Petrology and Mineral Chemistry of a Porphyritic Mafic Dyke, Jonnagiri Schist Belt, Eastern Dharwar Craton, India: Implications for Its Magmatic Origin

We present field, petrology and mineralogy of a porphyritic mafic dyke that traverses the granite-greenstone terrain of the Neoarchean Jonnagiri schist belt, eastern Dharwar craton, India. The undeformed porphyritic dyke is characterised by the presence of euhedral plagioclase megacrysts (0.5–3.5 cm) exhibiting primary magmatic alignment. At places, partial resorption is noticed in the plagioclase phenocrysts indicating crystal-melt interaction. The groundmass consists of andesine and titan augite. Sub-ophitic/ophitic textures are conspicuously noticed. Ilmenite, titano-magnetite, apatite and baddeleyite are the accessory phases. Exsolved ilmenite and titano-magnetite along with euhedral apatite is also observed in the porphyritic dyke. Mineral chemistry of plagioclase shows variation between Or3.50–6.44%, Ab43.18–62.98% and An30.57–52.46%. Clustering of biotite, is noticed at places. EPMA analyses of plagioclase reveals the presence of normal zoning; Ab45.73 in the core to Ab49.61 in the rim. The predominantly andesine composition of plagioclase, position of clinopyroxene in the Ca + Na versus Ti binary mineral chemistry diagram, and the micron size euhedral baddeleyite indicate transitional nature of the Jonnagiri porphyritic dyke. The clinopyroxene is compositionally a Ti-augite (Wo40En34Fs26). The temperature and oxygen fugacity estimates for the coexisting magnetite-ilmenite solid solution pair yielded an equilibration temperature of ~756 °C and 10−15.6atmfO2 for the Jonnagiri porphyritic dyke.