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

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.