Episodic subduction initiation triggered Jurassic magmatism in the Sanandaj–Sirjan zone, Iran

The Zagros suture zone in Iran is part of the Tethyan orogenic belt that records convergence between the Eurasian and Arabian plates during the Mesozoic-Cenozoic. Prior to collision, which may have initiated in the Late Eocene-Early Oligocene, much of the convergence was accommodated by the Late Triassic-Early Jurassic subduction of the Neotethys beneath the Iranian Cimmerides, terranes accreted on the southern margin of Eurasia during the Permo-Triassic. The Sanandaj-Sirjan zone (SaSZ) is located immediately at the northeast of the Zagros suture zone. In this study, we review the available geological information pertinent to the SaSZ, with a particular emphasis on the Jurassic igneous record, in order to understand the geodynamics under which the SaSZ subduction zones initiated and evolved during the Jurassic. Based on igneous activity, the SaSZ can be divided into three main sectors: a Central sector, a Northwestern sector and a Southeastern sector. The Central sector is dominated by calc-alkaline rocks, whereas the other two sectors consist of calc-alkaline rocks and subordinate amounts of alkaline rocks with OIB-like geochemical signatures. In all sectors, the dominance of calc-alkaline rocks that are largely subparallel to the Zagros suture are robust evidence for their formation in a continental arc setting. By integrating the existing geochemical data and the stratigraphic constraints, as well as the distribution of S-type granites, two episodes of orogenesis are described in this study. They are the early stage (195–168 Ma: early-middle Jurassic) and the late stage (168–140 Ma: middle-late Jurassic). We propose, on the basis of existing geochemical data, that subduction zone initiation was episodic and it involved periods of subduction and trench retreat that were interrupted by short-lived contraction events. Each of these stages is followed by a small-scale OIB-like magmatic activity, respectively, in the northwestern and southeastern sectors of the SaSZ. We interpret the OIB-like magmatism to be formed by melting of convecting asthenospheric mantle in response to slab tearing and is comparable to processes that operate in modern subduction zones. Our new tectonic model for the SaSZ provides a regionally coherent explanation for the temporal and spatial distribution of Jurassic magmatism in this part of the Tethyan orogenic belt.