A study of microstructures of granitoids from the Alvand plutonic complex, Sanandaj-Sirjan zone, Iran: with special reference to myrmekitedevelopment

Many plutonic complexes occur in the Sanandaj-Sirjan Zone (SSZ), Iran. The Alvand Plutonic Complex (APC), as a major complex, is located in the northwestern part of the SSZ (near Hamedan). The APC is composed of various mafic to felsic plutonic rocks, such as olivine gabbro, gabbro, norite, diorite, tonalite, granodiorite, monzogranite, syenogranite, leucocratic granitoids, aplite, and pegmatite. In this study, our research is concentrated on the granitoid part of the APC which is a major part of the pluton and consists of granodiorite and granites with porphyroid texture. Various microstructures, including magmatic, sub-magmatic, and solid-state structures can be distiguished in the APC. Alignment of feldspar phenocrysts and mafic enclaves are common magmatic structures. Fractures filled with quartz and micas inside deformed feldspar phenocrysts are major submagmatic microstructures. Flame perthite and myrmekite in deformed rocks are common solid-state features in the granitoids. Various mechanisms for the genesis of myrmekite are reviewed with special attention to myrmekite of the granitoids of the APC. Myrmekite was mostly generated from replacement of K-feldspar by sodic plagioclase in the presence of fluids with/without deformation and is more abundant in deformed rocks than in undeformed rocks, but occurs in some undeformed rocks, as well. Myrmekite was formed at the interface of K-feldspar/plagioclase, K-feldspar/K-feldspar, quartz/K-feldspar and biotite/K-feldspar; and within K-feldspar and plagioclase, as well. So, in contrast to ideas in several publications, plagioclase/K-feldspar interface is not prequisite factor for development of myrmekite, although it make easier the process.