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

M. J. D'Souza ¹, S. K. Chakraborty ²

Affiliations
1 Antarctica Division, Geological Survey of India, NH 5P, N.I.T., Faridabad- 121001, IN
2 Central Petrological Laboratory, Geological Survey of India, 15 A & D Kyd Street, Calcutta-700016, IN

Abstract

Lamprophyre dykes are conspicuous within the Proterozoic high-grade metamorphic rocks of Schirmacher Oasis. These dykes have been classified chemically into alkaline and calc-alkaline types on the basis of K₂O/Na₂O ratio and CIPW norm. The close association, age and geochemical similarities of both the-lamprophyres and melasyenite in thc area indicale their consanguinity and derivation from same source region. These lamprophyres have a high concentration of incornpatible elements like Ba, Rb, Sr and Zr. These rocks also have a high Mg# and depict a negative Ta-Nb-Ti anomaly. The chondrite normalised REE patterns show fractionated trends with large variation in LREE and small variation in HREE concentration in both the rock groups. The calc-alkaline lamprophyre is more enriched in LREE than the alkaline lamprophyre.

It is proposed that metasomatised phlogopite-bearing enriched mantle source has undergone partial melting to generate lamprophyric magma. This magma is generated in a continental within-plate tectonic setting. The collision of East and West Gondwana during Pan-African orogeny, resultant overthickening of continental lithosphere and generation of widespread magmatic activity in the region due to partial melting has culminated in the intrusion of lamprophyres around 450 Ma ago.

Keywords

Lamprophyre, Melasyenite, Schirmacher Oasis, East Antarctica, Pan-African Orogeny.

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R. Ravindra ¹, N. C. Pant ¹, M. J. D'souza ¹

Affiliations
1 Geological Survey of India, Antarctica Division, Faridabad, IN

Abstract

The Humboldt region of the Wohlthat Mountains in East Antarctica, depicts both polar and alpine type of morphological set-up. While the former is represented by vast stretch of antarctic ice sheet with hills and nunataks, the latter is marked by glacially fretted uplands with cirques, aretes, horns and moraines. These and other landforms associated with denudation and depositional processes have been used to evaluate the regional landscape of Humboldt area, which conforms with the composite erosional landscape of Sugden and John (1976).

The glacial geomorphological features are largely, due to Somovken and Humboldt glaciers flowing on either side of the Humboldt fiat. The interference of the movement between the two glaciers is reflected in the depositional pattern of moraines. The polar ice evidently covered much greater portions of the area in the past with ice level covering the ridges at 1900 m - 2000 m above sea level.

Keywords

Geomorphology, Antarctica, Wohlthat Mountains.

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Amitava Kundu ¹, M. J. D'souza ¹, Anand Mohan ²

Affiliations
1 Antarctica Division, Geological Survey of India, Faridabad, IN
2 Department of Geology, Banaras Hindu University, Varanasi, IN

Abstract

Preliminary study of metamorphic reaction textures coupled with thermobarometry suggests that the metapelites. garnet-clinopyroxene gneisses. calc-silicate gneisses and orthopyroxene bearing quartzo-feldspathic gneisses exposed in Dallmannfjellet-Conrad area, central Dronning Maud Land, East Antarctica were metamorphosed to granulite grade. Peak metamorphic conditions evaluated from core thermobarometry are ~900 - 1050°C and pressure ~9 kbar. Sillimanite and garnet growth over spinel in metapelites suggests cooling of the rocks to 800°C, followed by a mild decompression inferred on the basis of orthopyroxene+plagioclase moats around garnet in metanorite pods within metapelite. The metanorite pods record P-T conditions of 7-10 kbar and 750-850°C. Late component of the P-T segments is represented by cooling stage and eventual exhumation of the terrain.

Keywords

Petrology, Reaction Textures, Thermobarometry, Metamorphic History, East Antarctica.

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