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Crustal structure of the western Indian shield: Model based on regional gravity and magnetic data

Geoscience Frontiers 2013年第6期4卷 717-728页

作者： Suman Kilaru Bandaru Karunakar Goud Vijay Kumar Rao National Geophysical Research Institute

Regional surface gravity data and global satellite magnetic data have been utilized to generate a preliminary model of the crustal structure along a southwest-northeast profile （Gadra-Fatehpur） through western *** study area represents the western part of the Indian continental landmass which has undergone several major episodes of repeated subduction/collision,plume traces and rifting from Archaean to recent *** temporal and spatial relationship between the various geotectonic provinces is quite complex,thereby limiting the emergence of a suitable crustal structure model for this *** of the Malani Igneous Suite （MIS）,a product of bimodal volcanism （～780 Ma）,and considered to be the third largest felsic magmatic province of the world,is evident along the profile and also to the southwest of the study *** easternmost part of the profile is close to the DAFB （Delhi Aravalli Fold Belt）,a Proterozoic orogenic *** study probes the geometry of the different crustal units in terms of density and susceptibility variations in order to decipher the imprints of the major tectonic processes the region has *** order to decipher the crustal geometry of the Gadra-Fatehpur profile,two NW-SE gravity and magnetic profile vertical sections （A-A＇ in the south and B-B＇ in the north） are modelled on the basis of the constraints provided from previous seismic *** crustal model of the Gadra-Fatehpur profile is composed of alluvium,Tertiary sediments,MIS,Marwar Supergroup,low-density layers （LDLs） and the middle-lower crustal layers,with a distinct change in configuration from the southwest to *** Moho dips from SW to NE,the MIS in the SW gives way to the thick pile of the Marwar Supergroup to the *** evolution of MIS has been suggested to have occurred as a consequence of delamination of the upper *** are incorporated in Gadra-Fatehpur *** the SW,LDL （2550 kg/m3） lies below the MIS in the NE,another LDL （2604

关键词： Western Indian shield Gravity and magnetic modelling MIS Delamination bimodal volcanism

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Relationships between Basic and Silicic Magmatism in Continental Rift Settings:A Petrogeochemical Study of Carboniferous Post-collisional Rift Silicic Volcanics in Tianshan,NW China

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Acta Geologica Sinica(English Edition) 2005年第5期79卷 633-653页

作者： XIA Linqi XIA Zuchun XU Xueyi LI Xiangmin MA Zhongping WANG Lishe Xi＇ an Institute of Geology and Mineral Resources China Geological Survey Xi＇ an Shaanxi 710054

Petrogeochemical data are reported for silicic volcanic rocks from the Tianshan Carboniferous rift, with the aim of discussing the petrogenesis of silicic magmas. Incompatible element vs. incompatible element diagrams display smooth positive trends for the Tianshan Carboniferous rift-related volcanic rocks; the isotope ratios of the silicic lavas [^87Sr/^86S（t）=0.699880.70532; eNd（t）=4.76-8.00; ^206pb/^204pb（t）=17.435-18.017; ^207Pb/^204Pb（t）=15.438-15.509; ^208Pb/^204Pb（t） = 37.075-37.723] encompass those of the basic lavas. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modeling of incompatible vs. compatible elements excludes the possibility that silicic melts are generated by the melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks （AFC） starting from intermediate rocks to silicic rocks. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. The presence or absence of bimodal volcanism （the ＂Daly Gap＂） might be related to cooling rates of magma chambers. In central and eastern Tianshan, the crust was thinner and the cooling rates of the magma chamber within the crust were greater. These conditions resulted in a rapid fall in temperature within the magma reservoir and caused a narrow temperature interval over which intermediate melts formed, effectively reducing the volume of the intermediate melts.

关键词： Carboniferous rift-related silicic volcanic rocks geochemistry bimodal volcanism Tianshan NW China.

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Mid—Late Neoproterozoic rift-related volcanic rocks in China:Geological records of rifting and break-up of Rodinia

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Geoscience Frontiers 2012年第4期3卷 375-399页

作者： Linqi Xia Zuchun Xia Xueyi Xu Xiangmin Li Zhongping Ma Xi＇an Institute of Geology and Mineral Resources China Geological Survey Xi＇an Shaanxi 710054 China

Early Cambrian and Mid--Late Neoproterozoic volcanic rocks in China are widespread on several Precambrian continental blocks, which had aggregated to form part of the Rodinia supercon- tinent by ca. 900 Ma. On the basis of petrogeochemical data, the basic lavas can be classified into two major magma types： HT （Ti/Y 〉 500） and LT （Ti/Y 〈 500） that can be further divided into HT1 （Nb/La 〉 0.85） and HT2 （Nb/La ≤ 0.85）, and LT1 （Nb/La 〉 0.85） and LT2 （Nb/La ≤ 0.85） subtypes, respectively. The geochemical variation of the HT2 and LT2 lavas can be accounted for by lithospheric contamination of asthenosphere- （or plume-） derived magmas, whereas the parental magmas of the HT1 and LT1 lavas did not undergo, during their ascent, pronounced lithospheric contamination. These volcanics exhibit at least three characteristics：（1） most have a compositional bimodality; （2） they were formed in an intracontinental rift setting; and （3） they are genetically linked with mantle plumes or a mantle surperplume. This rift-related volcanism at end of the Mid-- Neoproterozoic and Early Cambrian coincided temporally with the separation between Australia-- East Antarctica, South China and Laurentia and between Australia and Tarim, respectively.

关键词： Neoproterozoic bimodal volcanism Continental rift Mantle plume Rodinia supercontinent China

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