Mineral chemistry of biotite and hornblende from mesoproterozoic quartz syenite intrusions of the Cuddapah Intrusive Province, Eastern Dharwar Craton, India: implications for their source characterization

Through petrological and mineral chemistry studies of the mafic hydrous phases of the Proterozoic quartz syenites along the NE margin of Cuddapah Intrusive Province (CIP) of the Eastern Dharwar Craton (EDC), we present the nature of the magmatic sources of the Mesoproterozoic Purimetla quartz syenite (PUQS) and the Vikurthi and Kottappakonda quartz syenites (VKQS). Earlier studies suggested that these rocks evolved from subalkaline magmas. However, the present study, based on amphibole and biotite mineral chemistry, demonstrates the contrasting magmatic sources of the PUQS and VKQS. Results of electron probe micro-analyzer (EPMA) chemical analyses indicate that the calcium amphiboles in the PUQS are ferro-pargasite-hastingsite and edenite in the VKQS. Biotite is Fe-rich (annite) in the PUQS and Mg-rich (phlogopite affinity) in the VKQS. Bulk rock, amphibole, and biotite geochemical ratios [e.g. MgO/(FeO T + MgO); with FeO T = total Fe oxide assuming all Fe to be Fe 2+ ] demonstrate the ferroan (PUQS) and magnesian (VKQS) nature of the respective magmas. Though both the plutons show variations in SiO 2 , FeO, MgO, and CaO contents, they present similarities in total alkalies. The mineral chemistry of the mafic hydrous minerals indicates the alkaline nature of PUQS and the subalkaline nature of the VKQS magmas. Temperature estimates for the crystallization of the PUQS and VKQS, based on amphibole Al 2 O 3 and TiO 2 contents, are 840–800 °C and 780–650 °C respectively. Lower-temperature estimates were obtained for the crystallization of biotite in the PUQS (760–730 °C) and VKQS (730–650 °C). Overall pressure estimates, based on Al in hornblende barometry, suggest that the PUQS formed at ~ 6–9 kbar and the VKQS at ~ 3– 6 kbar. Furthermore, the oxygen fugacity ( f O 2 ) estimates derived from amphibole chemistry suggest reduced conditions of crystallization for the PUQS and oxidized conditions for the VKQS. Likewise, the f O 2 estimates using biotite Fe 2+ , Fe 3+ , and Mg contents also indicate reduced conditions for the PUQS (close to the QFM – quartz-fayalite-magnetite buffer) and oxidized conditions for the VKQS (NNO – nickel-nickel oxide buffer). Based on bulk-rock and mineral chemistry data integration, we envisage that the PUQS evolved from anorogenic alkaline magmas, while the VKQS evolved from an orogenic, calc-alkaline to subalkaline magmas. Finally, the thermobarometric data (amphibole and biotite) for the two quartz syenites correspond to a lower