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

Fareeduddin ¹, V. P. Mishra ¹, S. K. Basu ¹

Affiliations
1 Geological Survey of India, AMSE Wing, Vasudha Bhavan, Kumaraswamy Layout, Bangalore - 560 078, IN

Abstract

A pictorial display of some of the kimberlites, lamproites and lamprophyres of India, is given in this work. This would give an idea about the texture and mineralogy of these rocks from different cratonic regions of the Indian shield.

Keywords

Kimberlites, Lamproites, Lamprophyres, Petrography, Atlas.

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S. K. Basu ¹

Affiliations
1 PPOD, AMSE Wing, Geological Survey of India, Eshwar Nagar, Bangalore - 560 070, IN

Abstract

No Abstract.

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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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S. K. Som ¹, K. C. Bandyopadhyay ¹, S. K. Basu ¹, D. K. Santra ¹, R. N. Ghosh ²

Affiliations
1 Geological Survey of India, Op. WB-S-A&N, GSI Office Complex, DK-Block, Sector 11, Salt Lake City, Kolkata -700 091, IN
2 Geological Survey of India, Zorem building, Nongrim Hills, Shillong - 793 003, IN

Abstract

The Li-Cs-Rb bearing Beku pegmatite occurs in a calc-silicate rock enclave within the Proterozoic Chhotanagpur Gneissic Complex (CGC). Its internal structure shows zonation that comprises an outer tourmaline-rich pegmatite and inner rare metal-bearing pegmatite containing spodumene-pollucite-lepidolite. The pegmatite is strongly peraluminous with high SiO₂, low Fe₂O₃(t), CaO, TiO₂, P₂O, and variable Al₂O₃, K₂O and Na₂O. Its Cs/Rb and Li/Rb values increase towards the centre and decrease towards the boundary. K/Rb value increases from pollucite-bearing pegmatite to tourmaline-rich pegmatite, depicting the more evolved nature of the pollucite-bearing pegmatite. Petrography and geochemistry of the Beku pegmatite suggest fractionation from a peraluminous S-type granitic magma. It is synchronous with shear development in the area, under a minimum formational P-T of 4 kbar and 500°C.

Keywords

Rare Metal (Li-Cs-Rb) Pegmatite, Calc Silicate Rock, Proterozoic, Chhotanagpur Gneissic Complex, Beku, West Bengal.

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