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

L. G. Gwalani ¹, R. Ramasamy ², S. C. Patel ³, P. Sangurmath ⁴

Affiliations
1 University Department of Geology, Nagpur, IN
2 Tamilnadu State Dept. of Geology & Mining, Madras, IN
3 Indian School of Mines, Dhanbad, IN
4 Mineral Exploration Corporation Ltd. Hyderabad, IN

Abstract

No Abstract.

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S. C. Patel ¹, D. Asthana ², C. S. Acharya ²

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai - 400 076, IN
2 Department of Applied Geology, Indian School of Mines, Dhanbad - 826 004, IN

Abstract

In this communication we report for the first time the occurrence and microprobe analyses of prehnite and pumpellyite in amygdules of metabasalt from Karu Pahar near Deori, Bhandara district, Maharashtra. The metabasalt belongs to the Sitagota Volcanics of the Late Archaean Khairagarh Group of the Dongargarh Supergroup. Pumpellyite occurs as fibrous aggregates, lining or filling amygdules, whereas prehnite occurs sporadically in amygdules as clusters of tiny prismatic crystals. Pumpellyite is iron-rich, between 10.3 and 13.1 wt% Fe₂O₃ Chlorite is found in close association with pumpellyite in a few of the amygdules. The absence of zeolite, and the presence of prehnite and pumpellyite in the Sitagota metabasalt indicate prehnite-pumpellyite facies metamorphism.

Keywords

Prehnite, Pumpellyite, Amygdule, Metabasalt, Maharashtra.

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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.

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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.

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S. C. Patel ¹, A. V. Subrahmanyam ², T. Desapathi ², M. Nageswara Rao ², Y. V. Rathaiah ², V. Anil Kumar ², D. Rajasekhara Reddy ³, Malathi Vemuri ³

Affiliations
1 Department of Earth Sciences, IIT, Powai, Mumbai - 400 076, IN
2 Atomic Mineral Directorate for Exploration and Research, Begumpet, Hyderabad - 560 016, IN
3 Department of Geology, Andhra University, Visakhapatnam - 530 003, IN

Abstract

No Abstract.

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S. C. Patel ¹, S. Behera ², A. Mohanty ¹, A. K. Singh ³, S. K. Patel ³

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai, 400076, IN
2 Department of Applied Geology, Indian School of Mines, Dhanbad, 826004, IN
3 Orissa Mining Corporation Ltd., Bhubaneswar, Orissa, 751001, IN

Abstract

Cordierite-phlogopite dominated Mg-Al rich rocks occur as small enclaves at the Hatimunda hill near Junagarh in the Orissa sector of the Eastern Ghats belt. Peak mineral assemblages in these rocks included phlogopite, bronzite, kyanite (now reliet) and quartz/corundum. Sapphirine typically occurs as corona around corundum or in chisters along with corundum. The sapphirine is slightly more aluminous than the MgO:Al₂O₃:SiO₂ ratio of 7:9:3. Textural criteria and phase analysis in the model system MAS indicate the follwing reactions: Opx+Sil+Qtz+Crd; Opx+Sil=Crd+Crn; and Opx+Crn=Spr+Crd. A qualitative petregenetic grid shows that these reactions may be crossed in sequence along a decompressional P-T trajectory. The rocks preserve textural evidence of formation of sapphirine, at least in part, from phlogopite and sillimanite. P-T conditions for peak metamorphism were 700-800° C and > 8kbar based on the stability field of mineral assemblages. Texturally, early kyanite and late sillimanite indicate that the decompression is possibly part of a clockwise P-T path. This interpretation is supported by the occurrence of Mg-rich garnet (X_prp + 0.47) in the amphibole-dominated Mg-Al rich schist of the neighbouring Jilindara area.

Keywords

Sapphirine, Cordierite, Phlogopite, Relict kyanite, Decompression, Eastern Ghats.

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S. C. Patel ¹, B. R. Frost ², I. Faizan ¹, R. Dey ³, T. K. Ghosh ⁴

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Bombay, Powai, Mumbai - 400076, IN
2 Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, US
3 MR Division, R & C Directorate, ACC Ltd., LBS Marg, Thane - 400 604, IN
4 University Science Instrumentation Centre, University of Roorkee, Roorkee - 247667, IN

Abstract

Massif type anorthositic rocks in Bolangir area of the Eastern Ghats belt contain garnet porphyroblasts, which are surrounded by reaction coronas. The corona usually comprises a vermicular intergrowth, which in one instance consists of radial orthopyroxene and anorthite-rich plagioclase, and in another of radial chlorite and anorthite-rich plagioclase. Some of the anorthositic rocks are layered with thin discontinuous mafic layers of clinopyroxene and biotite. Notable is the presence of retrograde ferro-anthophyllite in the mafic layers of some of the rocks. Thermobarometric calculations using assemblage in the orthopyroxene corona indicate decompression from minimum peak P-T condition of 8 kbar and 700°C.

Keywords

Anorthosite, Metamorphism, Corona, Decompression, Bolangir, Eastern Ghats.

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