Hansen, E. C. and Janardhan, A. S. and Newton, R. C. and Prame, W. K. B. N. and Ravindra Kumar, G. R. (1987) Arrested charnockite formation in Southern India and Sri Lanka. Contributions to Mineralogy and Petrology, 96 (2). pp. 225-244. ISSN 0010-7999
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Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic "gray gneiss" produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of "mixed-facies" exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO 2 and Na 2 O. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°-800° C and 5-6 kbar. Lower P(H 2 O) in the Ponmudi-type metamorphism was probably the definitive factor. CO 2 -rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO 2 from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO 2 -streaming hypothesis. CO 2 -rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic H 2 leakage or some systematic contamination. Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO 2 drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and CO 2 -rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin. Further evidence, such as oxygen isotopes, is necessary to test the CO 2 -streaming hypothesis for the Ponmudi-type localities. © 1987 Springer-Verlag.
| Item Type: | Article |
|---|---|
| Additional Information: | Publisher: Springer-Verlag |
| Subjects: | F Earth Science > Geology |
| Divisions: | Department of > Earth Science |
| Depositing User: | Dhruva Kumar |
| Date Deposited: | 06 May 2021 10:49 |
| Last Modified: | 06 May 2021 10:49 |
| URI: | http://eprints.uni-mysore.ac.in/id/eprint/13569 |
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