Petrogenesis of Bonai volcanic rocks from Singhbhum Craton (Eastern India): Geophysical and geodynamic implications for pervasive plume-lithospheric interaction

Abstract

• Petrogenesis and geodynamic evolution of Bonai volcanic, Singhbhum Craton. • Volcanic rocks generated by mantle plume within spinel lherzolite stability field. • Mantle potential temperature of 1300-1700°C and pressure 0.5-3.5 GPa. • Lower crustal exhumation, highly denudated lithospheric keel and magma underplating. • Crust-mantle thermal interaction, contamination and fractional crystallization. Singhbhum Craton of the Indian Shield is one of the oldest Eoarchean segments on the Earth, which contains several well-preserved Mesoarchean-Paleoproterozoic volcanic suites. In this study, we investigate petrogenesis of the Bonai suite of volcanic rocks located in the western part of the craton on the basis of detailed geological, geochemical and geophysical data, which provide a new understanding of its regional geodynamic evolution. These volcanics are generally fine to medium-grained, exhibit porphyritic texture and contain plagioclase and pyroxene as primary minerals and amphiboles, epidote, sericite and chlorite as secondary minerals. The accessory phases are mainly zircon, ilmenite, pyrite and chalcopyrite. They can be petrographically and geochemically classified as basalt and basaltic andesite of tholeiitic nature. They are characterized by moderate to high contents of SiO 2 (49.35-57.81 wt%), MgO (7.14-11.39 wt%), Al 2 O 3 (10.21-14.68 wt%), CaO (5.14-11.15 wt%), TiO 2 (0.48-2.03 wt%), Cr (32-483 ppm) and Ni (34-175 ppm). Their HFSE/REE ratios and REE patterns indicate their derivation from a relatively low melting of a similar mantle source within the spinel lherzolite stability field. They are formed as a result of decompressive melting of the mantle plume near the crust-mantle boundary, which underwent fractional crystallization and crustal contamination. The plume had an initial mantle potential temperature between 1300°C and 1700°C with corresponding pressure of 0.5 to 3.5 GPa. In Nb/Y-Zr/Y plot, almost all the samples fall in the field of oceanic plateau basalt with mantle plume affinity. This region is underplated by about 14 km thick magmatic rocks above the Moho, over which gravity field and regional elevation are consistently high and the lithosphere is extremely thin. These characteristics corroborate well with the mantle plume signatures. Subduction related geochemical signatures appear absent.