Charnockite-Gneiss-Complex Relationship in Southern Karnataka
Authors
-
Geological Survey of India, Karnataka (South) Circle, V Block, Jayanagar, Bangalore-560041
A. S. Ramiengar -
Geological Survey of India, Karnataka (South) Circle, V Block, Jayanagar, Bangalore-560041
M. Ramakrishnan
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Geological Survey of India, Karnataka (South) Circle, V Block, Jayanagar, Bangalore-560041
M. N. Viswanatha
Abstract
The charnockites of southern Karnataka occur at the fringes of the greenstonemigmatite terrain. The contacts of the individual pods and balls of charnockite with the associated migmatites are invariably diffuse. Replication of all types of migmatitic structures by the conformable charnockite masses and pods, and discordant 'tree-structure' of charnockites transecting the early rnigrnatitic fabric of the gneisses strongly suggest that the gneiss complex has been transformed in situ into charnockite without much attendant ductile deformation. The mineralogy of the transformation phenomenon reflects the transitional nature of the amphibolite-granulite facies boundary, rather than a craton-wide regional retrogression. These transformed charnockite buds probably represent nuclei of dehydration.The major 2900-3000 m.y. thermal event in the gneiss complex, involving the high-grade supracrustal rocks, is correlatable with the progressive metamorphism of amphibolite to granulite facies in the region. This event probably culminated in the production of charnockite around 2600-2700 m.y.
The progressive metamorphism in Karnataka from green schist to granulite facies is shown to be the cumulative effect of polymetamorphism and regional synclinorial structure plunging northward. Post-Dharwar uptilting of the Peninsula has served to accentuate this vertical zonation. As a result deeper crustal levels are exposed southward, involving older supracrustal rocks. The high-grade terrain in Karnataka is, therefore, older than the greenstone terrain. The greenstones and granulites were not juxtaposed against each other because of their differing levels. Parts of the high-grade terrain later evolved into a mobile belt of charnockite-khondalite association. This mobile belt has been interpreted variously as geosynclinal orogenic belt, collision feature similar to the Circum-Pacific belts of the Himalayas, or as a transform zone. The continent-continent collision model is preferred.
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