Field Evidence of Magma Mixing from Concentric Zoning and Mafic Microgranular Enclaves in Bunam Stock, Korea

Abstract

The Bunam Stock (29.5 km 2 area) is an outcrop of plutonic complex classified four facies: coarse-grained granite, quartz monzodiorite, granodiorite and fine-grained granite. Three facies except the last one exhibit very irregular boundaries with gradational compositional variations between both facies and show concentric zoning from the central quartz monzodiorite through granodiorite to outer coarse-grained granite. Mafic microgranular enclaves (MME) commonly occur in granodiorite. Some MMEs, have very fine-grained chilled margins and indentedly crenulate contacts, and display horizontally circular and vertically elongate shapes. Their shape and granularity indicate coeval flow and mingling of partly crystalline felsic and mafic magmas. MMEs exhibit dark fine-grained margins giving them a ellipsoidal form that has been attributed to undercooling of a mafic magma as blobs intruded into a felsic magma. The observed relations in the Bunam Stock identify that two endmembers are coarse-grained granite from a felsic magma and quartz monzodiorite from a mafic magma, and hybrid is granodiorite including MMEs.