Petrogenesis of the Mesoarchean (~3.05 Ga) mafic volcanics from the western Iron Ore Group volcano-sedimentary succession, Singhbhum craton, eastern India: Constraints from geochemical modelling and Sm-Nd geochronology

Abstract

Cratonic Mesoarchean volcano-sedimentary successions preserve compositionally diverse volcanic rocks which give insight into Archean mantle characteristics and crustal evolution. The late Archean western Iron Ore Group (W-IOG) succession in the Singhbhum craton represents deposition during this time period. As the W-IOG depositional age has remained poorly constrained, to between Paleo- and Mesoarchean, petogenesis and chronology of the interbedded volcanics would be an important geological constraint. Mafic volcanics, interlayered with BIF and Fe-Mn-rich phyllite/shale, were sampled from Kalta and Ongarbira, and petrogenetic modelling of their mantle source and Sm-Nd geochronology were conducted. The Ongarbira basalts show tholeiitic affinity, are depleted in LILE, LREE, Zr and Th and an absence of Nb-Ta-Ti anomalies that imply a depleted MORB-type mantle source. Tholeiitic basalts yield an isochron age of 3050 ± 71 Ma (2SD) with Ndi = 0.50885 ± 0.00010, MSWD = 0.17 (n = 10) and εNd(T) = +3.3 ± 1.6. Geochemical modelling indicates the most primitive tholeiitic basalt (Mg# = 61) formed by ~14% partial melting of depleted mantle whereas the most enriched variant formed by its subsequent assimilation-fractional crystallization (AFC) (~15% with r = 0.2 being the ratio between assimilation rate and fractional crystallization rate). In contrast, the Kalta basaltic andesite have calc-alkaline affinity, are enriched in LILE, LREE, Zr and Th with pronounced negative Nb-Ta-Ti anomalies indicating incompatible trace element enrichment in their petrogenesis. The basaltic andesite yield an isochron age of 3041 ± 94 Ma (2SD) with Ndi = 0.50875 ± 0.00009, MSWD = 0.62 (n = 10) and εNd(T) = +1.1 ± 1.6. Geochemical modelling indicates that their compositions were generated by 20–40% AFC (r = 0.2) of ~14% partial melt of depleted mantle. The source magma to the Ongarbira basalt is interpreted to have formed during lithospheric extension of the Singhbhum craton, by partial melting of depleted mantle followed by minor AFC, whereas the Kalta basaltic andesite represents higher extents of AFC of the Ongarbira-type tholeiitic magma.