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

M. N. Balasubrahmanyan ¹, M. K. Murty ¹, D. K. Paul ¹, Amitabha Sarkar ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

No Abstract.

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Amitabha Sarkar ¹, D. K. Paul ¹, M. N. Balasubrahmanyan ¹, N. R. Sengupta ²

Affiliations
1 Geochronology and Isotope Geology Division, Geological Survey of India, 29, Jawaharlal Nehru Road, Calcutta 700 016, IN
2 Chemical Laboratory, Geological Survey of India, 27, Jawaharlal Nehru Road, Calcutta 700016, IN

Abstract

Lamprophyric dyke rocks occurring in the Indian Gondwanas (Permian-Lower Cretaceous) form a consanguinous suite ranging from ultrabasic mica-peridotite to micro-syenitic assemblages. Chemical characters include low SiO₂ (ca. 40% or less), high (FeO+MgO) and high MgO/FeO and K₂O/Na₂O ratios. The petrographic and chemical characters have been discussed in a broad framework after Rock (1977). K-Ar radiometric ages on four biotite separates and one whole rock from three different localities range from 105 to 121 Ma and are believed to represent true crystallisation ages.

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M. N. Balasubrahmanyani ¹, Amitabha Sarkar ²

Affiliations
1 Geological Survey of India, 40th Cross, Jayanagar, Bangalore 560041, IN
2 Geochronology Division, Geological Survey of India, Calcutta 700 016, IN

Abstract

Four ⁴⁰K/⁴⁰Ar mica ages of the granitoid suite around Kadavur range from 475 to 527 Ma with a mean around 500 Ma. Since (i) there is a close agreement in mica ages between variants of the granitoid suite and between size fractions and (ii) the dated minerals are from post-tectonic, undeformed and unmetamorphosed rocks, this mean age appears to closely approximate the age of intrusion of the granitoids. One hornblende separate from the older invaded hornblende leuco-gabbro of the Kadavur gabbro-anorthosite complex with a ⁴⁰K/⁴⁰Ar age of 492 Ma is most probably a thermally reset age.

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