Geochemistry of gabbroic pockets of a mantle sequence in the Nain ophiolite (Central Iran): constraints on petrogenesis and tectonic setting of the ophiolite

The Nain ophiolite in central Iran is part of the Central Iranian ophiolite belt. Many rock pockets with various compositions can be seen in the peridotite. The main rock type of these pockets is gabbro. Clinopyroxene and plagioclase in the gabbros are Ca-rich. The chemistry of clinopyroxene relates the rocks to orogenic gabbros, generated at relatively low pressure. The parental magma of these rocks was tholeiitic, chemically similar to supra-subduction zone (SSZ) tholeiitic basalts. Whole rock geochemistry shows two groups of gabbros derived from different sources. On the basis of geochemistry, rocks of group I are depleted in HFSEs in comparison with primordial mantle and display convex upward REE pattern, characterized by LREE depletion (LaN/SmN = 0.05-0.8) and enrichment in MREE and HREE. Enrichment in LILs and REEs of group II rocks are relatively similar (LaN/SmN = 0.5-1.4) to MORB with small depletion in HFSEs and low enrichment in LILs. In this paper we show that the source materials of group I rocks were magmas derived from the lithospheric mantle or depleted asthenospheric mantle. Group II rocks were derived by mixing of basaltic magmas from the asthenospheric mantle with magmas from the lithospheric mantle. Rocks of each group are formed by various rates of partial melting and subsequent crystal fractionation. Geochemistry of the gabbros, their nature (SSZ character) and different source materials involved in their generation, suggest a marginal basin setting for the formation of the Nain ophiolite.