Discriminating Tectonic and Magmatic Fabrics in the Remal Granite Gneiss: Implications for Terrane Amalgamation Processes in Southeastern Singhbhum, India

Abstract

The Remal granite gneiss body in the southeastern part of the Singhbhum Craton, near its contact with the Rengali Province, preserves two orthogonal penetrative foliations. The subhorizontal foliation defines cross and trough bedding structures, and is characterized by (i) systematic grain-size variation between the layers, (ii) the presence of feldspar laths and (iii) graphic intergrowth textures, confirming that it is primary and of igneous origin (Sign). Sign has a non-planar geometry and shows a spread in orientations, while a later foliation of tectonic origin is defined by alternating biotite-rich and biotite poor bands that define a consistently oriented gneissic foliation (S1). S1 gneissic layering is associated with the alignment of biotite flakes and myrmekite formation, and operated under low grade metamorphic conditions. S1 shows a remarkable similarity in orientation with the shear fabric along the amalgamation front of the Rengali Province with the Eastern Ghats Province further to the south, indicating that strain related to this regional strike-slip event can be identified even in southeastern Singhbhum.

Origin of magmatic and tectonic fabrics in the Remal granite-gneiss, Singhbhum Craton, India

“…other parts of the Singhbhum granite) in the southern part of the Singhbhum Craton. There are no earlier folds to which this fabric is axial planar, and no associated shear sense markers or grain-size reduction related to the subhorizontal foliation; rather, the foliation shows curved surfaces that closely resemble trough cross-beds (Dobe & Gupta, 2018). A number of field observations (see Section 4) are consistent with a magmatic origin of the sub-horizontal fabric.…”

“…Individual cross-beds show finer laminar terminations against the bases of the overlying boundary layer. The schlieren bounded cross-beds are variably oriented, defining a dome and basin geometry with dips ranging from sub-horizontal to moderate (Dobe & Gupta, 2018). A number of magmatic deformation structures are recognizable including vertical magmatic shear fabrics (Fig.…”

“…The Singhbhum Craton is one of the oldest parts of the Indian shield (Chaudhuri et al 2018;Ranjan et al 2020), and has recently been postulated to represent one of the oldest places where continental crust emerged from the sea (Chowdhury et al 2021). The Remal granite-gneiss is located in the southeastern part of the craton, and contains a sub-vertical tectonic fabric superposed onto an earlier magmatic fabric (Dobe & Gupta, 2018). The supracrustal Iron Ore Group sequence lying just south of the area is multiply deformed, and therefore, the positive identification of a magmatic fabric within the granite is important.…”