Reappraisal of the ‘early proterozoic gabbro-anorthosite suite’ rocks from the eastern Singhbhum craton, India: Insights from field features, petrography–mineralogy and geochemistry

The present study focuses on six mafic–ultramafic units from eastern Indian Singhbhum craton: (i) gabbroic rocks of Galudih, (ii) dolerite from North of Bisoi, (iii) Bangriposi wehrlite (BW), (iv) dolerite from North of Kuliana, (v) Kuliana-layered gabbro (KLG) and (vi) dolerite of Jashipur. These rocks had been grouped earlier as a co-genetic unit titled ‘early proterozoic gabbro-anorthosite suite’. Dolerites of Jashipur, North of Bisoi and North of Kuliana are found as undeformed intrusives into the Mesoarchaean Mayurbhanj granite (MBG) unit and show hydrothermal alteration features and variable LREE enrichment. Multiply-deformed gabbroic rocks of Galudih also intrude the MBGs with similar hydrothermal alteration features and comparatively lower REE abundances. KLG and BW are found as dismembered rocks, emplaced within a multiply-deformed metasedimentary assemblage of Mid-Proterozoic North Singhbhum mobile belt. Through a critical assessment of their field features, petrography, mineralogy and geochemistry, the earlier grouping was found to be invalid. Instead, these rock units should be categorised into three groups: Group I (Galudih), Group II (Jashipur, North of Bisoi, North of Kuliana) and Group III (KLG and BW). Group I rocks are the oldest among them, but their stratigraphic affinity remains unclear. Group II mafic rocks possibly belong to the Proterozoic newer dolerite dykes, whereas Group III represents fragments of oceanic crust and mantle.