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3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry

Abstract : We present field, petrographic, Sm­Nd whole-rock isochron and whole-rock geochemical data for komatiites from Sargur Group greenstone belts of the western Dharwar craton. Field evidence such as pillow structure indicates their eruption in a marine environment. Petrographic data reveal that the igneous mineralogy has been altered during post-magmatic hydrothermal alteration processes corresponding to greenschist- to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene. A 16-point Sm­Nd whole-rock isochron gives an age of 3352 ± 110 Ma for the timing of eruption of komatiite lavas. About 60% of the studied komatiite samples show Al-depletion whilst the remaining are Al-undepleted. The Al-depleted komatiites are characterised by high CaO/Al2O3 ratios (1.01­1.34) and low Al2O3/TiO2 (5­16) whereas Al-undepleted komatiites show lower CaO/Al2O3 ratios (0.59­0.99) and higher Al2O3/TiO2 (17­26). Trace element distribution patterns of komatiites suggest that most of the primary geochemical and Nd isotopic compositions are preserved with only minor influence of post-magmatic alteration processes and negligible crustal contamination. The chemical characteristics of Al-depleted komatiites, such as high (Gd/Yb)N together with lower HREE, Y, Zr and Hf, imply their derivation from deeper upper mantle with garnet (majorite?) involvement, whereas lower (Gd/Yb)N slightly higher HREE, Y, Zr and Hf suggest derivation from shallower upper mantle without garnet involvement. The observed chemical characteristics (CaO/Al2O3, Al2O3/TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, REE) indicate derivation of the komatiite magmas from different depths in a plume setting, whereas sub-contemporaneous felsic volcanism and TTG accretion can be attributed to an arc setting. In order to explain the spatial association of komatiite volcanism with contemporaneous mafic-felsic volcanism and TTG accretion we propose a combined plume-arc setting. Nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early (>4.53 Ga) global differentiation of the silicate Earth as suggested by Boyet and Carlson [Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (>4.53 Ga) global differentiation of silicate Earth. Science 309, 577­581] or extraction of continental crust during the early Archaean.
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Submitted on : Thursday, May 15, 2008 - 11:15:54 AM
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M. Jayananda, T. Kano, Jean-Jacques Peucat, S. Channabasappa. 3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry. Precambrian Research, Elsevier, 2008, 162 (1-2), pp.160-179. ⟨10.1016/j.precamres.2007.07.010⟩. ⟨insu-00279724⟩



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