POLYPHASE SOLID INCLUSIONS IN GARNET: INDICATOR OF MELT INFILTRATION OF EARLIER MINERAL INCLUSIONS

Partial melting (anatexis) plays a key role in the geodynamic evolution of UHP/HP terranes. Evidence has accumulated that some UHP terranes have experienced partial melting during exhumation. Macroscopic evidence for melting events in regional UHP/HP metamorphic rocks includes granitic leucosomes and felsic veins in migmatites, S-type granites and mafic intrusions. In contrast, microscopic evidence for partial melting is elusive due to melt crystallization and re-equilibration with surrounding minerals. Polyphase solid (PS) inclusions comprising quartz, K-feldspar, and/or other minerals are common in peak minerals (such as garnet and clinopyroxene). Interpretations on the origin of PS inclusions have been controversial. These inclusions are recently interpreted as entrapped melts or supercritical fluids prior to, during or subsequent to peak metamorphism because these PS inclusions generally display features consistent with melting experiments on hydrous and carbonaceous minerals in garnet, such as intergrowth of various daughter minerals, radial wedge-like offshoots and negative crystal shapes. However, a primary entrapment is problematic with the random spatial distribution and the highly variable bulk composition of PS inclusions in host minerals, in addition to the coesite-like geometry of some PS inclusions. Therefore, the origin of PS inclusions still remains unclear and critical to resolve. Here we report microstructural and geochemical evidence of partial melting and PS inclusions in garnets of a migmatitic quartz-garnet restite from the Ganjialing, Dabie Mountains, showing typical mineral assembly of quartz, K-feldspar, ± other minerals as isolated inclusions or thin films along rutile inclusions in garnet. These PS inclusions and films often display shapes that are compatible with those of euhedral coesite grains or the rutile grains in the center, in addition to wedge-like offshoots, neck-down textures and inclusion-garnet interfaces controlled by the crystallography of garnet. These observations exclude the possibility of primary melt entrapments for the PS inclusions. We proposed that these PS inclusions are reaction products between infiltrated melt and earlier mineral inclusions along fractures during early exhumation.