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

S. Ray ¹, P. K. Gangopadhyay ¹

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

Abstract

Earlier studies on metamorphic zones in the metamorphic belts of Singhbhum, Manbhum and adjoining parts of Chhota Nagpur around longitude 86°E and between latitudes 22°45 Nand 23°40 N have been synthesised. Maps of Barrovian zones delineated by different workers in small areas have been coordinated into a regional zonal map.

Broad structural data of earlier workers plotted on the map depict that the pattern of extension of the zones over the whole region has a direct correlation with the plunges of the regional fold axes; higher grade zones occupy anticlinal cores with the lower grade ones succeeding them in the direction of the stratigraphic younging.

Granites, late syntectonic with tbe major folds on S" accompany most sillimanite zone outcrops. Andalusite occurs (i) across the chlorite to the kyanite zones along the southern fringes of the Dalma basic volcanics, and (ii) also sporadically. within sillimanite zone schists in association with sillimanite and kyanite.

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S. Ray ¹

Affiliations
1 Calcutta, IN

Abstract

Most of the Archaean rocks in India have been commonly referred to as having originated at plutonic or hypometamorphic depths. Data exist to prove that the Archaean crusts were decidedly thin and hotter. Thus, metamorphism in the remote Archaean time took place under lower load pressure around 4-7 kb and under higher temperature around 650°C-750°C. As the crust thickened metamorphism in depth took place under higher pressures and lower temperatures.

The Khondalite tract along the East Coast of India has mineralogical, petrological and structural singularity to prove that it was metamorphosed under the former set of P-T conditions.

This khondalite belt shows a trend of 'facies' variation which is strongly contrasted with the variation in Hietanen's Barrovian or other types. A new 'facies series' and metamorphic' type', viz, 'Khondalite type' is hence proposed to be added to Hietanen's eight 'types' of metamorphism.

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S. K. Sen ¹, S. Ray ¹, D. Dasgupta ¹

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

Abstract

In the type area of the charnockites, south of the city of Madras, foliation attitudes of marker horizons of leptynites and of basic granulite bands trace a map pattern characterized by four-fold hinges. The overall structure of the area is brought out by the swerve of axial planes of these folds from NNE in south-west through E-W in the central part to N-S in southern part in a broad sweep. The folding is noncylindrical; and a probable structural interpretation, as advanced here, envisages a series of tight isoclinal folds on which a folding around a N-S axial plane has been superposed.

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S. Ray ¹, P. Chakrabarti ²

Affiliations
1 Department of Geology, G. C. College, Silchar (Assam), IN
2 Department of Geology, Presidency College, Calcutta, IN

Abstract

During the Upper Vindhyan times there was a northwesterly flow of current prevailing throughout the Great Vindhyan Basin of Northern India.

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A. Acharyya ¹, S. Ray ², B. K. Chaudhuri ², S. K. Basu ³, S. K. Bhaduri ⁴, A. K. Sanyal ⁴

Affiliations
1 Geological Survey of India, 27 Jawaharlal Nehru Road, Kolkata - 700 016, IN
2 Geological Survey of India, Op WB-SK-AN, Eastern Region, DK-6, Salt Lake, Kolkata - 700 091, IN
3 Geological Survey of India, Flat#4, H/31M, BP Township, Kolkata - 700 094, IN
4 Geological Survey of India, Chemical Division, Eastern Region, DK-6, Salt Lake, Kolkata - 700 091, IN

Abstract

The South Purulia Shear Zone (SPSZ) is a part of the conspicuous -E-W to -ESE-WNW trending Tamar-Porapahar lineament. This borders the Meso-Proterozoic greenschist facies lithopackage of the Singhbhum Group (SG) comprising metasedimentaries, felsic volcanics, mafites/Ultramafites, granitoids, and alkaline/Carbonatite suite of rocks bounded by the Chhotanagpur Gneissic Complex (CGC) attaining amphibolite to granulite facies in the north CGC presumably had certatn loci of crustal weakness where pre-Rift crustal sagging led to the deposition of supracrustal rocks. Intracratonic rifting had then initiated in those pre-Existing zones of crustal weakness and troughs of the resident crust. The intracratonic basin was covered by huge volume of the felsic volcanics represented by layered tuffaceous volcaniclastic, felsic ignimbrites, crystal-Vitric tuff and tuffaceous phyllite. Meta-Greywacke has been considered as epiclastic deposit derived from felsic volcanics. The nature of welding, absence of phenocrysts typical of lava flows, parallel aligned lenticular pumice fragments exhibiting pumice-Lenticle foliation, fiamme and eutaxitic fabric and petrochemistry could imply that the felsic igniimbrite was close to welded pumiceous ash-Flow tuff having dacitic to rhyodacitic composition. Mafic-ultramafic suites of rocks in close proximity to felsic volcanics suggest a bimodal character of magmatic episode in the area Bands and lenses of tourmalinite disposed along the contact of the CGC and the SG, argued to be of volcanogenic sedimentary origin, possibly demarcates the rupture locales of a rift. The alkali-Feldspar granite(± richterite, ± aegirine-augite), syenite and carbonatites suggest tapping of deeper fractures in the rifted basin. This was coupled with the presence of granite plutonism in the setting. Geochemical signatures imply that felsic volcanics are chiefly of dacitic composition with strong LREE enrichment and narrow range of fractionation while the metabasic rock chemistry scatter in basalt to basaltic andesite field. Trace element data further corroborate 'volcanic arc granite', ' Within plate granite' and 'continental arc granite' composition for the felsic volcanics. The basin was then subscribed to compressional regime where ductile shear zones developed syn to post kinematic to F1 with the northern block (CGC) thrusting over the southern block (SG) on northerly dipping planes. Thus, an entire sequence of rifting, tourmalinite emplacement, felsic volcanism, mafic extrusion, granite plutonism, syenite/Carbonatite emplacement and sedimentation of pelites/Epiclastics during and after opening of the basin in an intracratonic setting is demonstrated. Subsequently, development of the shear zone (SPSZ) took place over the area at the close of the basin.

Keywords

Ignimbrite, Dacite, Tourmalinite, Rift, Shear, Petrography, Geochemistry, Purulia, West Bengal.

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