Deformation pattern in a proterozoic low pressure metamorphic belt near ramanujganj, western chhotanagpur terrane

摘要

The Ramanujganj area in eastern India exposes a Proterozoic metasedimentary belt (similar to 35 km x 18 km) which has been mapped in detail in terms of two pelitic and two psammitic lithounits. Dominant rock types in the belt include porphyroblastic argillite, phyllite and pelitic schist, feldspathic quartzite and orthoquartzite. The belt has an elongated oval shape with broadly NW-SE trend. Folds of three generations (F-1, F-2 and F-3) have been recorded. F-1 folds are rarely preserved as small isoclinal folds on bedding (S-0) associated with an axial planar cleavage (S-1). F-2 folds involve transposed S-0 + S-1 surfaces and vary from open to isoclinal, with steeply inclined axial plane and moderate plunge. Several large scale F-2 closures with NW to NNW striking axial planes are exposed. F-3 folds are steeply inclined to upright in nature with NE striking axial planes. Large scale F-3 folds are usually broad warps. Interference patterns of dome and basin type and hook type are observed in the area. F-1 and F-2 folds are broadly coaxial in nature, and their superposition has resulted in hook shaped pattern. F-2 and F-3 folds have superposed to produce dome-and-basin structures on both outcrop scale and large scale. The regional structure of the area is an elongate basin resulting mainly from the interference of F-3 folds on F-2 structures. The western and northern parts of the area are occupied by biotite quartzofeldspathic orthogneiss which intruded during or before F-1 deformation. In the eastern part mesocratic granodiorite intruded after F, deformation and before or during F-2 deformation. The Ramanujganj pelites are commonly characterised by large porphyroblasts of andalusite (chiastolite), whereas biotite and garnet form relatively small porphyroblasts. Andalusite occurs in the southeastern and central parts of the area, and transforms to sillimanite as we go towards northwest. This indicates increasing grade of metamorphism from southeast towards northwest: The metamorphism is of low pressure type.