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

T. Yellappa ¹, N. V. Chalapathi Rao ², T. R. K. Chetty ¹

Affiliations
1 National Geophysical Research Institute, CSIR, Uppal Road, Hyderabad - 500 007, IN
2 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN

Abstract

Occurrence of two lamproitic dykes intruding the basement granite near Khadka village at the northern margin of the Indravati Basin, Bastar craton is reported. Combined field, microscopy, XRD, EPMA and whole-rock geochemical investigations reveal that these lamproites were subjected to high degree of hydrothermal alteration as well as possible metamorphism. However, relicts of their original textures are well-preserved thereby providing important clues as to the nature of the protolith. Quartz, carbonate, chlorite and phlogopite constitute the bulk mineralogy whereas spinel, apatite and iron oxides are the accessory phases. Chemical composition of the groundmass spinels are strikingly similar to those from the lamproites. The Khadka lamproite dykes display high abundances of compatible elements such as Ni (238-396 ppm), Cr (484-892 ppm), and V (160-200 ppm) as well as high-field strength elements such as Zr (719-2057 ppm) and Nb (92-126 ppm) that resemble those in lamproites. Khadka lamproites also have high whole-rock REE abundances (ΣREE up to 1260 ppm) and display fractionated chondrite-normalized REE patterns (La/Yb= 113-237) which together with their average compatible and incompatible trace elemental ratios (e.g. Nb/Zr, Nb/La, Ba/Rb) are strikingly similar to those of the Mesoproterozoic Krishna lamproites of the Eastern Dharwar craton. Available field evidences suggest the Khadka lamproites to be of at least Palaeoproterzoic age (1.88 Ga) which makes them some of the oldest such rocks as yet documented from the Indian shield.

Keywords

Petrology, Geochemistry, Lamproite, Indravati Basin, Bastar Craton, Central India.

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T. R. K. Chetty ¹, D. P. Mohanty ¹, T. Yellappa ¹

Affiliations
1 National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad - 500 007, IN

Abstract

A new map of structural architecture has been compiled involving modern mapping techniques at the craton-mobile belt interface in the Western Ghats around the Coorg granulite massif revealing the occurrence of important shear zones. The shear zones are linked to the Moyar-Bhavani Shear Zone in Southern India. The nature, geometry and kinematics of the shear zones in the granulitic crust and the cratonic part are distinctly different.

Keywords

Dharwar Craton, Shear Zones, Coorg Massif, Western Ghats.

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