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

B. F. Windley ¹, T. A. Selvan ²

Affiliations
1 Department of Geology, The University of Leicester, GB
2 Geological Survey of India, Tamil Nadu Circle, 35 Haddows Road, Madras 6, IN

Abstract

No Abstract.

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S. Ramadurai ¹, M. Sankaran ², T. A. Selvan ³, B. F. Windley ⁴

Affiliations
1 Central Groundwater Board, Jaipur, Rajasthan,, IN
2 Central Groundwater Board, Madras, IN
3 Geological Survey of India, Tamil Nadu Circle, Madras, IN
4 Department of Geology, Leicester University, GB

Abstract

The Sittampundi Complex has the following igneous stratigraphy from the bottom upwards: gabbro with pyroxenite inclusions, chromite-layered hornblende anorthosite and clinozoisite anorthosite. This stratigraphy is only well preserved in the north-east of the area. The complex has been folded into an isoclinal antiform with the result that the stratigraphy is duplicated in two fold limbs which are locally separated by gneiss. A second phase of open folding has given rise to the present form of the complex as a major interference pattern. Linear mineral fabrics largely formed in association with the first deformation. The complex is a tectonic remnant of a layered igneous body that has been recrystallized and deformed. It is remarkably well preserved in more ductile, quartzo-feIdspathic gneisses that have suffered higher finite strain.

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C. V. R. K. Prasad ¹, N. Subba Reddy ¹, B. F. Windley ²

Affiliations
1 Department of Geology, Sri Venkateswara University, Tirupati 517 502, Andhra Pradesh, IN
2 Department of Geology, The University, Leicester LEI 7RH, GB

Abstract

In Archaean granulite-gneiss belts there are banded iron formations (BIF) less than 20m thick which are weakly banded magnetite quartzites occurring in a supracrustal sequence of quartzites, mica schists, marbles and metavolcanic amphibolites. This sequence forms stratigraphic markers within voluminous intrusive tonalitic to trondbjemitic gneisses. These rocks are highly folded and metamorphosed to high amphibolite or granulite facies at 7-12 kb pressure in the lower Archaean crust. Such iron formations are well represented in Tamil Nadu, Southern Karnataka, Kerala and Andhra Pradesh in South India; in Hebei Province in China; Kola Peninsula, Stanovoy Range, and Ukraine in USSR; Kambui schists of Sierra Leone; Nimba series of Liberia; Limpopo belt of southern Africa; Imataca complex of Venezuela; Uivak gneisses of Labrador; and Amitsoq gneisses of West Greenland. They are obviously different in type and environment from the Proterozoic Superior type of BIF; it is shown here how they are different from the Archaean Algoma type of BIF. They were deposited in a quartzite-pelite-carbonate association and they lack the silicate-carbopate-sulphide facies of the Algoma-type which, in contrast, was deposited with a greywacke-flyschconglomerate- shale association in subsiding basins (Greenstone belts). Because they are dissimilar to the Algoma-type of BIF in both their sedimentary facies and tectonic environment (the granulite-gneiss belts are not highly metamorphosed greenstone belts), they should be given a separate status and we tentatively suggest they be termed the Tamil Nadu type.

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R. D. Broughton ¹, B. F. Windley ¹

Affiliations
1 Department of Geology, University of Leicester, Leicester, LE1 7RH, GB

Abstract

The Central Himalayan Gneisses on the northern margin of the Indian plate in the Hazara and Lower Swat areas of N. Pakistan comprise; orthogneisses with minor schists, amphibolites and calc-silicates, overlain unconformably by a cover sequence of graphitic and calcareous schists, marbles and minor amphibolites. These rocks are bounded to the south by a major thrust (equivalent to the Himalayan Main Central Thrust), and to the north by lhe Main Mantle Thrust (Indus Suture). To the south of the MCT there is a thrust slice of basement paragneisses with no relics of cover. The dominant gneissose foliation parallels the main thrusts, and is modified by numerous, large-scale N-S trending open folds.

The ortho- and para-gneisses and cover sequence are isofacial, and underwent two phases of metamorphism, prior to 35 Ma. Mineral equilibria indicate conditions for the cover sequence in the range 550-620°C and 8.0-8.9 kb, and for the paragneiss basement at ∼650°C and 6.7-6.8 kb. The two pressure ranges come from different thrust slabs. These data indicate an average cooling rate of ∼17°C my^-1, and an average uplift rate of ∼0.8 mm yr^-1 over the last 35 My, for the relic cover sequence.

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