Abstract
Progressive development of charnockite through dehydration of amphibolite grade gneisses in southern India occurred exclusively in zones transitional to the deeper and pervasively granulitized crust. Isotope data document two distinct events of “arrested” charnockite formation: at ~2.5 b.y. in the Archaean gneiss terrane of southern Karnataka and at ~ 550 m.y. in the Proterozoic crustal segment of southern Kerala. A detailed study of field relations, petrological and geochemical characteristics in two selected exposures constrain possible mechanisms of arrested charnockitization.
Charnockitization in both terranes postdates the major event of penetrative deformation, high-grade metamorphism and migmatization. It occurred during uplift and slow cooling when the rhéologie properties of the gneiss complexes changed from ductile to brittle. The dehydration process proceeded along late ductile shears (Kabbaldurga, Karnataka) or conjugate fractures (Kottavattam, Kerala) as well as the foliation planes of the gneisses. It was evidently controlled by fluid-rock interaction in the tectonically generated fluid-pathways. The final product is a coarsegrained massive hypersthene-bearing rock of granitic composition (charnockite s. str.). While charnockitization of granitic biotite-garnet gneisses (Kottavattam) was nearly isochemical, marked changes in mineralogy and bulk chemistry occurred where charnockite developed in place of granodioritic lithologies (Kabbaldurga).
The results of the present investigation indicate that arrested charnockite formation in southern India was controlled by two different mechanisms of fluid-rock interaction: At Kabbaldurga, dehydration of the hornblende-biotite assemblages was caused by influx of external carbonic fluids along ductile shears and the concommi-tant decrease of water activity (at ~ 700 °C and 6 kb). It is assumed that the invading fluids were released by shear deformation from a “fossil” reservoir of carbonic fluid inclusions trapped in the deeper crustal granulites underlying the gneiss terrane. At Kottavattam, dehydration of the biotite-garnet gneiss (at 750 °C and 5–6 kb) was possibly triggered by a drop in fluid pressure resulting from the escape of internally derived and buffered CO2-N2-rich pore fluids into a system of conjugate fractures. It appears unlikely that these locally restricted and tectonically controlled processes caused the pervasive granulitization of extensive parts of Precambrian lower crust in southern India.
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Raith, M., Hoernes, S., Klatt, E., Stähle, H.J. (1989). Contrasting Mechanisms of Charnockite Formation in the Amphibolite to Granulite Grade Transition Zones of Southern India. In: Bridgwater, D. (eds) Fluid Movements — Element Transport and the Composition of the Deep Crust. NATO ASI Series, vol 281. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0991-5_3
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DOI: https://doi.org/10.1007/978-94-009-0991-5_3
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