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Geochemical evolution of the Mangalwar Complex,Aravalli Craton,NW India:Insights from elemental and nd-isotope geochemistry of the basement gneisses

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Geoscience Frontiers 2018年第3期9卷 931-942页

作者： Iftikhar Ahmad M.E.A.Mondal Rajneesh Bhutani M.Satyanarayanan Department of Geology Aligarh Muslim UniversityAligarh 202002India Department of Earth Sciences Pondicherry UniversityPuducherry 605014India CSIR-National Geophysical Research Institute Hyderabad 500007India

The Banded Gneissic Complex(BGC) of the Aravalli Craton is divided into BGC-I and BGC-Ⅱ; the BGC-Ⅱ(central Rajasthan) is comprised of the Sandmata Complex and the Mangalwar Complex. We report elemental and nd-isotope geochemistry of basement gneisses of the Mangalwar Complex and constrain its origin and evolution. Geochemically, the basement gneisses have been classified as low-SiO_2 gneisses(LSG) and high-SiO_2 gneisses(HSG). Both the LSG and HSG are potassic, calc-alkaline and peraluminous in nature. The LSG are enriched in incompatible(K, Sr, Ba, large ion lithophile elements) and compatible elements(MgO, Cr, and Ni). They display fractionated rare earth element patterns(***_N/Yb_N=12.1)with small Eu-anomaly(δEu=0.9), and exhibit negative anomalies of Nb and Ti in primitive mantlenormalized multi-element diagram. In terms of nd-isotope geochemistry, the LSG are characterized by_(εnd)(t)=4.2 and depleted mantle model age of 3.3 Ga. To account for these geochemical characteristics we propose a three-stage petrogenetic model for the LSG:(1) fluids released from dehydration of subducting slab metasomatised the mantle-wedge;(2) the subducting slab underwent slab-breakoff causing upwelling and decompression melting of the asthenosphere during waning stage of subduction; and(3)upwelling asthenosphere provided the requisite heat for partial melting of the metasomatised mantlewedge leading to generation of the LSG parental magma. Asthenospheric upwelling also contributed in the LSG petrogenesis which is evident from its high Mg#(avg. 0.53). The LSG formed in this way are contemporary and chemically akin to sanukitoids of the BGC-I and Archean sanukitoids reported elsewhere. This provides a basis to consider the LSG as a part of the BGC-I. Contrary to the LSG, the HSG are depleted in compatible elements(MgO=avg. 1.1 wt.%; Cr=avg. 8 ppm; Ni=avg. 6 ppm) but enriched in incompatible elements(Sr=avg. 239 ppm, Ba=avg. 469 ppm). Its_(εnd)(t) values vary from-9.5 ***

关键词： Aravalli Craton Mangalwar Complex nd-isotope geochemistry Grey gneisses Sanukitoid High-K granitoids

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PGE and isotopic characteristics of Shergol and Suru Valley Ophiolites,Western Ladakh:Implications for supra-subduction tectonics along Indus Suture Zone

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Geoscience Frontiers 2021年第3期12卷 473-488页

作者： Irfan Maqbool Bhat Talat Ahmad D.V.Subba Rao Srinivasan Balakrishnan N.V.Chalapathi Rao Department of Earth Sciences University of KashmirSrinagar 190006India Vice Chancellors Office University of KashmirSrinagar 190006India Geochemistry Division CSIR-National Geophysical Research Institute(NGRI)Hyderabad 500007India Department of Earth Sciences Pondicherry UniversityPuducherry 605014India Centre of Advanced Study in Geology Institute of ScienceBanaras Hindu UniversityVaranasi 221005India

Present study reports the PGE-geochemistry of mantle peridotites and nd-isotope geochemistry of arc related mafic rocks from the Indus Suture Zone(ISZ),western *** total PGE concentration of the Shergol and Suru Valley peridotites(∑PGE=96-180 ppb)is much higher than that of the primitive mantle and global ophiolitic mantle *** studied peridotites show concave upward PGE-distribution patterns with higher palladium-group PGE/Iridium-group PGE ratios(i.e.,0.8-2.9)suggesting that the partial melting is not the sole factor responsible for the evolution of these *** observed PGE-distribution patterns are distinct from residual/refractory mantle peridotites,which have concave downward or flat PGE-distribution *** enrichment of palladium-group PGE as well as other whole-rock incompatible elements(e.g.,LILE and LREE)and higher Pd/Ir ratio(1.1-5.9)reflects that these peridotites have experienced fluid/melt interaction in a supra-subduction zone(SSZ)tectonic ***,the Shergol mafic intrusives and Dras mafic volcanics,associated with the studied peridotites,have high^(143)nd/^(144)nd ratios(i.e.,0.512908-0.513078 and 0.512901-0.512977,respectively)and positiveε_(nd)(t)(calculated for t=140 Ma)values(i.e.,+5.3 to+8.6 and+5.1 to+6.6,respectively),indicating derivation from depleted mantle sources within an intra-oceanic arc setting,similar to Spongtang and Nidar ophiolites from other parts of Ladakh *** transition from SSZ-type Shergol and Suru Valley peridotites to Early Cretaceous tholeiitic Shergol mafic intrusives followed by tholeiitic to calc-alkaline Dras mafic volcanics within the Neo-Tethys Ocean exhibit characteristics of subduction initiation mechanism analogous to the Izu-Bonin-Mariana arc system within western Pacific.

关键词： PGE nd-isotope Mantle peridotites Ophiolites Ladakh Neo-Tethys Ocean

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