Geochemical constraints on the geodynamic setting of Alborz-Azerbaijan Cenozoic magmatism

Abstract

The Alborz Mountains in northern Iran form part of the Tethyan orogenic belt and surround the South Caspian Basin. The geology of the western Alborz Mountains is dominated by Eocene mafic to intermediate high-K calc-alkaline-alkaline shoshonitic and minor Oligo-Miocene magmatic rocks, displaying arc geochemical characteristics (e.g., negative Nb, Ta, Ti anomalies). Cenozoic magmatism across this region in western Asia has been explained by a diversity of contrasting geodynamic models involving (multiple slab) subduction and slab-breakoff. The aim of this study is to better constrain the geodynamic setting of magmatism during regional convergence through the investigation of the relatively unstudied Alborz-Azerbaijan magmatic belt. Incompatible trace element geochemistry of Eocene lavas from this belt is distinctive and indicates that they were generated by relatively low-degrees of partial melting of the subcontinental lithospheric mantle with a contribution of asthenosphere melts. Miocene lavas from the Alborz and northern Urmia–Dokhtar magmatic arc (UDMA) share a common arc geochemical signature. Zircon εHf(t) values of the Miocene magmatic rocks from the Alborz and northern UDMA range from −0.4 to 11.7, suggesting incorporation of older continental crust mixed with a more juvenile component. New thermochronological data (fission track and (U-Th)/He on apatite) from the late Eocene plutonic bodies in the Tarom area track exhumational cooling at moderate rates following rapid post-emplacement magmatic cooling at ca. 40 Ma. The geochemical data in conjunction with geological and published geophysical results imply a bending or disruption in the subducting slab under the Tarom area, associated with slab roll-back during the Eocene. This process led to the arc-front displacement and a greater contribution of deep enriched mantle in the Alborz magmas compared to those from the high-flux magmatic event along the Alborz and Urmia–Dokhtar magmatic arc (UDMA), triggered by asthenospheric upwelling and mixing with melts derived from earlier metasomatized subcontinental lithospheric mantle.