Abstract

Four dismembered massifs belonging to the Nain-Baft ophiolitic belt (Central Iran) stretch in a NW-SE direction parallel to the fossil active margin of the Iranian Continental Block (Sanandaj-Sirjan Zone). They are separated by huge transcurrent faults. The Nain, Dehshir, Shahr-e-Babak and Baft massifs are composed of associated slices of harzburgites, small bodies of gabbros and dike swarm complexes, accompanied by various extrusives from basaltic-andesitic lava flows and breccias to dacites and rhyolites. Trace element geochemistry of these lavas displays calc-alkaline and arc-tholeiite signatures, suggesting a back-arc origin for these ophiolites. This is in accordance with the position of these massifs, to the North of the Mesozoic Magmatic Arc crosscutting the Sanadaj-Sirjan Zone. Conventional K-Ar datings on amphibole within amphibolite and gabbros deliver ages between 93 Ma and 67 Ma. These ages are in good agreement with the stratigraphic age of the conformably Cenomanian to Maastrichtian sedimentary cover of the extrusives. The closure of these back-arc basins occurred in the Middle Paleocene as testified by the presence of neritic limestones, sealing all the tectonic contacts. The general geological setting of the Nain-Baft belt suggests that these massifs generated in a transtensional small back-arc basins separated by transcurrent faults. These short-lived transtensional basins result from the oblique subduction of the Tethyan Ocean under the Iranian Continental Block.