MIGMATITES MICROSTRUCTURES AND PARTIAL MELTING OF THE HAMADAN PELITES WITHIN THE ALVAND AUREOLE, WEST IRAN

Contact metamorphic migmatites comprise a substantial part of the high-grade part of the Alvand aureole in the Hamadan, western Iran, which is dominated by Al-rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+spinel±orthopyroxene and leucosomes contain garnet, plagioclase and K-feldspar. Metamorphic grade throughout the area is in the upper pyroxene hornfels to lower sanidinite facies. Field evidence, petrographic observations, deduced reactions and P-T estimation suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The first appearance of orthopyroxene in these rocks marks the transition from upper pyroxene hornfels to lower sanidinite facies conditions. Peak metamorphism took place at 650-750°C and ~2-4 kbar (HT/LP) reflecting the high heat flow. This metamorphism is mainly controlled by advective heat input through magmatic intrusions into all levels of the crust. The Hamadan metamorphic rocks have experienced multiple episodes of metamorphism driven by burial and heating during arc construction and collision following subduction of a Neo Tethyan seaway. The oblique collision of Afro-Arabia (Gondwana) with the Iranian microcontinent in Late Cretaceous–early Tertiary formed the acr magmatism and related contact metamorphism. These events are associated with local partial melting at high grades, adjacent to the Alvand complex pluton.