40Ar/39Ar mineral ages of eclogites from North Shahrekord in the Sanandaj–Sirjan Zone, Iran: Implications for the tectonic evolution of Zagros orogen

Abstract

Eclogites are high-pressure/low-temperature metamorphic rocks and are regularly considered as an indicator of ancient subduction zones. Eclogites have recently been found in the North Shahrekord metamorphic complex (NSMC) of the Sanandaj–Sirjan zone and represent the only ones within the Zagros orogen. Their occurrence and timing are important for the reconstruction of convergence history and geodynamic evolution of the Neo-Tethys Ocean and Zagros orogen. White mica from the eclogites and an associated paragneiss give 40Ar/39Ar ages ranging from 184.3 ± 0.9 to 172.5 ± 0.8 Ma and represent the age of cooling through the closure temperature for phengitic white mica. The NSMC also comprises the ductile NW–SE trending North Shahrekord Shear Zone (NSSZ), which is located in the northeast of the Main Zagros Reverse Fault. The NSMC consists mainly of various metasedimentary rocks, orthogneiss and small-sized bodies of metabasic rocks containing also the eclogites. Furthermore, pre-metamorphic granitoids represent part of the NSMC. The North Shahrekord eclogites are composed of garnet, omphacite, zoisite, Ca–Na amphibole, phengite and rutile. The highly deformed and metamorphosed granitoids yield hornblende and biotite 40Ar/39Ar ages 170.1 ± 0.9 Ma and 110.7 ± 0.3 Ma, respectively. According to the new age dating results of eclogites, the rocks are the oldest high-pressure metamorphic rocks in the Zagros orogenic belt testifying the Neo-Tethys Ocean subduction. Our new data indicate that the eclogites formed during Early Jurassic subduction of a Panafrican microcontinental piece from the northern margin of the Neo-Tethyan Ocean under the Central Iranian microplate. We suggest that initiation of subduction in Neo-Tethyan Ocean occurred a few million years prior to 184 Ma (Pliensbachian stage).