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

C. R. L. Friend ¹, A. P. Nutman ¹

Affiliations
1 Department of Geology, Oxford Polytechnic, Oxford, OX3 OBP, GB

Abstract

Zircons separated from a sample of Peninsular gneiss (82-KD 14) from Kabbaldurga quarry, and a homogeneous Closepet granite (KO84-13) from near Ramnagaram have been dated with the ion microprobe SHRIMP. The Peninsular gneiss sample, a metatexite, contains an arrested charnockite transition and constitutes an amphibolite-granulite close pair. It was split into amphibolite facies, transitional and granulite facies (charnockite) domains prior to zircon separation.

Zircon populations separated from each domain are morphologically similar and have the same U-Pb isotopic systematics suggesting that they record events common to both the amphibolite facies and granulite facies domains. The pooled data yield a U-Pb age of 2963 ± 4 Ma (20σ) for the gneiss protolith at Kabbaldurga, with a few older inherited zircons found as cores in magmatic zircon. A concordant U-Pb age of 2528 ± 5 Ma was obtained for newly grown zircon and overgrowths, interpreted to have formed during a migmatisation event which led to the formation of the Closepet granite.

Zircons separated from the Closepet granite sample gave a concordant age of 2513 ± 5 Ma, interpreted as the time of crystallisation of the granite. This age was obtained from both zircon cores and overgrowths which are indistinguishable in age and arc both interpreted to have formed during the melting event. In preserved cores no indication of any inherited grains with a significantly older history was found.

This is the first precise age on the Closepet granite. The slight age difference between the metatexite event (2528 ± 5 Ma) and the granite age 2513 ±5 Ma) may indicate the length of time of the event during which the Closepet granite formed. The data provide a minimum constraint on the age of the charnockite event in the region.

Keywords

Geochronology, Closepet Granite, Peninsular Gneiss. Karnataka SHRIMP U - Pb Age Data.

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A. P. Nutman ¹, B. Chadwick ², M. Ramakrishnan ³, M. N. Viswanatha ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra ACT 2601, AU
2 Earth Resources Centre, University, Exeter EX4 4QE, GB
3 Geological Survey of India, 2 Church Street, Bangalore 560 001, IN
4 17 Rajamahal Vilas Extension, Bangalore 560080, IN

Abstract

New limits have been set on the age of the provenance and the depositional period of the oldest known Archaean supracrustal rocks (Sargur Group) in southern India. Detrital zircon grains from a pelitic schist and a quartzite within major tracts of supracrustal rocks older than their host regional grey orthogneisses (peninsular Gneiss, c. 3000-2900 Ma) have yielded U-Pb ages in the range 3580-2960 Ma. The data indicate that granitoid rocks in the age range 3580-3130 Ma were a significant component of the provenance of the sedimentary protoliths. Ages younger than 3130 Ma are attributed to effects of high-grade metamorphism during emplacement of the igneous precursors to the host orthogneisses. Exhumation of the granitoid provenance, deposition of the sedimentary protoliths, intrusion of major gabbroic and peridotitic complexes and possible basaltic volcanism represented by amphibolites in the tracts of supracrustal rocks took place in the period 3130-2960 Ma. This age range is at variance with previous suggestions that the Sargur Group represents early Archaean or primitive crust.

Keywords

Geochronology, Zircon-Dating. U-Pb Ages, Sargur Supracrustals, Karnataka.

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A. P. Nutman ¹, Brian Chadwick ², B. Krishna Rao ³, V. N. Vasudev ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, A.C.T. 0200, AU
2 Earth Resources Centre, University of Exeter, Exeter EX4 4QE, GB
3 Department of Geology, University of Mysore, Mysore - 570 006, IN
4 120/45(A), III Block, Thyagarajanagar, Bangalore - 560 028, IN

Abstract

SHRIMP U/Pb dating of zircon grains from a schistose acid volcanic rock from the Daginkatte Formation in the Chitradurga Group of the Late Archaean Dhwar Supergroup in western Karnataka has yielded a precise concordant age of 2614 ± 8 Ma which indicates the time of melt crystallisation. In contrast, zircons from similar schistose acid volcanic rocks in the Vibhuti Gudda Formation of the Sandur Group in the Sandur schist belt of eastern Karnataka yielded less precise concordia intercept ages of 2658±14 and 2691±18 Ma.

Zircons from high-strain grey gneisses which appear to be the oldest recognisable component of the Late Archaean granite complex adjacent to the Sandur schist belt have an imprecise age of 2719 ± 40 Ma. Zircons from the youngest recognisable granite adjacent to the belt have also yielded an imprecise, but younger, age of 2570 ± 62 Ma. The ages show that granite emplacement and deformation adjacent to the Sandur schist belt took pIace in a period of c.150 Ma.

The imprecise ages of zircon in the acid volcanic rocks in the Sandur Group and the adjacent granites are related to Neoproterozoic loss of lead which may have been an effect of either weathering or a regional thermal event. We favour the latter in the light of the record of Pan-African thermal effects in the east and south of southern Peninsular India.

Keywords

Geochronology, Zircon, Acid Volcanics, Sandur Schist Belt, Karnataka.

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A. P. Nutman ¹, B. Chadwick ², M. Ramakrishnan ³, M. N. Viswanatha ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, AU
2 Earth Resources Centre, University Exeter EX4 4QE, GB
3 Geological Survey of India, Bandlaguda, Hyderabad 500 660, IN
4 17, Rajamahal Vilas Extension, Bangalore 560 080, IN

Abstract

No Abstract.

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Brian Chadwick ¹, V. N. Vasudev ², G. V. Hegde ³, A. P. Nutman ⁴

Affiliations
1 17 St Mary's Park, Ottery St Mary EX 11 IJA, GB
2 Australian Indian Resources Group Companies, 125/45, 3rd Block, 6th Cross, 6th Main, Tyagarajanagar, Bangalore-560 028, IN
3 Department of Mines and Geology, Government of Karnataka, "Khanija Bhavan", Race Course Road, Bangalore 560001, IN
4 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AU

Abstract

Neoarchaean granites adjacent to the Chitradurga schist belt were emplaced in the inner margin of the foreland in the context of the Neoarchaean oblique convergent setting of the Dharwar craton. Two previously unreported granites, one 50 km and the other 80 km NW of Chitradurga town, and a mylonitised granite in the hanging wall of a duplex in the NW of the schist belt yielded SHRIMP U/Pb zircon emplacement ages of 2648±40 Ma, 2598±19 Ma, and ca. 2600 Ma, respectively, the large errors being due to radiogenic Pb loss during an unidentified Neoproterozoic event. Some discrete zircon grains and xenocrystic cores yielded ≥3000 Ma ages that were derived from older rocks during anatexis or emplacement. The granites NW of Chitradurga town were emplaced as steep sheets trending NW·SE. The Chitradurga granite has a similar form, bifurcating N of Chitradurga town into two separate, steeply dipping, NW-SE sheets. Magmatic-and solid-state fabrics in these granites show that emplacement took place during, but was outlasted by, sinistral and dextral strike-Parallel shear. Emplacement of the granite above the hanging wall of the duplex in the NW of the schist belt was outlasted by top-SW displacement.

The shapes of the granites and their emplacement in relation to the structure of the Ranibennur and Chitradurga schist belts in the west of the craton are modelled as a mid-Crustal part of a craton-Wide imbricate fold-Thrust belt. The relationships show that whereas some Neoarchaean granites in the craton were emplaced prior to, or during, SW-vergent thrust thickening, most granites and related plutonic suites in the foreland and accretionary complex were emplaced later as multipulse injections in steep NW-SE sheets or wedges during orogen-parallel. sinistral and dextral shear. Steep highstrain zones in the foreland and accretionary complex are interpreted as listric structures that ischolar_main into an attachment at a depth of ca. 18-20km in accord with the depth of the boundary between upper and lower crust placed at ca. 23 km from seismic reflection data published in 1979 and in more recent studies. The new structural observations and zircon dating, combined with published isotopic age data, show that the inner margin of the foreland in the west of the craton and the outer margin of the accretionary complex in the east are linked in a diffuse, steeply dipping, Orogen-parallel boundary zone at least 200 km wide.

Keywords

Neoarchaean, Dharwar Craton, Fold-Thrust Belt, Transpressive Orogen, Oblique Convergence, Attachment, Chitradurga Schist Belt, Karnataka.

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