Discrimination of the age and tectonic setting for magmatic rocks along the Zagros thrust zone, northwest Iran, using the zircon U–Pb age and Sr–Nd isotopes

Abstract

The Zagros thrust is a suture zone 10–15 km wide and 800 km long between the Arabian and Iranian plates, extending NW–SE in the west of Iran. Along this zone, some dismembered ophiolite, Eocene volcanic and some gabbroic to dioritic bodies crop out. The basaltic rocks have a zircon U–Pb age of 54.6 Ma; some gabbroic to dioritic bodies have ages of 36.75 ± 0.51 Ma and 36.63 ± 0.40 Ma, and they are rich in LILE and LREE relative to the N-MORB. High ( 87Sr/ 86Sr)i and low ( 143Nd/ 144Nd)i ratios relative to the MORB show that the slab fluid had a greater effect in the evolution of the source magma. The geochemical data infers that the basaltic rocks in the western part, Kamyaran basalt, were produced in an active margin such as an island arc. Geochemical features of gabbroic bodies and Oskul basalts indicate back arc basin-like N-MORB activity. Their chemical compositions and initial isotope ratios of 87Sr/ 87Sr and 143Nd/ 144Nd confirm that all the basaltic and gabbroic bodies along the Zagros fault in the study area were originated from the depleted mantle in the Cenozoic. It is possible that the complex was part of the supra subduction zone ophiolite, which cropped out in the collision of Arabian and Iranian plateau, probably in the Miocene or later.