followed by groundmass quartz + alkali feldspar + minor sodic plagioclase during dome emplacement, and terminated with quartz + topaz + vapour bubbles forming small scattered miaroles. Fluorine partitioning into the fluid phase occurred only in the final stage of groundmass crystallization. The magmatic origin of topaz indicates the presence of a fluorine-rich highly differentiated magma in the early history of the Chivinar volcano and suggests the possibility of rare metals mineralizations related to the cooling and solidification of a silicic magma chamber. A late fluid circulation phase, pre-dating the andesitic phase of the Chivinar volcano, affected part of the topaz rhyolite lavas. The presence of Nb, Ta and Mn minerals as primary accessories in the rhyolites and as secondary minerals in veins suggests a connection of the fluid circulation phase with the silicic magmatic system. Although at the edge of the active volcanic arc, the Chivinar topaz rhyolites are in correspondence of the transtensive Calama-Olacapato-El Toro fault system, suggesting preferred extensional conditions for the formation of magmatic topaz in convergent settings, consistently with evidence from other known cases worldwide.

  3. Friction in volcanic environments

    Kendrick, Jackie E.; Lavallée, Yan


    Volcanic landscapes are amongst the most dynamic on Earth and, as such, are particularly susceptible to failure and frictional processes. In rocks, damage accumulation is frequently accompanied by the release of seismic energy, which has been shown to accelerate in the approach to failure on both a field and laboratory scale. The point at which failure occurs is highly dependent upon strain-rate, which also dictates the slip-zone properties that pertain beyond failure, in scenarios such as sector collapse and pyroclastic flows as well as the ascent of viscous magma. High-velocity rotary shear (HVR) experiments have provided new opportunities to overcome the grand challenge of understanding faulting processes during volcanic phenomena. Work on granular ash material demonstrates that at ambient temperatures, ash gouge behaves according to Byerlee's rule at low slip velocities, but is slip-weakening, becoming increasingly lubricating as slip ensues. In absence of ash along a slip plane, rock-rock friction induces cataclasis and heating which, if sufficient, may induce melting (producing pseudotachylyte) and importantly, vesiculation. The viscosity of the melt, so generated, controls the subsequent lubrication or resistance to slip along the fault plane thanks to non-Newtonian suspension rheology. The shear-thinning behaviour and viscoelasticity of frictional melts yield a tendency for extremely unstable slip, and occurrence of frictional melt fragmentation. This velocity-dependence acts as an important feedback mechanism on the slip plane, in addition to the bulk composition, mineralogy and glass content of the magma, that all influence frictional behaviour. During sector collapse events and in pyroclastic density currents it is the frictional properties of the rocks and ash that, in-part, control the run-out distance and associated risk. In addition, friction plays an important role in the eruption of viscous magmas: In the conduit, the rheology of magma is integral

  4. Catastrophic volcanic collapse: relation to hydrothermal processes.

    López, D L; Williams, S N


    Catastrophic volcanic collapse, without precursory magmatic activity, is characteristic of many volcanic disasters. The extent and locations of hydrothermal discharges at Nevado del Ruiz volcano, Colombia, suggest that at many volcanoes collapse may result from the interactions between hydrothermal fluids and the volcanic edifice. Rock dissolution and hydrothermal mineral alteration, combined with physical triggers such as earth-quakes, can produce volcanic collapse. Hot spring water compositions, residence times, and flow paths through faults were used to model potential collapse at Ruiz. Caldera dimensions, deposits, and alteration mineral volumes are consistent with parameters observed at other volcanoes.

  5. Geological and geophysical interpretation of the Espirito Santo Basin and the Abrolhos region: petrography, radiometric dating and seismic visualization of volcanic rocks; Interpretacao geologica e geofisica da Bacia do Espirito Santo e da regiao de Abrolhos: petrografia, datacao radiometrica e visualizacao sismica das rochas vulcanicas

    Mohriak, Webster Ueipass [PETROBRAS S.A., Rio de Janeiro, RJ (Brazil). E e P. Gestao de Projetos Exploratorios Sul-Sudeste], E-mail:


    Geologic and geophysical interpretation of structural, magmatic and sedimentological features outcropping in the Abrolhos Archipelago (offshore of the Espirito Santo - Mucuri basins along the eastern Brazilian margin), and petrographic and geochronologic studies of igneous rocks, characterized a sequence of volcanic (basalts, diabase, breccia, etc.) and sedimentary rocks (fine grained to coarse grained sandstones and conglomerates in channels, sandstones with concordant stratification and locally discordant with the volcanic rocks, siltites, silty shales, etc.) which were uplifted after their original deposition between the Late Cretaceous and the Paleogene. The geochronological dating, based on the Ar-Ar method, suggests an interval of magmatism between 40 and 60 Ma. The field works suggest that siliciclastic rocks outcropping in the islands constitute a unique example of Paleogene Tertiary turbidity sedimentation in the Brazilian continental margin. Thick layers of massive sandstones with an external lobate geometry, overlying silty rhythmites, particularly in the eastern portion of the Santar Barbara Island, are probable deposits of dense flows (hyperpycnal) deposited in mid platform to upper bathyal environments. The tectonic and seismic stratigraphic interpretation suggests that these rocks were uplifted in a compressional event, probably associated with salt tectonics, affecting the sedimentary layers and creating angular unconformities that reach up to the Neogene. (author)

  6. Subterranean fragmentation of magma during conduit initiation and evolution in the shallow plumbing system of the small-volume Jagged Rocks volcanoes (Hopi Buttes Volcanic Field, Arizona, USA)

    Re, Giuseppe; White, James D. L.; Muirhead, James D.; Ort, Michael H.


    Monogenetic volcanoes have limited magma supply and lack long-lived sustained magma plumbing systems. They erupt once, often from multiple vents and sometimes over several years, and are rarely or never re-activated. Eruptive behavior is very sensitive to physical processes (e.g., volatile exsolution, magma-water interaction) occurring in the later stages of magma ascent at shallow crustal depths (flowing through dikes fragmented and conduits were formed. We have identified three main types of fragmental deposits, (1) buds (which emerge from dikes), (2) pyroclastic massifs, and (3) diatremes; these represent three different styles and intensities of shallow-depth magma fragmentation. They may develop successively and at different sites during the evolution of a monogenetic volcano. The deposits consist of a mixture of pyroclasts with varying degrees of welding and country-rock debris in various proportions. Pyroclasts are commonly welded together, but also reveal in places features consistent with phreatomagmatism, such as blocky shapes, dense groundmasses, and composite clasts (loaded and cored). The extent of fragmentation and the formation of subterranean open space controlled the nature of the particles and the architecture and geometry of these conduit structures and their deposits.

  7. Paleomagnetic study of Siluro-Devonian volcanic rocks from the central Lachlan Orogen: Implications for the apparent pole wander path of Gondwana

    VéRard, Christian; Tait, Jennifer; Glen, Richard


    The apparent pole wander (APW) path for Gondwana is still not clearly established, in particular, for Silurian-Devonian times. A controversial debate places authors who argue for an "X path," running directly through Africa on a reconstruction of Gondwana against those who advocate a large loop passing by southern South America, the "Y path." Most of the paleomagnetic data used to draw this loop come from the Lachlan Orogen (Australia). A paleomagnetic study was carried out in the well-dated Ambone and Ural volcanics in the central subprovince of Lachlan Orogen, New South Wales. Anisotropy of magnetic susceptibility measurements confirms detailed mapping of the region and shows that these massive dacitic sills and/or lava flows are flat lying. Among the different localities studied, only one yields interpretable paleomagnetic results. Two components of magnetization can be identified: a midtemperature direction yielding a corresponding pole in Australian coordinates λ = 67.9°S/ϕ = 084.4°E (B = 5; n = 21; dp = 17.5°/dm = 23.1°) and a high-temperature direction with a corresponding VGP λ = 24.4°S/ϕ = 060.6°E (B = 5; n = 25; dp = 1.4°/dm = 2.5°). The first is interpreted as corresponding to an Early Carboniferous pole position and can be regarded as an overprint probably related to the Early Carboniferous Kanimblan orogenic event. The second does not correspond to any expected Silurian-Devonian or younger pole position. This magnetization is thought to be primary in origin; however, secular variation has apparently not been averaged out in the single lava flow sampled. Therefore the earliest Devonian paleopole position probably lies in a 30° cone around the obtained VGP, and this position can only match the X-type APW path for Gondwana. It is in particular very different from coeval poles obtained in the eastern subprovince of the Lachlan Orogen, and it is mostly used as key poles supporting the Silurian-Devonian loop for the APW path of Gondwana

  8. The Impact of Space Flight on Survival and Interaction of Cupriavidus metallidurans CH34 with Basalt, a Volcanic Moon Analog Rock.

    Byloos, Bo; Coninx, Ilse; Van Hoey, Olivier; Cockell, Charles; Nicholson, Natasha; Ilyin, Vyacheslav; Van Houdt, Rob; Boon, Nico; Leys, Natalie


    Microbe-mineral interactions have become of interest for space exploration as microorganisms could be used to biomine from extra-terrestrial material and extract elements useful as micronutrients in life support systems. This research aimed to identify the impact of space flight on the long-term survival of Cupriavidus metallidurans CH34 in mineral water and the interaction with basalt, a lunar-type rock in preparation for the ESA spaceflight experiment, BIOROCK. Therefore, C. metallidurans CH34 cells were suspended in mineral water supplemented with or without crushed basalt and send for 3 months on board the Russian FOTON-M4 capsule. Long-term storage had a significant impact on cell physiology and energy status (by flow cytometry analysis, plate count and intracellular ATP measurements) as 60% of cells stored on ground lost their cell membrane potential, only 17% were still active, average ATP levels per cell were significantly lower and cultivability dropped to 1%. The cells stored in the presence of basalt and exposed to space flight conditions during storage however showed less dramatic changes in physiology, with only 16% of the cells lost their cell membrane potential and 24% were still active, leading to a higher cultivability (50%) and indicating a general positive effect of basalt and space flight on survival. Microbe-mineral interactions and biofilm formation was altered by spaceflight as less biofilm was formed on the basalt during flight conditions. Leaching from basalt also changed (measured with ICP-OES), showing that cells release more copper from basalt and the presence of cells also impacted iron and magnesium concentration irrespective of the presence of basalt. The flight conditions thus could counteract some of the detrimental effects observed after the 3 month storage conditions.

  9. The Impact of Space Flight on Survival and Interaction of Cupriavidus metallidurans CH34 with Basalt, a Volcanic Moon Analog Rock

    Natalie Leys


    Full Text Available Microbe-mineral interactions have become of interest for space exploration as microorganisms could be used to biomine from extra-terrestrial material and extract elements useful as micronutrients in life support systems. This research aimed to identify the impact of space flight on the long-term survival of Cupriavidus metallidurans CH34 in mineral water and the interaction with basalt, a lunar-type rock in preparation for the ESA spaceflight experiment, BIOROCK. Therefore, C. metallidurans CH34 cells were suspended in mineral water supplemented with or without crushed basalt and send for 3 months on board the Russian FOTON-M4 capsule. Long-term storage had a significant impact on cell physiology and energy status (by flow cytometry analysis, plate count and intracellular ATP measurements as 60% of cells stored on ground lost their cell membrane potential, only 17% were still active, average ATP levels per cell were significantly lower and cultivability dropped to 1%. The cells stored in the presence of basalt and exposed to space flight conditions during storage however showed less dramatic changes in physiology, with only 16% of the cells lost their cell membrane potential and 24% were still active, leading to a higher cultivability (50% and indicating a general positive effect of basalt and space flight on survival. Microbe-mineral interactions and biofilm formation was altered by spaceflight as less biofilm was formed on the basalt during flight conditions. Leaching from basalt also changed (measured with ICP-OES, showing that cells release more copper from basalt and the presence of cells also impacted iron and magnesium concentration irrespective of the presence of basalt. The flight conditions thus could counteract some of the detrimental effects observed after the 3 month storage conditions.

  10. Sr and O isotopes in western Aleutian seafloor lavas: Implications for the source of fluids and trace element character of arc volcanic rocks

    Yogodzinski, Gene M.; Kelemen, Peter B.; Hoernle, Kaj; Brown, Shaun T.; Bindeman, Ilya; Vervoort, Jeffrey D.; Sims, Kenneth W. W.; Portnyagin, Maxim; Werner, Reinhard


    High Mg# andesites and dacites (Mg# = molar Mg/Mg + Fe) from western Aleutian seafloor volcanoes carry high concentrations of Sr (>1000 ppm) that is unradiogenic (87Sr/86Sr 0.7030). Data patterns in plots of 87Sr/86Sr vs Y/Sr and Nd/Sr imply the existence of an eclogite-melt source component - formed by partial melting of MORB eclogite in the subducting Pacific Plate - which is most clearly expressed in the compositions of western Aleutian andesites and dacites (Nd/Sr and Y/Sr viewed in combination with inversely correlated εNd and 87Sr/86Sr, these patterns rule out aqueous fluids as an important source of Sr because mixtures of fluids from altered oceanic crust with depleted mantle and sediment produce compositions with 87Sr/86Sr higher than in common Aleutian rocks. The unradiogenic nature of Sr in the western Aleutian andesite-dacite end-member may be understood if H2O required to drive melting of the subducting oceanic crust is transported in fluids containing little Sr. Mass balance demonstrates that such fluids may be produced by dewatering of serpentinite in the mantle section of the subducting plate. If the eclogite-melt source component is present throughout the Aleutian arc, melting of the subducting plate must extend into minimally altered parts of the sheeted dike section or upper gabbros, at depths >2 km below the paleo-seafloor. Oxygen isotopes in western Aleutian seafloor lavas, which fall within a narrow range of MORB-like values (δ18 O = 5.1- 5.7), are also consistent with this model. These results indicate that the subducting Pacific lithosphere beneath the Aleutian arc is significantly hotter than indicated my most thermal models.

  11. Variation of olivine composition in the volcanic rocks in the Songliao basin, NE China: lithosphere control on the origin of the K-rich intraplate mafic lavas

    Zhang, L.-Y.; Prelević, D.; Li, N.; Mertz-Kraus, R.; Buhre, S.


    Lithospheric thickness and the heterogeneity of the mantle lithosphere are two major parameters that play a role in determining the final composition of the mafic melts and their minerals. The Songliao basin in northeast China represents an ideal natural laboratory to study the effect of these two parameters on early Pliocene to Holocene K-rich mafic lavas (K2O > 4 wt.%; K2O/Na2O > 1). A series of Cenozoic volcanic edifices (Erkeshan, Wudalianchi, Keluo and Xiaogulihe) are tentatively divided into three groups (Group 1 - thin, Group 2 - middle, and Group 3 - thick) according to the lithosphere thickness. They are located in the northern region of the Songliao basin extending in a near north-south direction along a broad zone where the lithosphere thickness increases gradually. We present a detailed petrographical and geochemical study on olivine macrocrysts in combination with new geochemical data on their host lavas, including major and trace element abundances as well as Sr, Nd, and Pb isotopic signatures. Our ultimate aim is to quantitatively and qualitatively determine the role of lithospheric mantle thickness (named as "lid effect") and composition in the variation of mafic lavas and olivine composition. When corrected to Mg# = 0.72, a number of major elements in the lavas correlate with increasing lithospheric thickness (L): Si72 and Al72 decrease, whereas Mg72, Fe72, Ti72 and P72 increase. Sm/Yb ratios in the lavas increase, implying that lithospheric thickness exerts an important control. Group 3 mafic lavas are ultrapotassic (showing lamproite affinity) with K2O/Na2O > 4: their La/Sm and Pb isotope ratios deviate from the above correlations, indicating that the lavas from the thickest part of the basin exhibit the highest extent of metasomatic enrichment of the mantle source. Several parameters (e.g. [Ni], Ni/Mg, Ni/(Mg/Fe), Mn/Fe and Ca/Fe) in melt-related olivine from Group 1 and Group 2 lavas are controlled by variable lithosphere thickness. Olivine

  12. High-Temperature, Perhaps Silicic, Volcanism on Mars Evidenced by Tridymite Detection in High-SiO2 Sedimentary Rock at Gale Crater, Mars

    Morris, R. V.; Vaniman, D. T.; Blake, D. F.; Gellert, R.; Chipera, S. J.; Rampe, E. B.; Ming, D. W.; Morrison, S. M.; Downs, R. T.; Treiman, A. H.; Yen, A. S.; Achilles, C. N.; Bristow, T. F.; Crisp, J. A.; Des Marais, D. J.; Farmer, J. D.; Fendrich, K. V.; Frydenvang, J.; Graff, T. G.; Grotzinger, J. P.; Morookian, J. M.; Schwenzer, S. P.


    The Mars Science Laboratory (MSL) rover, Curiosity, has been exploring sedimentary rocks within Gale crater since landing in August, 2012. On the lower slopes of Aeolis Mons (a.k.a. Mount Sharp), drill powder was collected from a high-silica (74 wt% SiO2) outcrop named Buckskin (BK). It was a surprise to find that the Buckskin sample contained significant amounts of the relatively rare silica polymorph tridymite. We describe the setting of the Buckskin sample, the detection of tridymite by the MSL Chemistry and Mineralogy (CheMin) X-ray diffraction instrument, and detection implications. Geologic setting: The Buckskin outcrop is part of the Murray formation exposed in the Marias Pass area. The formation was previously studied by CheMin in the Pahrump Hills member [1] where three samples of drill fines were analyzed (Confidence Hills (CH), Mojave2 (MJ) and Telegraph Peak (TP) [2]). Assuming approximately horizontal bedding, the Buckskin outcrop is approx.15 m stratigraphically above the bottom of the Pahrump Hills member. Mudstone, generally characterized by fine lamination, is the dominant depositional facies [1]. Buckskin Mineralogical and Chemical Composition: The CheMin instrument and XRD pattern analysis procedures have been previously discussed [3-6]. The diffraction pattern used for quantitative XRD analysis (Fig. 1) is the sum of the first 4 of 45 diffraction images. The remaining images are all characterized by both on-ring and off-ring diffraction spots that we attributed to poor grain motion and particle clumping. Coincident with particle clumping was a significant decrease in the intensity of the tridymite diffraction peaks (Fig. 2a). The derived mineralogical composition of the crystalline component (derived from the first 4 diffraction images) is given in Table 1. The tridymite is well-crystalline and its pattern is refined as monoclinic tridymite (Fig 1). Mineral chemical compositions were derived from XRD unit cell parameters or obtained from

  13. Facies-controlled volcanic reservoirs of northern Songliao Basin, NE China


    Volcanic rocks of the late Mesozoic are very important reservoirs for the commercial natural gases including hydrocarbon, carbon dioxide and rare gases in the northern Songliao Basin. The reservoir volcanic rocks include rhyolite,andesite, trachyte, basalt and tuff. Facies of the volcanic rocks can be classified into 5 categories and 15 special types.Porosity and permeability of the volcanic reservoirs are facies-controlled. Commercial reservoirs were commonly found among the following volcanic subfacies: volcanic neck (Ⅰ1), underground-explosive breccia (Ⅰ3), pyroclastic-bearing lava flow (Ⅱ3), upper effusive (Ⅲ3) and inner extrusive ones (Ⅳ1). The best volcanic reservoirs are generally evolved in the interbedded explosive and effusive volcanics. Rhyolites show in general better reservoir features than other types of rocks do.

  14. The volcanic and geochemical development of São Nicolau, Cape Verde Islands

    Duprat, Helene Inga; Holm, Paul Martin; Sherson, Jacob Friis;


    We present 34 new age results from 40 Ar/39 Ar incremental heating analyses of groundmass separates from volcanic rocks from São Nicolau, Cape Verde. Combining the age results with field observations, we show that the volcanic activity that formed the island occurred in four separate stages: 1: >6...... of 207 volcanic rocks representing all volcanic activity stages is presented. The rocks are alkaline and mostly primitive (MgO >8 wt.%) basic to ultrabasic ranging from nephelinites through basanites to picrobasalts. Evolved rocks range to phonolites. During all four volcanic stages predominantly high Mg...... assumed. With more ages of only around 50 ka the youngest volcanic rocks found on the island indicate that stage 4 may still be active. The cyclieity of São Nico volcanism suggests that mantle plume material arrived under São Nicolau in batches separated by nonfertile material....

  15. Palaeomagnetic dating of widespread remagnetization on the southeastern border of the French Massif Central and implications for fluid and Mississippi Valley-type mineralization

    Henry, B.; Rouvier, H.; Goff, M.L.; Leach, D.; Macquar, J.-C.; Thibieroz, J.; Lewchuk, Michael T.


    Palaeomagnetic dating techniques have been applied to determine the age of fluid migration that produced the Mississippi Valley-type (MVT) Pb-Zn-Ba-F deposits in the Ce??vennes region of southern France. 15 sampling sites in two gently deformed areas around the Largentie??re and Croix-de-Pallie??res mines on the Ce??vennes border were selected for palaeomagnetic study. They yielded a very well-defined direction of remagnetization corresponding to an Early-Middle Eocene age. This remagnetization cannot be related to the formation of magnetic as a result of the transformation of smectite to illite because the latter has been well dated as a Mesozoic event. The magnetic overprint in this area is related to a chemical phenomenon during fluid migration. The age of remagnetization corresponds to a major uplift in the Pyre??ne??es mountains, located to the south of the Ce??vennes. This implies that fluid migration occurred from the south to the north as a result of hydraulic head established in the Pyre??ne??es orogenic belt during orogenesis and suggests that the MVT deposits in the Ce??vennes region formed from a gravity-driven fluid system as described by Garven & Freeze (1984a,b).

  16. Palaeomagnetic dating of widespread remagnetization on the southeastern border of the French Massif Central and implications for fluid flow and Mississippi Valley-type mineralization

    Henry, Bernard; Rouvier, Henri; le Goff, Maxime; Leach, David; Macquar, Jean-Claude; Thibieroz, Jacques; Lewchuk, Michael T.


    Palaeomagnetic dating techniques have been applied to determine the age of fluid migration that produced the Mississippi Valley-type (MVT) Pb-Zn-Ba-F deposits in the Cévennes region of southern France. 15 sampling sites in two gently deformed areas around the Largentière and Croix-de-Pallières mines on the Cévennes border were selected for palaeomagnetic study. They yielded a very well-defined direction of remagnetization corresponding to an Early-Middle Eocene age. This remagnetization cannot be related to the formation of magnetite as a result of the transformation of smectite to illite because the latter has been well dated as a Mesozoic event. The magnetic overprint in this area is related to a chemical phenomenon during fluid migration. The age of remagnetization corresponds to a major uplift in the Pyrénées mountains, located to the south of the Cévennes. This implies that fluid migration occurred from the south to the north as a result of hydraulic head established in the Pyrénées orogenic belt during orogenesis and suggests that the MVT deposits in the Cévennes region formed from a gravity-driven fluid system as described by Garven & Freeze (1984a,b).

  17. Distribution and characteristics of volcanic reservoirs in China

    HUANG Yulong; WANG Pujun; CHEN Shuming


    About forty productive oil/gas fields hosted in volcanic reservoirs have been found since 1957 in fourteen basins of China. They can be simply subdivided into two groups, the east and the west. Reservoir volcanic rocks of the east group are predominantly composed of Late Jurassic to Early Cretaceous rhyolite and Tertiary basalt, preferred being considered as rift type volcanics developed in the circum-Pacific tectonic regime. Those of the west are Permo-Carboniferous intermediate/basic volcanic rocks, being island-arc type ones developed in paleo-Asian Ocean tectonic regime.

  18. Cretaceous Volcanic Events in Southeastern Jilin Province, China: Evidence from Single Zircon U-Pb Ages

    CHEN Yuejun; SUN Chunlin; SUN Yuewu; SUN Wei


    Mesozoic volcanic rocks in southeastern Jilin Province are an important component of the huge Mesozoic volcanic belt in the northeastern area. Study of the age of their formation is of great significance to recognize Mesozoic volcanic rule in northeastern China. Along with the research of rare Mesozoic biota and extensive Mesozoic mineralization in western Liaoning, a number of researchers have focused on Mesozoic volcanic events. The authors studied the ages of the Cretaceous volcanic rocks in southeastern Jilin Province using single Zircon U-Pb. The result shows that the Sankeyushu Formation volcanic rocks in the Tonghua area are 119.2 Ma in age, the Yingcheng Formation in the Jiutai area 113.4±3.1 Ma, the Jinjiatun Formation in Pinggang Town of Liaoyuan City and the Wufeng volcanic rocks in the Yanji area 103.2±4.7 Ma and 103.6±1 Ma, respectively. Combined with the data of recent publication on volcanic rocks ages; the Cretaceous volcanic events in southeastern Jilin Province can be tentatively subdivided into three eruption periods: 119 Ma, 113 Ma and 103 Ma. The result not only provides important chronology data for subdividing Mesozoic strata in southeastern Jilin Province, establishing Mesozoic volcanic event sequence, discussing geological tectonic background, and surveying the relation between noble metals to the Cretaceous volcanic rocks, but also otters important information of Mesozoic volcanism in northeastern China.

  19. Geochemical study for volcanic surveillance

    Panichi, C.; La Ruffa, G. [Consiglio Nazionale delle Ricerche, International Institute for Geothermal Research Ghezzano, PI (Italy)


    For years, geologists have been striving to reconstruct volcanic eruptions from the analysis of pyroclastic deposits and lava flows on the surface of the earth and in the oceans. This effort has produced valuable information on volcanic petrology and magma generation, separation, mixing, crystallisation, and interaction with water in phreatomagmatic and submarine eruptions. The volcanological process are tied to the dynamics of the earth's crust and lithosphere. The mantle, subducted oceanic crust, and continental crust contain different rock types and are sources of different magmas. Magmas consist primarily of completely or partially molten silicates containing volatile materials either dissolved in the melt or as bubbles of gas. The silicate and volatile portions affect the physical properties of magma and, therefore, the nature of a volcanic eruption.

  20. 蒙古国东南部额尔登查干地区晚古生代火山岩地球化学特征及成因%Geochemical Characteristics and Genesis of Late Paleozoic Volcanic Rocks in Erdenetsagaan Region of Southeastern Mongolia

    贾文臣; 张宝林; 沈晓丽; 郭志华; 黄雪飞; 徐永生


    在蒙古国苏赫巴托省东南部额尔登查干地区古生代地层中新发现一套火山岩,其Rb-Sr同位素年龄为(370±21) Ma,与前人划分的地层年代(志留纪—泥盆纪)比较吻合.岩石学和地球化学综合研究表明:该套火山岩基本同期同源演化,但岩性变化较大,以安山岩类为主,还有少量玄武岩类和流纹岩—英安岩类.熔融地幔源区受到了来自俯冲板片的改造,原始岩浆起源于俯冲带流体交代的地幔,并在上侵过程中发生地壳物质混染.岩浆的不均匀性和分异演化是产生多类型火山岩组合(安山岩、玄武岩、流纹岩—英安岩)的主要原因.推测该套火山岩形成于古亚洲洋闭合过程中的岛弧环境,表明火山岩形成时(晚古生代)中朝板块和西伯利亚板块还没有碰撞.%A set of volcanic rocks was newly discovered in Paleozoic strata in the southern Mongolia. The Rb-Sr age of the volcanic rocks is (370 ± 21) Ma, being comparatively consistent with their host stratum age proposed by former researchers. Based on petrologic and geochemical study, the volcanic rocks are contemporaneous and homologous basically. However, their lithology varies significantly, and they are composed mainly of andesites and minorly of basalts, rhyolite and dacites. Their mantle source has apparently modified by the subducting slab. The original magma for the volcanics came from the mantle modified by fluids originally contained in the subducted slab, and was contacted late by the overlying crust during its ascending. It is suggested that the inhomogenous nature and strong fractionation of the magmas are the main reasons for the diversity of varied volcanic rocks (andesite, basalt, rhyolite and dacite) in the set. It is suggested that these volcanic rocks were formed in an island arc environment during the closing of the ancient Asian Ocean. This implies that the formation of these volcanic rocks took place before the collision between the

  1. Petrotectonic assemblages and LA-ICP-MS zircon U-Pb age of Early Paleozoic volcanic rocks in Qimantag area, Tibetan Plateau%青藏高原祁漫塔格地区早古生代火山岩岩石构造组合和LA-ICP-MS锆石U-Pb年龄

    王秉璋; 罗照华; 潘彤; 宋泰忠; 校培喜; 张志青


    A study of the Early Paleozoic volcanic rocks in Qimantag area reveals that they were formed in the Middle-Late Ordovi-cian period. The volcanic rocks can be assigned to four volcanic rock belts, i.e., Gonglugou - luanshigou belt, Kuangou - Xi-aolangyashan belt, Bayan Gol-Suhait belt and Naling Gol belt. The first volcanic rock belt belongs to a combination of Nb-enriched basalts characterized by high content of Nb (7.38×10-6-10.1×10-6), TiO2 (1.56%-2.24%) and P2O5 (0.24%-0.34%). The combinations of the volcanic rocks in the second belt are complicated, the SiO2 mass fractions of these rocks are concentrated on two end members, and these rocks can be further divided into two sub-combinations of basalt+basatic andesite and dacite+lithoidite. U-Pb LA-ICP-MS analyses of zircons in basalt and lithoidite yielded the data of 440.2±2.4Ma and 450.3±1.2Ma, respectively. The content of TiO2 (1.1%) and K2O(0.14%) is low while that of Na2O(2.17%) is high for the rocks of the tholeiite combinations in the third belt. Although the content of TiO2 (1.64%) is relatively high, the tholeiite combinations in the fourth belt are very similar to those in the third belt.%祁漫塔格地区早古生代海相火山岩发育,初步确定该地区早古生代海相火山岩的形成时代为中晚奥陶世,可划分出4个火山岩带,分别为公路沟-乱石沟带、宽沟-小狼牙山带、巴音郭勒-苏海图带和那陵格勒河带.公路沟-乱石沟带火山岩为富铌玄武岩组合,具有相对高的Nb(7.4×10-6~10.1×10-6)、TiO2( 1.56%~2.24%)和P2Os(0.24%~0.34%)含量.宽沟-小狼牙山带火山岩岩石类型复杂,SiO2质量分数集中分布于2个端元,可进一步分为玄武岩+玄武安山岩组合和英安岩+流纹岩组合,玄武岩中获得了440.2Ma±2.4Ma的LA-ICP-MS锆石U-Pb年龄值,流纹岩中获得了450.3Ma±1.2Ma的年龄值.巴音郭勒-苏海图带为拉斑玄武岩组合,具低钛(1.1%)、富钠(2.17%,)、低钾(0.14%)的特

  2. Volcanic caves of East Africa - an overview

    Jim W. Simons


    Full Text Available Numerous Tertiary to recent volcanoes are located in East Africa. Thus, much of the region is made up volcanic rock, which hosts the largest and greatest variety of East Africas caves. Exploration of volcanic caves has preoccupied members of Cave Exploration Group of East Africa (CEGEA for the past 30 years. The various publications edited by CEGEA are in this respect a treasure troves of speleological information. In the present paper an overview on the most important volcanic caves and areas are shortly reported.

  3. A cryptoendolithic community in volcanic glass.

    Herrera, Aude; Cockell, Charles S; Self, Stephen; Blaxter, Mark; Reitner, Joachim; Thorsteinsson, Thorsteinn; Arp, Gernot; Dröse, Wolfgang; Tindle, Andrew G


    Fluorescent in situ hybridization (FISH) and 16S rDNA analysis were used to characterize the endolithic colonization of silica-rich rhyolitic glass (obsidian) in a barren terrestrial volcanic environment in Iceland. The <