Amphibole crystallization and its role in the origin of the adakitic rocks from the Malayer-Boroujerd igneous complex

The Malayer-Boroujerd plutonic complex (MBPC) is part of the Sanandaj-Sirjan continental arc of Iran resulted from subduction of Neotethys oceanic crust below Central Iran microcontinent. A number of adakitic samples have recently been reported in the MBPC. This study shows that among the various petrogenetic processes responsible for the formation of adakitic rocks, two have been suggested in the genesis of those from the MBPC. i) interaction of crust-derived magma with ultramafic rocks and ii) crystallization of amphibole from a more basic magma. The former process leads to formation of intermediate to basic magma that subsequently crystallized amphibole in the ultramafic rocks. Amphibole growth was associated with textural and chemical changes toward the rim. Chemical modeling shows that amphibole growth was coeval with assimilation of olivines that their residue are remained as inclusions in the amphibole. The magma that formed the rim of amphiboles had higher Th, Zr, Ta and LREE but lower Ti and HREE than the core-forming magma. Assimilation of olivine during the amphibole growth prevented a decrease in Ni and Co content of the remained magma. Comparable chemistry of the rim-forming melt and MBPC adakitic rocks suggest that they are of the same origin.