Petrogenesis of rhyolites and trachytes from the Deccan Trap: Sr,Nd and Pb isotope and trace element evidence |
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| Authors: | P C Lightfoot C J Hawkesworth S F Sethna |
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| Institution: | (1) Department of Earth Sciences, The Open University, MK7 6AA Milton, Keynes, England;(2) Department of Geology, St. Xaviers College, 400001 Bombay, India;(3) Present address: Department of Geology, University of Toronto, M5S 2A1 Toronto, Ontario, Canada |
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| Abstract: | Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking compositional gap between 50 and 65% SiO2 suggests that the high SiO2 rocks evolved by 10–15% partial melting followed by variable amounts of fractional crystallisation. The source material could have been basalt within the Deccan Trap, or related gabbroic rocks in deep crustal sill complexes. The rhyolites yield an Rb-Sr whole rock age of 61.5±1.9 Ma, with a slightly high initial 87Sr/86Sr=0.7085±18. It is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks. |
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