Low pressure migmatites from the Sanandaj-Sirjan Metamorphic Belt in the Hamedan region (Iran)

Migmatites with evidence for low pressure metamorphism and partial melting occur adjacent to the Alvand Plutonic Complex in the Hamedan region of Iran. They show stromatic, schollen, diktyonitic and massive structure. Sillimanite/andalusite/(kyanite)-garnet- and cordierite-K-feldspar-andalusite-spinel-bearing migmatites are the most common rock types. Some of the granitic intrusions contain xenocrysts which resemble the porphyroblasts of nearby migmatites (e.g. sillimanite, andalusite, cordierite and garnet). Although migmatitic rocks of the region are located near the granitic intrusions, the degree of partial melting is not related to intrusions and is irregular. It appears that partial melting and migmatization pre-date the intrusion of major granitic bodies in the region. Leucosomes in stromatic migmatites are commonly parallel to bedding planes and are mostly formed by metamorphic segregation and/or in situ partial melting (showing mafic selvedges, pinch and swell structures). The melt fraction and migmatite type depend on the chemical composition of parent rocks and the distribution of high strain zones. The formation of thin leucosomes in the stromatic migmatites was controlled by short-range melt movement along the grain boundaries. Melt-rich layers are constrained by pre-existing compositional layering and foliation. Peak metamorphic conditions of similar to 650 degrees C and similar to 300 MPa are consistent with the observed mineral assemblages and the presence of melt in the investigated migmatites.