MESOPROTEROZOIC FERROSYENITES OF EASTERN GHATS, INDIA - THOLEIITIC OR ALKALINE? A GEOPHYSICAL SOLUTION

Mesoproterozoic tholeiitic and alkaline magmatism is widespread in the southern segment of Eastern Ghats Belt, India, and is particularly well recorded in the cospatial Chimakurti and Uppalapadu plutons. The Mesoproterozoic Chimakurti-Uppalapadu plutons represent an unusual association of clinopyroxenite-gabbro-anorthosite-ferrosyenite and nepheline syenite association in the Prakasam Province of the Eastern Ghats Mobile Belt, India. These two plutons are co-spatial in nature and are believed to be formed during mesoproterozoic time with different magamtic processes, intruding into the deep Precambrian continental crust composed of amphibolites, quartzite and khondalite. The magmatic association of ferrosyenite exposures in between these two contrasting plutons is a debating issue. Different geological and geochemical studies visualized diversified magmatic origin, for these embedded ferrosynites. Now the geological problem is that the probable status of ferrosynites - whether belong to tholeiitic or Alkaline in nature? This paper is an attempt to find out a solution to this geological problem, through geophysical approach. Gravity and magnetic anomalies supported by density, magnetic susceptibility, palaeomagnetic and AMS are used in this study. Density and magnetic susceptibility show a wide variation among the lithologies and are correlated with changing mineral assemblages and abundances. The density and susceptibility values show conspicuous intra-pluton, inter-pluton and pluton-country rock contrast. Palaeomagnetic ages on these rocks help in understanding these rare magmatic processes. The Geophysical anomalies predict subsurface shapes of and nature of the contact with neighboring plutons. The AMS studies help in understand the magnetic fabric of different plutonic bodies. This integrated geophysical approach provided an explicit solution to the problem and in all probability the ferrosynites are associated with gabbros in this unusual plutonic set up.