Petrogenesis of Neoarchean (2.80–2.75 Ga) Jagannathpur volcanics and the Ghatgaon and Keshargaria dyke swarms, Singhbhum craton, eastern India: Geochemical, Sr[sbnd]Nd isotopic and Sm[sbnd]Nd geochronologic constraints for interaction of enriched-DMM derived magma with metasomatised subcontinental lithospheric mantle

Abstract

Petrogenesis of Neoarchean dyke swarms and dispersed volcanic successions in the Singhbhum craton can potentially constrain mantle heterogeneity. Their arc-like trace element contents, that have a mismatch with their intracratonic extensional setting, were interpreted to propose widely varied genesis from either an enriched mantle (EM I) or wholly from a metasomatised subcontinental lithospheric mantle (SCLM). In order to test the type of enriched source(s) of magma to the dykes swarms and volcanics, geochemical modeling, Sr and Nd isotopic analyses and geochronology were conducted. The Jagannathpur amygdular volcanics, one such unmetamorphosed volcanic succession, yielded an SmNd isochron age of 2799 ± 67 Ma (2σ) with Ndi = 0.50895 ± 0.00006, MSWD = 0.36 (n = 16) and εNd(T) = −1.12 ± 0.50. As the eruptive age is indistinguishable within error to the Ghatgaon (2762.4 ± 2.0 Ma, 2752.0 ± 0.9 Ma) and the Keshargaria (2800.2 ± 0.7 Ma) dyke swarms, the dykes are interpreted as coeval feeder conduits to the volcanics. Enriched LILE and LREE, Nb/Th and Zr/Th ratios coupled with highly radiogenic present-day Sr isotopic ratios which have mantle-like 87Sr/86Sri ratios (0.7031–0.7055) and negative εNd(T) values (between −1.8 and − 5.6 for the dykes and − 1.3 and − 1.4 for the Jagannathpur volcanics) indicate that the source mantle components underwent enrichment processes likely by pre-2800 Ma subduction. The primitive mantle-normalized trace element patterns for these dykes and volcanics have pronounced negative Nb-Ta-Ti anomalies and their arc-like geochemistry suggest interaction with metasomatised SCLM. Generation of primitive magmas has been modeled by two end member components involving 18% partial melt from enriched-DMM (depleted MORB mantle) [100–70%] and interaction with low degree (5%) partial melt of metasomatised SCLM [0–30%]. These magmas ponded at Moho depths and underwent assimilation-fractional crystallization (AFC) forming the diverse dykes and basaltic andesitic-andesitic volcanics. We link the geophysically imaged Mid Lithospheric Discontinuity to depth of metasomatised SCLM beneath the Singhbhum craton that formed ≥2800 Ma ago.