Petrogenesis of adakites from the Sheyda volcano, NW Iran

The Sheyda volcano study area, located in the Hamedan-Tabriz volcanic belt of western Iran, is composed of intermediate to acidic volcanic rocks of Miocene age and is partially covered by Plio-Quaternary sedimentary rocks. The igneous rocks have primarily microlithic, porphyrytic and glomeroporphytic textures and less frequently, trachytic texture involving phenocrysts of plagioclase feldspar and hornblende with minor alkali feldspar, quartz and biotite. The Sheyda high-potassium, calc-alkaline trachydacites and dacites have been called "adakites" on the basis of their high SiO2 (67.5-51.7 wt %), Al2O3 (18-15.6 wt %), Sr/Y (67-49) and La/Yb (28-23), and low Mg# (average 41.5). Rare earth elements geochemistry data, demonstrate that these rocks do not originate as melts of subducted oceanic crust. Rare earth element abundance variations and the absence of a significant positive relationship between Sr/Y and La/Yb ratios and SiO2 and MgO contents indicate that fractional crystallization or partial melting can played role in their genesis. The existence of the subducted oceanic slab beneath the studied area and the thin continental crust as well as thinning in the lithosphere suggested that these adakites originated from melting metasomatized continental lithosphere that could be garnet or dinopyroxene bearing amphibolite in composition.