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The study region forms the western part of the Madurai block (southern block) and shares several lithological characteristics of the Proterozoic exhumed South Indian Granulite Terrain (SGT). The crustal structure of the area has been derived from gravity data, constrained partly by aeromagnetic data. The Bouguer anomaly map of the region prepared based on detailed gravity observations shows a number of features (i) the Periyar lineament separates two distinctly different gravity fields, one, a high gravity gradient tending to be positive towards the coast in south west and significant gravity lows ranging from − 85 to as low as − 150 mGal in the NE covering a large part of the Periyar plateau (ii) within the broad gravity low, three localised circular anomalies of considerable amplitude occur in the region of Munnar granite. A magnetic low region in the central part coincides with the area of retrogressed charnockites and the major lineaments suggestive of a genetic link and considerable downward extent. The crustal models indicate that the upper layer containing exhumed lower crustal rocks (2.76 gm/cc) is almost homogeneous, most part of the gravity field resulting from variations in intracrustal layers of decharnockitised hornblendic gneisses and granite bodies. Below it, a denser layer (2.85 gm/cc) of unknown composition exists with Moho depth ranging from 36 to 41 km. The structure below the region is compared with that of two other segments of the SGT from which it differs markedly. The Wynad plateau forming the western part of the Northern Block of the SGT is characterised by a heterogeneity due to the presence of contrasting crustal blocks on either side of the Bavali shear zone, possibly a westward extension of the Moyar shear zone and presence of high density material in the mid-to-lower crustal portions. The crust below the Kuppam–Palani transect has a distinctive four-layer structure with a mid-crustal low density layer. The differences in crustal structure are consistent with the different tectonic settings of the three regions discussed in the paper. It is suggested that the crustal structure below the Kuppam–Palani transect corridor is not representative of the SGT as a whole, an aspect of great relevance to intra-continental comparisons and trans-continental reconstructions of continent configurations of the Gondwanaland.

Twelve zircon separates from diverse geological settings in peninsular India have been analyzed for their Zr, Hf, Sc, REE, Th and U contents employing neutron activation techniques. The whole rock samples of acid charnockite and a related rock of syenitic composition were also analyzed for these elements after separation of zircons. The study reveals that 1. a) the zircons exhibit a wide range of variation in elemental abudances and their ratios, 2. b) their chondritic normalised REE patterns exhibit negative or positive Ce and negative Eu anomalies in addition to smooth HREE enriched patterns without anomalies, and 3. c) there are significant differences in the REE partition coefficients of zircons from the charnockite and the associated syenite. The overall elemental abundances and REE patterns of the zircons could be explained in terms of crystallochemical factors, bulk compositional differences of parent melts and paragenetic histories.

Whole-rock gamma-ray spectrometric data for U, Th and K contents of 133 samples belonging to the granulite belts of North Arcot-Pallavaram (NAP), Eastern Ghats (EG) and Kondapalli and the greenstone gneiss belts of Sargur, Holenarsipur and Kclar are presented. The data reveal that the grarulite and greenstone belt rocks have distinctive radioelement abundances and characrerisric enrichment patterns for U and Th in relation to K. The main rocks aralysed from. the granulite belts include pyroxene granulites, charrockites, associated anorthcsites and migmatite gneisses. They are divided into two broad groups on the basis of their silica content; (a) intermediate and basic rooks with SiO 2 , content less than 65%, and (b) more acidic rocks with SiO 2 content greater than 65%. Compared with normal magmatic rocks the more basic granulites and charnockites are depleted in U and Th in relation to K. The more acidic rocks, however, have higher concentrations of all three elements and high Th/U ratios, indicating enrichment of Th over U. Later quartzo-feldspathic rocks intrusive into the charnookite-pyroxene granalite group shew higher concentrations of all three radioelements and a higher Th/U ratio. These show depletion in U but enrichment in Th in relation to K. The main rocks from the Dharwar schist belts include amphibolites and associated gneisses, granites and the “Champion gmeiss”, These show a relavive enriohment of U and Th in relation to K, in marked contrast to the patterns of the radioelemental ratios in the granulite belt. The significance is discussed of the contrasting trends of the radioelement distribution in the granulite and greenstone belts.