Post-collisional Miocene adakitic volcanism in NW Iran: Geochemical and geodynamic implications

摘要

In NW Iran, about 30 subvolcanic porphyritic dacitic to rhyodacitic domes (1-5 km~2) are intruded into a variety of rock sequences from Permian to Early Miocene in age. These subvolcanic domes occur along the North Tabriz, North Misho and Darediz dextral faults in the northern part of the Urumieh-Dokhtar magmatic arc (UDMA) of Iran. The UDMA contains intrusive and extrusive rocks of Eocene-Quaternary age. Geochemical data indicate that the subalkalic dacitic to rhyodacitic rocks have an adakitic composition with Na_2O/K_2O > 1, high Sr (346-737 ppm), Mg# = 0.48 and low Y (10-20 ppm) and HREE. Fractionated REE patterns, (Ce/Yb)_N = 9-76, absence of negative Eu anomaly, low content of Y, Nb, Ti, and high Sr/Y (20-58) and (Ce/Yb)_N ratios suggest that the source was probably amphibole-eclogite or garnet-eclogite, possibly generated during subduction of the Neo-Tethyan oceanic slab beneath the Central Iran microplate. The adakitic volcanism was followed by eruption of alkaline magmas including ultrapotassic, shoshonitic, and lamprophyric volcanic rocks. Slab melting occurred after cessation of subduction, possibility from the detached slab. Transtensional tectonics accompanied by a locally extensional stress regime may account for magma genesis and ascent.