Redox Condition and Mineralogical Evidence of the Magma Mixing Origin of the Mafic Microgranular Enclaves (MMEs) from Sircilla Granite Pluton (SGP), Eastern Dharwar Craton (EDC), India

Abstract

Abstract Mafic microgranular enclaves (MMEs) and Syn-plutonic dykes (SPD) are commonly observed in granitoids and are believed to have a similar period of origin as that of the host. Here the petrographical and mineralogical characteristics of the MMEs and Syn-plutonic dykes from Sircilla granite pluton (SGP), eastern Dharwar craton (EDC), Southern India is presented. The contact relationships of MMEs with the host granite are diffusive or gradational, implying under-cooling and disaggregation of MMEs. Petrographic features such as quartz ocelli, bladed biotite, and acicular apatite grains in mafic enclaves suggest magma mixing/ mingling-related processes. Consequently, crystallization temperature and pressure of hornblende and biotite from SPD indicate rapid undercooling and hence suggest their emplacement towards the final stage of pluton evolution. Mineral chemistry studies reveal that the plagioclase in host SGP compositionally varies from An 23 -An 27, while the Fe# in biotite vary from 0.47-0.52. The composition of plagioclase and mafic phases in MMEs and the SPDs exhibit variation in compositional range, plagioclase (An 26 -An 30), hornblende Mg# (0.42-0.55), and biotite Fe# (0.46-0.54). A decreasing trend in fO2 value in MMEs and SPD during progressive crystallization suggest an orogenic tectonic setting of magma formation. Intermediate composition of biotite and plagioclase from MMEs suggest control of magma mixing processes for their formation.