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Abstract
Abstract The Prakasam alkaline province is essentially composed of three major nepheline syenite plutons at Elchuru, Purimetla and Uppalapadu, and one silica-oversaturated (syenite and quartz syenite) pluton at Settupalle. These four post-orogenic plutons, all occurring in the Prakasam District of Andhra Pradesh, display dissimilar internal structures and variable rock associations; they are almost exclusively salic and generally show evidence of passive emplacement. The gross petrographic characteristics of the individual plutons are presented, and a generalised discussion on the petrogenetic aspects is included. The undersaturated and oversaturated syenites of the province are supposed to have formed from a critically undersaturated hornblende syenite magma by branching differentiation mechanism. The gabbros represent tholeiitic magmatism preceding the emplacement of alkaline rocks, and the ferrosyenites represent the final product of a closed system fractionation (with Jow fO2) of magma that gave rise to gabbro-anorthosite complexes. The geological setting of the plutons is portrayed and the tectonic significance of the province, especially in its regional context, is highlighted. The feldspathoidal plutons of the province are conspicuously confined to an extremely narrow linear belt which is close to the known basement fracture zone, with which the major Bouguer gravity axis runs in close paralJelism ; this belt lies at the junction between two (granitegreenstone and gneiss-granulite) terrains. The localisation of feldspathoidaI plutons at the western margin of the middle Proterozoic mobile belt and their absence in the cratonic region is significant. The over-all development of the mobile belt is speculated in the light of currently fashionable tectonic models. Crustal shortening and thickening of the mobile belt due to collision tectonics, predate the emplacement of the alkaline plutons; the lithosphere controls the character of the magmatism, and pre-existing abyssal faults determine the location of alkaline magmatism.
Published Version
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