Deformation Pattern in a Proterozoic Low Pressure Metamorphic Belt near Ramanujganj, Western Chhotanagpur Terrane

The Ramanujganj area in eastern India exposes a Proterozoic metasedimentary belt (-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 (F₁, F₂ and F₃) have been recorded F₁ folds are rarely preserved as small isoclinal folds on bedding(S_o)associated with an axial planar cleavage (S₁) F₂ folds involve transposed S_o+S₁ surfaces and vary from open to isoclinal, with steeply inclined axial plane and moderate plunge. Several large scale F₂ closures with NW to NNW striking axial planes are exposed F₃ folds are steeply inclined to upright in nature with NE striking axial planes. Large scale F₃ folds are usually broad warps. Interference patterns of dome and basin type and hook type are observed in the area F₁ and F₂ folds are broadly coaxial in nature, and their superposition has resulted in hook shaped pattern F₂ and F₃ 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 F₃ folds on F₂ structures.

The western and northern parts of the area are occupied by biotite quartzofeldspathic orthogneiss which intruded during or before F₁ deformation. In the eastern part mesocratic granodiorite intruded after F₁ deformation and before or during F₂ 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.