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

D. S. Bhattacharyya ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

Three phases of high grade metamorphism are recognised here. These are dated with reference to the four phases of deformation recognised in this area. It is concluded that the index minerals are progressive in time and space; the metamorphisms are also synkinematic to post-kinematic. Zonal patterns are analysed and it is seen that these show synchronous fold shapes. Folding of the geoisothermals seems to be the best explanation for the metamorphic episodes.

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S. Pasayat ¹, D. S. Bhattacharyya ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal, IN

Abstract

Earlier workers recognised two phases of deformation. Evidences are presented here which indicate three phases of deformation. The imprint of the last phase of deformation is strongest.

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A. N. Sarkar ¹, D. S. Bhattacharyya ²

Affiliations
1 Geological Survey of India, Map Division, 29, Jawaharlal Nehru Road, Calcutta-700 016., IN
2 Department of Geology and Geophysics, Indian Institute of Techonology, Kharagpur 721 302, IN

Abstract

The study reveals three generations of deformation which have produced two splanes and a host of linear structures with complex geometric relations. All the three fold systems are, however, not equally developed all over the area and the associated structures also regionally vary in orientation and angular relation; this has resulted in diverse geometric patterns in different subareas.

The first deformation produced subhorizontal folds (F₁) on bedding (S₁) with strongly developed axial plane schistosity (S₂). These folds are refolded, along with S₂, about a set of steeply plunging axes (F₂) at high angles with the F₁ axes (60° to 90°). The main structural closures defined by Hesadi antiform and Kamrora synform are caused by the second folds (F₂). On these structures are superposed a subhorizontal E-W trending set of folds (F₃) with northerly dipping axial planes, resulting in dispersal of all the earlier lineations. The F₃ folding was also accompanied by shearing in several tracts producing shear cleavage (S₃) with downdip striation and groove lineations. The F₃ folds show somewhat regular plunge reversals on macroscopic scale forming highly elongated domes and basins.

An attempt has been made to restore the original attitudes of the fold systems.

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A. N. Sarkar ¹, D. S. Bhattacharyya ²

Affiliations
1 Geological Survey of India, Map Division, 29, Jawaharlal Nehru Road, Calcutta-700016, IN
2 Indian Institute of Technology, Kharagpur, IN

Abstract

The rocks record four phases of deformation, of which the imprints of the first two are not readily recognisable. The deformation during the third phase was the most intense and is manifested in the northern part of the area by a set of E-W folds on an earlier schistosity and its transposition into a later schistosity. Towards the south, the transposition increases and culminates in the formation of the shear zone. The fourth deformation, restricted within the shear zone, indicates a relative downsliding movement of the northern block over the southern block. Two phases of regional metamorphism are recognised: (i) a progressive cycle, synchronous with the first deformation, and (ii) a progressive-retrogressive cycle, syn-to post-kinematic with respect to the third deformation. The later cycle shows a general increase of grade towards tbe north. The study records two highly contrasting, but grossly simultaneous, tectonometamorphic environments during the third deformation: (i) low stresses, low strain-rates, more of grain rotation and progressive metamorphism in the areas of folding in the north, and (ii) high stresses, high strain-rates, more of flowage and slip and retrogressive metamorphism in the shear zone in the south.

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D. S. Bhattacharyya ¹, D. Dasgupta ¹

Affiliations
1 Indian Institute of Technology, Kharagpur, IN

Abstract

No Abstract.

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D. S. Bhattacharyya ¹, D. Dasgupta ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, IN

Abstract

The Precambrian Dalma metabasalts stretching for more than 200km represent original oceanic tholeiites. The metabasalts can be represented by eleven different groups of mineralogical assemblages ranging from low greenschist to high amphibolite facies. Spatial distribution of these six groups indicates increasing P-T conditions from east to west.

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P. Sanyal ¹, D. S. Bhattacharyya ¹, D. Dasgupta ¹

Affiliations
1 Department of Geology and Geophysics, IIT, Kharagpur, IN

Abstract

The Arkasani granite gneiss occurs as detached elongate outcrops close to the northern margin of the Chakradharpur granite gneiss. Both show imprints of the same deformation and same textural and structural characters. While the Chakradharpur gneiss ranges from tonalite - granodiorite - granite - alkali feldspar granite, the Arkasani gneiss is an alkali feldspar granite. Major element compositions of the two are either gradational or overlapping. The former has higher Na₂O values and the latter higher K₂O values. The two groups of gneisses are genetically related.

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