Different geodynamic settings for Sistan suture zone ophiolitic units: discussion of textural evidences and mineral chemistry of crustal sequence ultramafic-mafic associations

Sistan suture zone ophiolitic rocks are remnants of Tethyan oceanic lithosphere between Lut and Afghan continental blocks. This oceanic basin has been existed at least during the Cretaceous. Mineral chemistry from crustal sequence ultramafic-mafic associations and petrographical evidences from parts of these ophiolitic complexes (the ophiolitic rocks located between the south of Birjand and Tchehel Kureh ophiolites) suggest distinct geodynamic setting for the formation of oceanic lithosphere. Plutonic rocks from crustal sequence mainly include ultramafic cumulates (dunite and pyroxenite)، cumulate gabbro، gabbronorite and isotropic gabbro. The crystallization trend in these rocks is variable including earlier crystallization of pyroxene relative to plagioclase and vice versa which implies SSZ (supra-subduction zone) and MORB (Mid-Oceanic Ridge Basalt) - type trends، respectively. The composition of plagioclase in cumulate gabbros and gabbronorite is bytownite and anorthite، respectively. Moreover، the contents of compatible and incompatible elements of clinopyroxene and orthopyroxene in ultramafic cumulates، cumulate gabbros and gabbronorite represent significant differences. The chemical composition of these minerals and crystallization trends indicate that pyroxenites-gabbronorite and troctolite-cumulate gabbros-isotropic gabbros have been formed from two different magma series with SSZ and MORB affinities، respectively. In the study area، it seems that the MORB-type magmatism resulted from partial melting beneath mid ocean ridge produced a depleted mantle. Subsequently، consequence of intra-oceanic subduction the MORB mantle has been suffered higher depletion due to fluid-induced melting in supra subduction zone setting and produced SSZ-type magmatism.