http://www.sciencedirect.com/science/article/pii/S1674987112001296 |
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| Authors: | Tushipokla MJayananda |
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| Institution: | Department of Geology, Centre of Advanced Studies, University of Delhi, Delhi 110007, India |
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| Abstract: | We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites (northern extension of Nagamangala belt) and further northwest in Nuggihalli belt and Kalyadi belt we speculate ca. 3.2–3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influence of post-magmatic alteration and/or contamination. About 1/3 of studied komatiites show Al-depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, (Gd/Yb)N ratios show low CaO/Al2O3 ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaO/Al2O3 ratios. The elemental characteristics of Al-depleted komatiites such as higher (Gd/Yb)N (>1.0), CaO/Al2O3 (>1.0), Al2O3/TiO2 (<18) together with lower HREE, Y, Zr and Hf indicate their derivation from deeper upper mantle with minor garnet (majorite?) involvement in residue whereas lower (Gd/Yb)N (<1.0), CaO/Al2O3 (<0.9), higher Al2O3/TiO2 (>18) together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics (CaO/Al2O3, Al2O3/TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE) indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle (depleted to primitive mantle) at different depths in hot spot environments possibly with a rising plume. The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca. 3.2 Ga which in turn imply an earlier episode of mantle differentiation, greenstone volcanism and continental growth probably during ca. 3.6–3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton (WDC). |
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| Keywords: | Komatiites Dharwar craton Geochemistry Greenstone volcanism Mantle evolution |
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