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

R. K. Goel ¹, N. G. K. Nair ¹

Affiliations
1 Dept. of Geology & Geophysics, University of Roorkee, IN

Abstract

Classic pin valley section in Spiti (Himachal Pradesh) India, has been re-examined and formal stratigraphic nomenclature in terms of the Lower Paleozoic succession of Spiti is proposed.

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Pradeep K. Aggarwal ¹, N. G. K. Nair ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee 247672, IN

Abstract

Data on the distribution of Cd, Mn, Cu, Ag, Ni and Co in four monomineralic fractions each of sphalerite and pyrite from five ore samples of the Precambrian stratiform zinc-lead-copper deposit of Deri, Rajasthan are presented. The minor element geochemistry of the sulphides suggests possible volcanic exhalative origin.

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K. Soman ¹, N. G. K. Nair ², V. N. Golubyev ³, M. M. Arakelyan ³

Affiliations
1 Department of Geology, Friendship University, Moscow, RU
2 Centre for Earth Science Studies, Trivandrum 695010, IN
3 IGEM, Academy of Sciences, Moscow, USSR, RU

Abstract

K/Ar mica age of chrysoberyl-bearing pegmatites from four localities of Trivandrum district, Kerala ranges from 445 to 474 m.y., with a mean around 460 m.y. There is a close agreement between the ages of the concordant and cross-cutting pegmatites, between muscovite and biotite ages, as also between various size fractions. Chrysoberyl occurs within feldspars in association with quartz and muscovite. Since the formation of chrysoberyl-quartz association requires a higher pressure or temperature, the reported age would indicate the time of emplacement of the chrysoberyl-bearing pegmatites at the closing phase of a major tectonomagmatic event. It is interesting to note that these pegmatites of south-western India are younger compared to those of other localities viz., Rajasthan, Bihar. Nellore and Mysore.

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M. Santosh ¹, N. G. K. Nair ¹

Affiliations
1 Centre for Earth Science Studies, P. B. No. 2235, Trivandrum 695010, IN

Abstract

The granite of Chengannoor, Alleppey district, Kerala, having an intrusive relationship with Precambrian charnockite-cordierite gneiss sequence is inferred to be a post-kinematic granite of magmatic origin. The major mineral constituents are mesoperthitic K-feldspar with quartz and two generations of plagioclase. A late/post intrusive potash metasomatism caused changes in the mineralogy and chemical composition of the granite.

Recent K-Ar dating of hornblende from the Chengannoor granite assigns an age of 550 m.y. for the granite (Soman, Golubyev and Santosh. Ind. Jour, Earth Science) (in press).

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N. G. K. Nair ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

No Abstract.

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M. Santosh ¹, N. G. K. Nair ¹

Affiliations
1 Centre for Earth Science Studies, P. B. No. 2235, Sasthamangalam. Trivandrum 695010, IN

Abstract

A syenite pluton occurring ncar Angadimogar in Cannanore district. northern Kerala and emplaced close to the continental margin and along a NW-SE trending faultlineament is reported. Alkali feldspar constitutes the dominant mineral with subordinate albitic plagioclase and quartz. corresponding to nordmarkitic composition. The syenite shows unique assemblage of accessory minerals. namely hornblende, riebeckite. crocidolite and acmite with diopside, bi.otite. zircon, sphene and calcite. High alkali content, with dominance of Na₂0 over K₂0, moderate transition dement contents and low Rb values are characteristic. Petrochemical features suggest melt equilibration from a K-rich, Rb-depleted mantle source, with subsequent amphibole and alkali feldspar fractionation. Significant increase in fO₂, fH₂O and peralkalinity towards the residual stage is deduced. The syenite belongs to the group of Late Precambrian-Early Palaeozoic alkaline intrusives of the region, with its generation and emplacement correlatable with the taphrogenesis of the western continental margin of lndia.

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K. Soman ¹, N. G. K. Nair ¹, A. V. Druzhinin ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695 010, IN
2 Peoples Friendship University, Moscow, RU

Abstract

Recent investigations prove that the southern part of Kerala and the adjoining areas further to the south in Tamil Nadu constitute a chrysoberyl-pegmatite field in the granulite terrain of southwestern India. Gem stones like chrysoberyl and its varieties, sapphire, topaz and zircon are mostly derived from pegmatites, though some come from the alluvial beds also. Chrysoberyl-pegmatites occur in a predominantly garnet-sillimanite ± graphite gneissic terrain with intermittent bands of garnet-biotite gneiss and enderbites. Geochronological data reveal that the pegmatites were emplaced 445-474 m.y. ago. Fluid inclusion studies show that the pegmatite minerals contain liquid carbon dioxide and give an entrapment temperature of 500-570°C. CO₂ densities permit a pressure estimate of 250 Mpa and more, which is also consistent with the sillimanite-andalusite and quartz-chrysoberyl associations in the pegmatites. The similarities in geology, the ages of pegmatites and granites and also the mineral composition of pegmatites of southwestern India and the alluvial gem gravels of Sri Lanka suggest that the al1uvial gem beds of Sri Lanka may also be of pegmatitic origin.

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M. Santosh ¹, N. G. K. Nair ¹, K. Pande ², K. Gopalan ²

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Sasthamangalam, Trivandrum 695 010, IN
2 Physical Research Laboratory, Navarangpura, Ahmedabad 380 009, IN

Abstract

Rubidium and strontium determinations are reported for seven whole-rock samples of the Ambalavayal granite of northern Kerala. The data yield a well defined Rb-Sr isochron, corresponding to an age of 595 ± 20 Ma and initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7171 ± 0.0022. The age marks a regime of Late Precambrian-Early Palaeozoic granite magmatism in the southwestern Indian shield. The agreement of the Rb/Sr age with the previously measured K/Ar age of 560± 30 Ma indicates no significant secondary thermal event in the region. The evolved initial Sr ratio suggests either an exclusive origin or significant crustal contamination of magma derived from a deeper source.

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N. G. K. Nair ¹, M. Santosh ¹

Affiliations
1 Centre for Earth Science Studies, P. B. No. 2235, Trivandrum 695010, IN

Abstract

The E-W elongated pluton of Peralimala, Cannanore district, Kerala, which is classified here as an alkali granite, occurs along the Bavali lineament, spatially associated with the late Proterozoic anorthosite-gabbro-granite-granophyre suite of the Ezhimala Igneous Complex. Perthitic K-feldspar, plagioclase of albite-oligoclase composition, quartz alkali amphibole, aegirine and sphene constitute the main mineralogy of the alkali granite. Chemically, the rock shows high content of alkalies with K₂O values ranging up to 10.96 wt. percent. The extremely high levels of K, AI, Ba and Ba/Sr suggest the existence of a phlogopite-rich pocket in the mantle beneath the Bavali lineament from where the melt equilibrated. As peralkaline plutonism is a keynote of pre-rift tectonics, the occurrence of the alkali granite along the Bavali lineament is a manifestation of rift-related alkaline magmatism in this part of the western continental margin of India.

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