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

A. K. Banerji ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta, IN

Abstract

The Singhbhum shear zone represents a deep seated fracture that probably extended up to the upper mantle through which basic rocks poured out continuously and extensively together with some ultrabasic rocks. The northern block gradually subsided and received sediments interlayered with basic rocks and tuffs, with a conglomerate horizon at tbe base. Subsequently the movements were reversed along the line of fracture and it was transformed into a zone of shearing within which migma-magma activity and mineralisation took place. Residual stresses within the shear zone were released through wrench faulting after which the shearing movements resumed locally and continued even up to recent times at some places.

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A. K. Ghosh ¹, A. K. Banerji ²

Affiliations
1 Department of Geology, University of Calcutta, IN
2 Department of Geology, Presidency College, Calcutta, IN

Abstract

No Abstract.

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A. K. Banerji ¹, A. K. Talapatra ², A. V. Sankaran ³, T. K. Bhattacharyya ³

Affiliations
1 Geology Department, Presidency College, Calcutta, IN
2 Geological Survey of India, Patna, IN
3 Petrology Laboratory, Atomic Minerals Division, New Delhi, IN

Abstract

Study of the trends of geochemical migration of some major and minor elements during progressive migmatisation within part of the Singhbhum shear zone supports the view that apatite-magnetite, uraninite and copper sulphide deposits and the associated wall rock alterations are related to processes of migmatisation therein. During migmatisation extensive mobilisation of elements took place when Si, Al, Na and small quantities of Ga, U and Zr were introduced into the pre-existing basic metavolcanics within the shear zone, and Fe, Mg, Ca, K, Ti, Mn, P, Ba, Rb, Sr, Li, Ni, Co, Cr, V, Cu, and Y were expelled from these rocks. The addition of the former group of elements brought about transformation of these rocks, into felspathic schists and gneisses (the soda-granites) while the expulsion of the latter group of elements from these rocks and their addition to the migmatising fluids caused progressive changes in the chemical composition of these fluids. At the closing stages of the process these fluids were transformed into mineralising solutions that gave rise to apatite-magnetite, uraninite and copper sulphide deposits and to extensive sericitisation, chloritisation, biotitisation and silicification within the shear zone. The various elements, present in traces, entered the crystal lattices of these minerals, in accordance with their geochemical preferences and in proportion to their concentration in the mineralising solutions. The resulting deposits should be regarded as 'metasomatic hydrothermal'.

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A. K. Banerji ¹

Affiliations
1 Geology Department, Presidency College, Calcutta, IN

Abstract

The iron ore bearing rocks of Singhbhum and the adjoining areas of Bihar and Orissa consist of two groups with distinct lithological characters. One of these occurs within the Noamundi basin in the western part of the Singhbhum granitic platform, while the other occurs in the Gorumahisani-Badampahar ranges in the eastern part of the platform. Detailed studies indicate that the iron ore bearing rocks of the Noamundi basin (Noamundi group) rest with a slight break on the Jagannathpur lava which is considered equivalent to the Dhanjori lava. Hence the Noamundi group is assigned a stratigraphic position above the Dhanjori group. The iron ore bearing rocks of the Gorumahisani-Badampahar region (Gorumahisani group) underlie the Dhanjori group and hence are assigned a stratigraphic position below these rocks. Each of these rock groups has been affected by independent cycles of deformation, metamorphism and igneous activity as a consequence of which these rocks have been welded into the fabric of the Singhbhum granitic platform. The successive orogenic cycles have been provisionally dated on the basis of geochronological data provided by Sarkar et al, and a revised stratigraphic succession for the rocks of the region has been suggested.

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A. K. Banerji ¹, N. C. Bhattacharyya ¹, B. Chattopadhyaya ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta 700073, IN

Abstract

The granophyre bodies have massive monzo- to syenogranitic core and gneissose to schistose borders where the composition varies from granodioritic to tonalitic. These bodies appear to have crystallised from granitoid melts generated within the shear zone during an earlier phase and were replaced by albitic liquids, around their borders, during a later phase. The evolution of these rocks seems to be related to the tectonic history of the shear zone which represents the boundary between a southerngranitic platform and a northern mobile belt and evolved as the platform progressively underthrust below the mobile belt during the course of four successive phases of deformation. It is suggested that the granitoid melt was derived through the partial melting of the metapelitic rocks of the shear zone during the first phase under pressure of about 2 Kbar while the albitic liquid was derived during a later phase when pressure exceeded 5 Kbar. Temperature, well above 650°C, required for such melting could have been provided by the higher geothermal gradient along the shear zone, which primarily represents a zone of deep seated fracture, possibly extending up to the upper mantle; frictional heat generated during thrusting and shearing, rise of isogeotherms on either side and within the shear zone during concomitant regional metamorphism, and radioactive heat as indicated by the occurrence of uraniferous minerals along this zone.

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A. K. Banerji ¹

Affiliations
1 Department of Geology, Calcutta University, 35, Ballygunge Circular Road, Calcutta 700019, IN

Abstract

A number of massive pegmatoid kyanite veins occur along shear planes at Lapsa Buru, in the western part of the Singhbhum shear zone, Bihar. The host rocks are Precambrian metatuff and metachert, now represented by muscovite-biotite-chlorite schist and quartzite, which have been affected by four phases of deformation and metamorphism. During the first two phases granitic rocks were emplaced within the shear zone at temperature between 650°-700°C and pressure around 2 Kbar. During the 3rd phase albite metasomatism took place within the shear zone under pressure of about 5 Kbar. At the same time, kyanite veins were emplaced along the northern boundary of the shear zone under slightly higher pressure ranging between 5-6 Kbar. A review of experimental data indicates that temperature around 450°-500°C prevailed during kyanite segregation. It has been suggested that under these P-T conditions alumina and silica were mobilised from the alumina rich volcanogenic rocks of the shear zone. Presence of tourmaline, topaz, and dumortierite in the kyanite veins indicates that boron and flourine may have aided the mobilisation process. The resulting kyanite was affected by minor shearing and local retrogressive metamorphism during the fourth phase of deformation in the region.

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