New geochronological and geochemical data for the Eocene shoshonitic trachyandesites in NW Iran: Constraints on their petrogenesis and tectonic setting

The Eocene magmatic flare-up in NW Iran provides valuable information on the tectonic evolution of the region. This study presents detailed geochronological and geochemical data from Eocene trachyandesitic rocks from the Ahar region of NW Iran. 40Ar/39Ar ages indicate that the Ahar trachyandesites were produced during the Eocene (44 Ma). The Ahar trachyandesites have a limited range of SiO2 contents (53.4-54.4 wt%) and low MgO contents (1.57-2.78 wt%), and they belong to the shoshonitic series with high Na2O + K2O contents (9.1-9.8 wt%) and K2O/Na2O ratios (0.4-1). The Ahar trachyandesites are characterized by arc-like trace element patterns, including enrichment in large-ion lithophile and light rare earth elements and depletion in high field strength elements. They have depleted Sr-87/Sr-86((i)) ratios (0.704616-0.704736) and epsilon Nd-(t) (+2.0 to +3.5) and epsilon Hf-(t) (+6.6 to +6.8) values and high Th/La and Ba/La ratios, similar to those of previously studied Eocene K-rich mafic-intermediate rocks in NW Iran. The geochemical characteristics of the Ahar trachyandesites suggest that they originated from an enriched, fertile mantle source that was produced by the reaction between mantle wedge peridotite and oceanic slab-derived components (sediment-derived melts and slab-derived fluids) during the subduction of the Neo-Tethys Oceanic slab along the Zagros segment. Taking the regional tectonic evolution into consideration, we propose that NW Iran was in an extensional setting caused by the rollback of the Neo-Tethys Oceanic slab during the Eocene. Partial melting of the enriched, fertile mantle source would have produced the Ahar trachyandesites and other contemporaneous K-rich mafic-intermediate rocks in NW Iran. Accordingly, the Ahar trachyandesites and contemporaneous K-rich mafic-intermediate rocks provide a geochemical record of interaction between the crust and mantle in an oceanic subduction channel.