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

C. Srikantappa ¹, C. R. L. Friend ², A. S. Janardhan ¹

Affiliations
1 Department of Geology, Manasa Gangotri, University of Mysore, Mysore 570006, IN
2 Dept. of Geology & Physical Sciences, Oxford Polytechnic, Headington, Oxford OX3 OBP, GB

Abstract

Mineralogical and chemical studies of chromite deposits of Sinduvalli ultramafic body, Mysore District, show definite evidence of layering and other crystal settling processes. Stretching of occluded olivines, chain structures in chromite and rotation of chromite layers indicate their participation in the deformational episodes which affected the area. Structure and chemistry of chromites classify the host ultramafics as belonging to stratiform type. Presence of gabbroic anorthosites confirms this and indicates that they represent layered complexes characterizing the Sargur high-grade terrain.

The presence of recrystallised orthopyroxenes and development of chrome amphibole indicate their formation under metamorphic conditions. Olivine-spinel geothermometry suggests re-equilibration at sub-solidus temperatures during high grade metamorphism. The formation of 'Ferritchromit' zones is connected to magnesite formation.

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C. Srikantappa ¹, C. R. L. Friend ²

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore 570005, IN
2 Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford, OX3 OBP, GB

Abstract

The occurrence of chromian edenite, confined to the degraded chromitite layers of the Sinduvalli ultramafic body is reported. Field, petrographic and chemical studies suggest that its development has occurred during the prograde metamorphism of the layered body. The enrichment of silicate phases in chrome-amphibole and chloritesis attributed to re-equilibration of chromite during the metamorphic episodes affecting the area and the formation of 'ferritchromit'.

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C. R. L. Friend ¹

Affiliations
1 Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford, OX3 OBP, GB

Abstract

The origin of the late Archaean Closepet granites by in situ anatexis of amphibolite facies Peninsular gneisses is discussed.

The Closepet granites which are essentially post-tectonic, cut the high-grade metamorphic fabrics found in the Peninsular gneisses. They show no effects of the regional deformation and of the foliation, forming events found in the Peninsular gneisses. Additionally, the granites have escaped metamorphism except in the south where they are statically overprinted by charnockite. Since the charnockite may be shown to be syn- and post-Closepet granite it follows that there may be two granulite facies events: (1) belonging to the evolution of the Peninsular gneiss; and (2) the late-stage formation of charnockite, which is in some places static whilst in others may be accompanied by deformation.

The Closepet granites may thus be used to clearly separate the charnockite from the early evolution of the Peninsular gneisses.

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C. R. L. Friend ¹, B. Mahabaleswar ²

Affiliations
1 Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford OX3 OBP, GB
2 Department of Geology, Bangalore University, Jnana Bharathi, Bangalore 560056, IN

Abstract

Field evidence is presented which allows the syenitic and granitic rocks occurring at Kunduru Betta to be reinterpreted as a ring intrusion. The syenitic rocks were intruded first and are divided into three major types based upon their field, textural and mineralogical characteristics which may be related to the conditions of crystallization.

Crystallization began with saturated syenite and ended with markedly silica oversaturated Quartz syenite. The granitic stock in the centre of the complex clearly crosscuts the syenitic rocks and is itself cut by later aplite dykes which also extend into the syenite and the country rocks. Both the syenite and the granite display lithological and sub-vertical mineralogical layering which is described for the first time.

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B. Mahabaleswar ¹, I. R. Vasant Kumar ¹, C. R. L. Friend ²

Affiliations
1 Department of Geology, Bangalore University, Bangalore, IN
2 Department of Geology & Physical Sciences, Oxford Polytechnic, Headington, Oxford OX3 OBP, GB

Abstract

The area of present investigation forms a part of the Archaean high-grade gneiss terrain of Karnataka. The rocks comprise both acid and basic lithologies and involve the late Archaean Closepet granite. Petrographically the acid rocks span the range, tonalite to granite, whilst the basic rocks are amphibolites and two-pyroxene granulites. The acid 'rocks have calc-alkaline affinities and resemble those from the transition zone to the east. Trace elements show them to be relatively undepleted with K/Rb ratios of >500. Simulated REE patterns of gneisses reveal that there are three broad groups: two of which are steep, but at different REE abundances, suggesting high pressure evolution; and one relatively flat group suggesting low pressure evolution. The basic rocks have a tholeiitic affinity, but are clearly separate, the granulite facies types being slightly more magnesian. The trace element chemistry of these basic rocks has been disturbed by migmatization. The REE patterns of Closepet granite are steep and may be related to the partitioning of accessory minerals such as allanite. An attempt is made to establish the relative position of the granulite facies metamorphism with respect to the regional fabric and retrogression.

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C. R. L. Friend ¹

Affiliations
1 Department of Geology and Physical Sciences, Oxford Polytechnique, Oxford OX3 OBP, GB

Abstract

No Abstract.

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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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