Geochemical characteristics of basaltic rocks from the Nain ophiolite (Central Iran); constraints on mantle wedge source evolution in an oceanic back arc basin and a geodynamical model

Abstract The Nain ophiolitic complex is situated at the north west of the Central Iran Micro-continent (CIM) block. The basaltic rocks of this complex consist of both mantle and crustal suites and include pegmatitic and isotropic gabbros, gabbroic–dibasic dykes, dyke swarm complex and pillow lavas. The chondrite-normalized rare earth element (REE) patterns of most of these rocks show LREE depletion and the primary mantle-normalized incompatible elements indicate depletion in HFSEs (Nb, Ta) and enrichment in LILEs. The rocks show characters of island arc tholeiite/mid-ocean ridge basalt magma types. Whole rock chemistry of the rocks shows that they are originated in an oceanic back arc basin, and subsequently have been enriched by slab-derived fluids. Abundances of HFSE and HREE in most of the basaltic samples, suggest a slow subduction rate. Opening of Nain–Baft Ocean, which was probably a marginal basin, occurred during Lower Jurassic. After generation of an inter-oceanic island arc in the Nain–Baft Ocean during the Late Jurassic, a second rifting phase started within the inter-ocean island arc during Late Cretaceous (Cenomanian–Senonian). The inter-ocean island arc was developed and formed an oceanic back arc basin, the site of generation of most of the Nain ophiolitic rocks. The Nain–Baft Ocean finally closed in Maastrichtian. According to the new tectono-magmatic evolution model proposed here, the arc volcanic-like magmas were produced at the early stage (producing gabbros, gabbroic–diabasic dykes and dyke swarm complex) and then MORB-like basalts (producing pillow lavas) were generated at the later stage of evolution of the Nain ophiolitic complex.