REMNANTS OF THE PALEOTETHYAN SUTURE ZONE IN NW IRAN (MISHO MOUNTAINS) AND REGIONAL CORRELATIONS

The closure of the Paleotethyan Ocean was marked by a collisional event between the Turan and Central Iran plates along an E-W suture zone (PTSZ), which is well displayed in the Misho Mountains in NW Iran. The Misho Mountains are bounded both in the north and the south by two dextral, oblique-slip Cenozoic fault systems along which they have been uplifted as a major positive flower structure. Basement rocks in the Misho Mountains are composed mainly of the Precambrian Kahar formation, which consists of pelitic micaceous shale and fine-grained sandstone and tuff that are locally intercalated with limestone. These rocks are locally contact-metamorphosed to hornfels along the periphery of a series of mafic to ultramafic plutons. Mafic plutonic rocks consist of gabbro, dunite, norite, pyroxenite, troctolite, anorthosite, and are locally covered by basaltic extrusive rocks. These basaltic rocks contain mainly plagioclase, orthopyroxene, clinopyroxene and olivine, and show tholeiitic to calc-alkaline compositions. Micro-sond analyses (ARL-EMX computer) show that pyroxenes of the massif are made of enstatite to hypersthene, plagioclases are labradorite to bytownite and olivines are hyalosiderite. The FeO content is high in the mafic plutons and their LREE/HREE >1 is related to their mantle origin. Positive correlation between CaO and Al₂O₃ and HREE suggests an origin of partial melting of the upper mantle. All mafic rock types are likely to have been derived from the same parental magmas since their spider diagrams and multi-element patterns are similar. Diabasic dykes intrusive into mafic and ultramafic plutons and are crosscut by S-type and A-type granitoids. S-type granitoid plutons are syn-collisional whereas the A-type granitoid plutons are post-collisional in origin. Permian dark-gray limestone and massive dolomitic limestone units (Routeh Formation) unconformably cover all these plutonic rocks and provide an upper age limit for the collision-related magmatism in the region.

Regional correlations suggest that the mafic-ultramafic plutons along the PTSZ in the Misho Mountains continue to NE Iran and into Afghanistan, where an early Carboniferous-early Permian subduction occurred NW of the Hindookosh basin forming the Carboniferous granitoids (Bulin 1973). The Hercynian orogenic phase in Turan-Russia (NE Iran), northern Pamir and Tianshan also produced mafic and ultramafic rocks in this orogenic system. The Carboniferous sedimentary rocks, meta-ophiolites and meta-flycsh deposits representing Paleotethyan remnants in south of Mashhad (Binalood zone) are intruded by Triassic to Cretaceous syn-collisional, S type-granitoids. The ophiolites here have been dated at 281-277 Ma (Karimpour et al. 2002) to 268 Ma (Ghazi et al. 2003) by ⁴⁰Ar-³⁹Ar methods, and the S type-granitoids at 256 Ma (Madjidi 1978) through K-Ar dating. The Hercynian phase in the Agh Darband area is represented by lower Permotriassic sedimentary rocks and Hercynian folded rocks (folded belt). To the south and southwest of the Caspian Sea, mafic and ultramafic complex in the Talesh mountains (Gasht-Masooleh ophiolites) consists of gabbro, pyroxenite and wherlite with ages around 297 Ma (Ghazi et al. 2003) and the Lahijan metamorphic rocks consist of slate, phyllite, schist and gneiss with ages of 390-375 Ma (Crawford 1972), all part of the PTSZ. Syenitic and granitic plutons in the Moro and Misho Mountains (Moro is separated from Misho via the Tabriz fault) and metamorphic rocks in NW Orumieh are part of the PTSZ. The PTSZ continues into the Greater Caucasus, where it includes the 330 Ma metamorphic rocks of Arax (Berberian & King 1981). Remnants of the Paleotethyan Ocean are likely to continue under the eastern part of the Black Sea.