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

Jayananda, M. and Kano, T. and Peucat, J.J. and Channabasappa, S. (2008) 3.35 Gyr komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-​rock geochemistry. Precambrian Research, 162 (2). pp. 160-179. ISSN 03019268

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Official URL: https://doi.org/10.1016/j.precamres.2007.07.010

Abstract

We present field, petrog., Sm-​Nd whole-​rock isochron, and whole-​rock geochem. 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. Petrog. 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 while the remaining are Al-​undepleted. The Al-​depleted komatiites are characterized 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 geochem. and Nd isotopic compns. are preserved with only minor influence of post-​magmatic alteration processes and negligible crustal contamination. The chem. 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 obsd. chem. 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 assocn. 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, M. and R.W. Carlson (2005) or extn. of continental crust during the early Archean.

Item Type: Article
Subjects: Faculty of Science > Earth Sciences > Geology
Divisions: Jnana Bharathi / Central College Campus > Department of Geology
Depositing User: Ms. Deepa M Kolli
Date Deposited: 15 Sep 2016 06:04
Last Modified: 15 Sep 2016 06:04
URI: http://eprints-bangaloreuniversity.in/id/eprint/5657

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