Potasium-Argon Ages and Geochemical Characters of some Basic Igneous Rocks of Cuddapah Basin

The Proterozoic intracratonic Cuddapah Basin contains different phases of igneous intrusives associated with sediments and lava flows from Vempalle Formation and basaltic and dolerite sills from Tadpatri Formation which occur in an arcuate fashion parallel to the basin margin. K-Ar ages suggest that the igneous activity commenced at about 1840 Ma after the initial sedimentation in the basin. Some dolerite sills show ages of about 1000 Ma may represent reequilibrated ages and an interesting younger age of 177±7 Ma is obtained for a norite dyke emplaced in the Cumbum shales of Pullampet Formation in Nallamalai Group of sediments. Basaltic lava flows contain plagioclase (39-48% )and clinopyroxene (18-40%) and opaques (1.5-4%), orthopyroxene (1-2%) and olivine (1.5-4%) are the common accessory phases. A few picrite samples occur at the base of sills in the Tadpatri Formation of Chitravati Group showing cumulus textures and contain olivine, orthopyroxene, clinopyroxene and small amounts of plagioclase. The lava flows and the sills exhibit continental tholeiitic nature and range from basalt to basaltic andesite in composition. They are olivine or quartz normative in character with strong magmatic differentiation trend of iron enrichment during their crystalization. The rocks are enriched in light rare earth elements and exhibit positive Eu anomaly (Eu/Eu^*=1.34-2.33). Plagioclase, clinopyroxene ± olivine phenocryst mineralogy, distinctive negative Sr anomaly suggest a relatively low-pressure fractionation during their evolution.