Thermo-tectonic evolution of the North Singhbhum Mobile Belt (eastern India): A view from the western part of the belt

Abstract

The arcuate North Singhbhum Mobile Belt (NSMB) comprises multiply-deformed greenschist–amphibolite facies phyllites and schists flanking the centrally-located belt of the Dalma meta-igneous suite of rocks. The NSMB is separated from the Archaean Singhbhum Craton in the south, and the suite of granulite–amphibolite facies gneisses and foliated granites of the Chotanagpur Gneissic Complex to the north, by ductile shear zones. Three fabrics (S 1S, S 2S, and S 3S) formed due to D 1S, D 2S and D 3S deformation events and three metamorphic stages (M 1S: pre-S 2S; M 2S: syn-S 2S; and M 3S: syn/post-S 3S) were identified in phyllites and schists to the south of the Dalma meta-igneous in the NSMB. Reverse-sense transport along north-dipping imbricate thrusts (S 3S) emplaced the high-grade garnet–kyanite–staurolite schists centrally located in the orogen over low-grade muscovite–biotite–chlorite schists in the foreland. The M 1S stage (∼1.5 Ga; EPMA monazite date) in the high-grade gneisses characterized by high P/ T prograde metamorphism ( P > 10 kbar) was followed sequentially by post-loading prograde heating ( T max ∼650 °C) and exhumation – cooling at amphibolite facies (M 2S) and greenschist facies (M 3S: ∼1.3 Ga; EPMA monazite date) conditions. In the northern part of the NSMB, mica schists fringing the CGC document a northward increase in strain (sequential cleavage formation) and temperature (greenschist to amphibolite facies). The strain–temperature spatial gradient is correlated with crustal shortening and heating ( T ∼620 °C, P ∼7 kbar) synchronously with reverse-sense transport of schists along southerly-dipping thrust planes. The structural framework and thermal structure across the mobile belt is correlated with (a) collisional thickening of the orogen (>1.5 Ga), followed by convective removal of the lithospheric root resulting in post-burial heating and (b) syn-collision and exhumation of the orogen (∼1.3 Ga). The exhumation was caused by outward-directed extrusion of the crustal slices along imbricate foreland-vergent thrusts dipping towards the centre of the orogen.