Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. S. Thakur ¹, S. C. Patel ¹

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai - 400076, IN

Abstract

This communication discusses the metamorphism of low pressure granulites in the Sukma supracrustal suite near Nagaras in the southern Bastar craton. The area shows occurrence of Mg-Al rocks,of both amphibolite and granulite facies. Amphibolite facies Mg-Al schist usually contains cordierite and a low-Ca amphibole, which is commonly cummingtonite and rarely gedrite. Low-Ca amphibole and sillimanite are never found together in the same rock. Mg-Al granulite is characterised by the presence of orthopyroxene in lieu of low-Ca amphibole. Most of the Mg-Al rocks are silica saturated.

Keywords

Cummingtonite, Cordierite, Granulite, Bastar Craton.

Full Text

     

S. C. Patel ¹, S. Sundararaman ¹, R. Dey ², S. S. Thakur ¹, M. Kumar ³

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai - 400 076, IN
2 Intersight Consulting Pvt Ltd, 12th Main Road, Vasanthnagar, Bangalore - 560 052, IN
3 Geological Survey of India, Sector 33B, Chandigarh - 160 047, IN

Abstract

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.

Keywords

Structral Analysis, Polyphase Deformation, Pelite, Chhotanagpur Gneissic Complex.

Full Text

     

S. S. Thakur ¹, S. C . Patel ¹, S. Dasgupta ¹

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai- 400 076, IN

Abstract

The Precambrian Sukma supracrustal sequence near Nagaras, southern Bastar Craton, has been mapped as two quartzite lithounits and an intervening undifferentiated unit comprising Mg-Al rich granulite, mafic granulite, amphibolite and hornblende schist. The rocks bear imprints of three phases of deformation. The structures of the first generation are represented by rarely occurring small isoclinal folds(F₁) on bedding (S₀), and pervasive axial planar foliation (S₁). The folds of second generation (F₂) vary from open to close, and upright to locally overturned with NNWSSE to NNE-SSW striking axial planes Gentle warps with ESE-WNW striking axial planes represent folds of the third generation (F₃) Superposition of F₃ folds on F₂ folds has resulted in Dome-and-Basin interference pattern in both small and large scales. The map pattern is elongated in the N-S direction because of control by F₂ folds, and slight concave geometry towards west is the result of F₃ folding.

Keywords

Structural Geology, Sukma Supracrustals, Granulite, Bastar, Chhattisgarh.

Full Text