Relative contributions of crust and mantle to the origin of the Bijli Rhyolite in a palaeoproterozoic bimodal volcanic sequence (<Emphasis Type="Italic">Dongargarh Group</Emphasis>), central India |
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| Authors: | S Sensarma S Hoernes D Mukhopadhyay |
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| Institution: | 1.Department of Geology,St. Anthony's College,Shillong,India;2.Mineralalogisch-Petrologisches Institut,der Universit?t Bonn,Bonn,Germany;3.Department of Geology,University of Calcutta,Calcutta,India |
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| Abstract: | New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal
volcanic sequence (Dongargarh Group) in central India, and one of the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli
Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic K-feldspar + albite ± anorthoclase
set in fine-grained micro-fragmental matrix of quartz-feldspar-sericite-chlorite-iron-oxide ± calcite. The rocks are largely
metaluminous with high SiO2, Na2O + K2O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to an ‘A-type granite’ melt. The
rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible element concentrations 2–3 times more than the upper
continental crust (UCC). LREE is high (La/Yb ∼ 20) and HREE 20–30 times chondritic. δ18Owhole-rock varies between 4.4 and 7.8‰ (mean 5.87±1.26‰).
The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt.
It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting,
and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower
crustal component with low CaO and Al2O3 to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ∼ 30% partial melting of AAG under anhydrous
condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better
matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065)
vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal
fluid are attributed for the spread in δ18Owhole-rock values. The fast eruption of high temperature (∼ 900°C) rhyolitic melts suggests a rapid drop in pressure of melting related
to decompression in an extensional setting. |
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| Keywords: | Bimodal volcanism high-silica melts granulite melting crust-mantle interaction central India |
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