The role of heterogenetic mantle in the genesis of adakites northeast of Sanandaj, northwestern Iran

In the northern part of the Sanandaj-Sirjan Zone (SSZ), northwestern Iran, there are a number of young (10-11 Ma) dismembered volcanic complexes that extend parallel to the Zagros Suture Zone in a northwest-southeast direction. The major lithologies associated with these complexes include trachyte, andesite, dacite and rhyolite. These rocks are characterized by high SiO2 (>60 wt.%) and Al2O3 (>15 wt.%), low MgO (<2 wt.%) and high Sr/Y (>30) and La/Yb (>25) ratios. Initial 87Sr/86Sr ratios based on an age of 11 Ma (Ar-Ar dating) are relatively high (0.7070-0.7079), and εNdt values are relatively low (between −4 and −3). Values of fSm are negative and fluctuate between −0.5 and 0; values of fRb vary between +3.5 and +8. Analyses of chemical compositions confirm that these rocks belong to the adakite group. The Sr-Nd isotope ratios indicate that these rocks are distinct from adakites derived from partial melting of oceanic slabs or lower continental crust and suggest that the parent magma originated from the metasomatic mantle wedge above the oceanic slab in an active continental margin. Furthermore, the eruption of high-Nb basalts (HNBs) in this area supports the role of metasomatic mantle in the genesis of these adakites. Adakites in the northern SSZ are likely produced by (1) crystallization at high pressure of minerals such as garnet or amphibole from normal basaltic magma (e.g., high-Mg basalt) above the subduction zone, or (2) partial melting of metasomatic mantle at high pressure in the garnet stability field.