Gravity and Magnetic Studies on Cospatial and Coeval Mesoproterozoic Plutons - Prakasam Alkaline Province, India

Chimakurti-Uppalapadu plutons is an unusual association representing a cospatial, coeval but believed to be not comagmatic clinopyroxenite-gabbro-anorthosite and nepheline syenite in the Prakasam Province of the Eastern Ghats Mobile Belt, India. The petrologic status of the Errakonda ferrosyenite occurring amidst the Chimakurti-Uppalapadu plutons is not worked out unambiguously by the petrological and geochemical studies. In addition to the petrological problems, the subsurface infrastructure of these co-spatial plutons is absolutely not known. Therefore, detailed gravity and magnetic studies are carried out in this area to delineate the subsurface infrastructure and subsurface extension of the exposed plutons. Density and magnetic susceptibility measured on over 1200 specimens covering all the lithounits of these plutons are used in evaluating gravity and magnetic anomalies. AMS studies carried out to address the petrologic status of the ferrosyenites. The gravity and magnetic anomaly interpretation brought out a funnel-shaped body for the Chimakurti complex and a sheet-like structure for the Uppalapadu complex; ferrosyenite and anorthosite appear to be thin layers overlaying the gabbronorite. The petrophysical riddle with respect to the status of ferrosyenite, occurring between gabbronorite and nepheline syenite plutons, is clearly resolved with the AMS studies. These results are highly useful in understanding analogous magmatic systems elsewhere