Origin of myrmekite in high-grade gneiss in the Zhongshan Station area, East Antarctica

Abstract：Myrmekite is commonly developed in the high-grade gneisses of the Chinese Antarctic Zhongshan Station area (Larsemann Hills). The occurrence of myrmekite is usually the symbol of the association of plagioclase and K-feldspar. Some plagioclase-quartz rims and K-feldspar rims along mineral grains are generally related to the texture. The anorthite content of the plagioclase in myrmekite may be lower or higher than the content of the main plagioclase in the rock, and the former shows a wider composition range than the latter. The origin of myrmekite is mainly related to the change of felsic minerals. Others factors like the decomposition and exsolution of K-feldspar, decomposition of quartz and biotite and even decomposition and exsolution of plagioclase sometimes may be involved. The exsolved components corrode and replace the preexisting minerals (predominantly K-feldspar) and myrmekite is produced simultaneously. Although not a necessary condition, deformation is favorable for the formation of myrmekite. The formation of myrmekite in felsic rocks also implies the decompression process in PT evolution. The components responsible for the formation of myrmekite were mainly derived from the system itself, rather than from the outside environment. In combination with its correlation with related textures and analysis, the formation of myrmekite is the result of differentiation of felsic components in the late stage: the differentiation of K and Na (Ca) resulted in the precipitation of Na (Ca)(2)+ and mobilization of K+ as well as migration of SiO2 to some degree. The released and migrating components tend to be more alkaline. This differentiation is the result of adjustment of the composition and textures inside the rocks, but the activity of the components is confined to the scale of several grains. The presence of myrmekite may represent cotectic crystallization between plagioclase and quartz and exerts a buffering effect on the composition of neogenic plagioclase. The process approaches the solution feature rather than the magma feature. In addition, the presence of myrmekite suggests the essential end of the activities of all the components except that of H2O.