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

S. K. Sen ¹, S. Ray ¹, D. Dasgupta ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

In the type area of the charnockites, south of the city of Madras, foliation attitudes of marker horizons of leptynites and of basic granulite bands trace a map pattern characterized by four-fold hinges. The overall structure of the area is brought out by the swerve of axial planes of these folds from NNE in south-west through E-W in the central part to N-S in southern part in a broad sweep. The folding is noncylindrical; and a probable structural interpretation, as advanced here, envisages a series of tight isoclinal folds on which a folding around a N-S axial plane has been superposed.

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S. K. Sen ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, IN

Abstract

Available good quality data on the major element chemistry of charnockitic rocks of the type area and their minerals, though few, permit generalising on some significant aspects of their geochemistry hitherto not focussed upon. Rocks belonging to even the 'charnockite suite' of Subramaniam (1959) exhibit migmatitic rather than magmatic chemistry; migmatisation is also reflected in subtle changes in the chemistry of some minerals such as hornblendes. The grand average of all charnockitic rocks of the type area correspond to an intermediate composition like that ofa diorite and is similar to the average compositions of other well-studied charnockite areas. The lower crust, according to current belief, is probably of such composition; and in as much as charnockites-granulites make substantial contribution to the Precambrian shield areas, they should be included in computing the average chemical composition of the shields and the continental crust. This would result in lower Si for shield averages, and lower K, Ti, Fe and higher Si for the continental crust.

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