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

Jayananda, M. ; Kano, T. ; Peucat, J. J. ; Channabasappa, S. (2008) 3.35Ga komatiite volcanism in the western Dharwar craton, southern India: constraints from Nd isotopes and whole-rock geochemistry Precambrian Research, 162 (1-2). pp. 160-179. ISSN 0301-9268

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Official URL: http://www.sciencedirect.com/science/article/pii/S...

Related URL: http://dx.doi.org/10.1016/j.precamres.2007.07.010


We present field, petrographic, Sm–Nd 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 Sm–Nd 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 (1.01–1.34) and low Al2O3/TiO2 (5–16) whereas Al-undepleted komatiites show lower CaO/Al2O3 ratios (0.59–0.99) and higher Al2O3/TiO2 (17–26). 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, 577–581] or extraction of continental crust during the early Archaean.

Item Type:Article
Source:Copyright of this article belongs to Elsevier Science.
Keywords:Middle Archaean Komatiites; Dharwar Craton; Nd Isotopes; Whole-Rock Geochemistry; Mantle Evolution; Crustal Growth
ID Code:102868
Deposited On:13 Mar 2017 08:35
Last Modified:13 Mar 2017 08:35

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