Origin and evolution of metamorphosed mantle peridotites of Darreh Deh (Nain Ophiolite, Central Iran): Implications for the Eastern Neo-Tethys evolution

The Nain Ophiolite is one of the most complete harzburgitic-ophiolite suites exposed along the Nain-Baft Fault, around the Central-East Iranian Microcontinent (CEIM). It is a remnant of the Nain Ocean (Early Jurassic to upper Early Cretaceous-Paleocene). During the Cretaceous (similar to Al-bian to Cenomanian), ascending melts and serpentinization overprinted the Jurassic melt-rock reactions and metamorphism of peridotites in most of the ophiolite. However, metamorphosed peridotites of the Darreh Deh massif in the eastern part of Nain Ophiolite remained intact, with the lowest degree of alteration and serpentinization and slightly higher degrees of partial melting and melt - rock reactions. Clinopyroxene and Cr-spinel compositions together with bulk rock geochemistry of Darreh Deh peridotites indicate a mid-ocean ridge affinity for lherzolite but later compressional tectonics led to an intra-oceanic subduction and supra-subduction zone (SSZ) in which harzburgite and dunite formed due to ascending melt reacting with the mantle lherzolite. Further indications are the presence of replacive olivine, incongruent melting of orthopyroxene and increase of Cr# of Cr-spinel in harzburgite. The extensional to compressional evolution in the Jurassic ended with accretionary and/or collisional processes induced by convergence between the Sanandaj - Sirjan zone and the CEIM. These caused a regional metamorphism that affected the Darreh Deh massif by forming antigorite, talc, metamorphic olivine, tremolite and chlorite metamorphic assemblages in mantle peridotites at amphibolite facies (similar to 630-700 degrees C / 7-15 kbar) conditions, similar to the metamorphosed diabasic dikes, basalts and pillow lavas of this ophiolite.