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

M. Ramakrishnan ¹, M. N. Viswanatha ¹, N. Chayapathi ¹, T. R. Narayanan Kutty ²

Affiliations
1 17, Raja Mahal Vilas Extension, Bangalore-560006, IN
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, IN

Abstract

Anorthosites of Karnataka occur in two distinct tectonic milieu, within a major arcuate zone, namely, (i) as minor differentiates of gabbro in the ultramafic-mafic complexes emplaced in Archaean high-grade supracrustal belts and the 'Dharwar type' greenstone belt of Shimoga; and (ii) as layered concordant sheets, and as cumulates in a layered basic pluton occurring within the gneissic complex of Archaean high grade terrain. Their absence in 'Keewatin type' greenstone belts and the other  'Dharwar type' belts of Bababudan and Chittadurga is probably due to their location away from the arcuate zone in Karnataka Craton. This arcuate zone reflects a progressive vertical zonation of the crust to deeper levels from northwest to southeast.

The anorthositic rocks of mafic-ultramafic complexes are nonlayered and extensively deformed, whereas those of the high grade terrain show layering and cumulus textures. In the transitional zone, the anorthositic rocks of mafic-ultramafic complexes show cumulus textures, thereby indicating a genetic connection between the anorthosites of two tectonic milieu.

Whole-rock chemistry of the anorthosites of Karnataka indicates differentiation trend of iron-poor environment comparable to other Archaean anorthosites. They are enriched in CaO and A1₂O₃ Niggli mg against Cr or Ni shows positive correlation, corresponding to an igneous parentage for the original rock. Similarly, the plot of Niggli c against mg, c against (al-alk) and 100 mg-c-(al-alk) diagrams exhibit a well defined igneous trend. A common differentiation trend is shown by anorthosites of Karnataka, even though they occur in two distinct tectonic milieu. The singularised trends support the view that the ultramafic-mafic complexes are possibly derived by the dismembering of the basal parts of the layered complexes. The Archaean anorthosites of Karnataka, although comparable broadly to lunar anorthosites, are not related to the early history of the Earth-Moon system.

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A. S. Ramiengar ¹, G. R. Devadu ¹, M. N. Viswanatha ¹, N. Chayapathi ¹, M. Ramakrishnan ¹

Affiliations
1 Geological Survey of India, Jayanagar, Bangalore-560011, IN

Abstract

A small enclave of ancient supracrustal rocks (Sargur Group) in Kamataka contains an interesting occurrence of fuchsite quartzite with layers of chromite and other heavy minerals like magnetite, tourmaline, rutile and zircon. The opaque and non-opaque heavy mineral suite is suggestive of a source region rich in sialic components with subordinate ultramafic rocks. This is consistent with the spatial association of these supracrustal rocks with a vast gneissic complex containing numerous enclaves of chromite-rich ultramafic rocks in the vicinity.

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