Boninitic metavolcanic rocks and island arc tholeiites from the Older Metamorphic Group (OMG) of Singhbhum Craton, eastern India: Geochemical evidence for Archean subduction processes

Abstract

The metavolcanic rocks of the Paleo-Mesoarchean Older Metamorphic Group (OMG) from Jagannathpur area of the Singhbhum Craton occur as enclaves within the Singhbhum granites (SBG). The primary mineralogical composition of these rocks has been overprinted by plagioclase–chlorite–tremolite–actinolite assemblage reflecting lower to medium greenschist facies metamorphism. Disseminated sulphides and minor amount of chromite are present as opaque phases. Clinopyroxene compositions correspond to augite. Major, trace and rare earth element compositions of these rocks are characterized by high Mg# (80–82), MgO (30.6–32.9 wt.%), Ni (490–1262 ppm) and Cr (2404–4385 ppm) contents, high Al₂O₃/TiO₂ (49–67), Zr/Hf (10–100) ratios with low TiO2 (0.1–0.2 wt.%), Zr (1.0–3.1 ppm) contents, (La/Sm)_N >1 and (Gd/Yb)_N <1. These geochemical features in conjunction with distinct negative Nb, Ta, Zr, Hf and Ti anomalies on N-MORB normalized multi-element diagram, relative enrichment in LILE and La collectively attest to boninitic signatures and magma generation in an intraoceanic subduction zone setting. These boninites are associated with low Ti island arc tholeiites (IAT) that have geochemical characteristics inherited from subduction zone processs. Relative MREE depletion in the OMG boninites compared to LREE and HREE is reflected by their high ((La/Sm)_N = 0.7–6.7) and low (Gd/Yb)_N = 0.4–0.6) ratios. These REE signatures corroborate a multi-stage petrogenetic process involving a refractory mantle source that was previously depleted by melt extraction episodes and subsequently enriched by subduction-related slab-dehydration process. The LILE and LREE replenishment of the refractory mantle wedge is attributed to influx of slab-derived hydrous fluids that lowered the solidus temperature and triggered partial melting of mantle wedge at shallow level under hydrous conditions. A mantle domain of spinel peridotite composition has been inferred for partial melting and generation of magmas parental to OMG boninites and IAT. The PGE chemistry of the studied boninites and IAT, marked by higher Pd and Pd/Ir over lower Pt and Pt/Pd and S-undersaturated nature attest to fluid-fluxed metasomatism of mantle wedge and sulphide retention in the residual mantle due to high fluid pressure and high degree of mantle melting at shallow depth