The rocks within the Singhbhum shear zone in the North Singhbhum fold belt, eastern India, form a tectonic melange comprising granitic mylonite, quartz-mica phyllonite, quartz-tourmaline rock and deformed volcanic and volcaniclastic rocks. The granitic rocks show a textural gradation from the least-deformed variety having coarse-to medium-grained granitoid texture through augen-bearing protomylonite and mylonite to ultramylonite. Both type I and type II S-C mylonites are present. The most intensely deformed varieties include ultramylonite. The phyllosilicate-bearing supracrustal rocks are converted to phyllonites. The different minerals exhibit a variety of crystal plastic deformation features. Generation of successive sets of mylonitic foliation, folding of the earlier sets and their truncation by the later ones results from the progressive shearing movement. The shear sense indicators suggest a thrust-type deformation. The microstructural and textural evolution of the rocks took place in an environment of relatively low temperature, dislocation creep accompanied by dynamic recovery and dynamic recrystallization being the principal deformation mechanisms. Palaeostress estimation suggests a flow stress within the range of 50–190 MPa during mylonitization.

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.