Magma Mixing and Mingling during Pluton Formation: A Case Study through Field, Petrography and Crystal Size Distribution (CSD) Studies on Sirsilla Granite Pluton, India

Field, petrography, and crystal size distributions (CSD) of different lithological variants from Sirsilla granitic pluton (SGP), southern India, is described here to understand operative magmatic processes. The SGP contains many mafic microgranular enclaves (MMEs) and syn-plutonic dykes. The contact relationship between MMEs and the host granite is often diffusive or gradational and rarely sharp, implying disaggregation and under-cooling of MMEs. Petrographic features like resorption textures, quartz ocelli, and the poikilitic nature of the large K-feldspar grains enclosed within plagioclase indicate interaction and magma mixing/mingling processes in an open magma chamber. Bladed biotite and acicular apatite grains in MMEs are due to rapid crystallization during the magma mingling process. The CSD curves generated for plagioclase provide an inverse relationship between population density and crystal size. Multiple crystal populations, i.e., a gently sloping line for the core samples and a steeply sloping line for margin samples, are interpreted to be caused by the mafic — felsic magma mixing and mingling processes.