Mantle heterogeneity and crust-mantle interaction in the Singhbhum craton, India: New evidence from 3340 Ma komatiites

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Research Article

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The Singhbhum Craton in eastern India preserves the records of Paleoarchean - Mesoarchean oceanic crust dominated by ultramafic-mafic extrusive rocks (greenstones), and sialic rafts composed of tonalite-trondhjemite-granodiorite, together with younger granitoids constituting the composite Singhbhum granite batholith. The greenstone sequences, intruded by younger granitoids are entrained as synformal keels in the batholith. The lower greenstone sequence of eastern Iron Ore Group (IOG) is composed of peridotitic komatiite, and komatiitic basalt with minor chert and BIF, whereas the upper sequence has tholeiite to tholeiitic andesite. Here we report results from first direct dating of the Karanjharan spinifex-textured komatiite from the lower part of Badampahar greenstone sequence of IOG. LA-ICPMS U-Pb analysis of zircon grains in the komatiites yielded consistent weighted mean 207Pb/206Pb age of 3340 Ma, with closely similar Hf depleted mantle age (TDM) between 3357 Ma and 3432 Ma. The high MgO (25–35%), and low Al2O3/TiO2 (<10) and CaO/Al2O3 (2.3–3.7) ratios in the serpentinized Karanjharan komatiites suggest strong similarity with the Al-depleted komatiites from Barberton Mountainland, South Africa. The Badampahar komatiites have Gd/YbN in the range 2–3, and show depletion in incompatible elements including LREE, slight enrichment of MREE and near chondritic HREE, suggesting derivation from a depleted mantle source with garnet in the residue. Juvenile Paleoarchean mantle source is also suggested by positive εHf(t) (3.0–4.6) and broadly similar 207Pb/206Pb age and TDM age. Geochemical features of basalts from the upper greenstone sequence overlap with those of the lower greenstone sequence, but are more enriched in HFSE (Th, U) and LREE possibly due to contamination with continental crust or oceanic sediments. The variation in Al2O3/TiO2 ratios as well as Gd/YbN ratios from the Karanjharan komatiite and other coeval komatiites in the eastern IOG suggest Paleoarchean mantle heterogeneity and/or melt extraction from different depths.



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