SERPENTINIZED ULTRAMAFICS OF THE KHOY OPHIOLITIC MÈLANGE IN NORTHWESTERN IRAN. A VIEW OF THEIR TECTONIC EMPLACEMENT

The Khoy ophiolitic mélange in northwestern Iran consists of different intrusive igneous rocks and some metamorphic rocks mixed with red pelagic and dark gray Paleocene limestone. Most of the area (up to 70%) is covered by greenish color, serpentinized (up to 90% in some samples) harzburgite, dunite, pyroxenite, and chromitite juxtaposed in fault contact with quartzite, schist, and other metamorphic rocks. Study of many harzburgite samples shows that olivine and psuedopyroxene are the major minerals in these rocks while chlorite, epidote, tremolite, talc, and calcite are the secondary constituents. The ophiolitic mélange together with volcanic and metamorphic units are highly tectonized and are marked by numerous fractures, veins, and faults. The evidence for serpentinization are (1) tremolite, talc schist, and talc vein lets, at the contact between ultramafics and amphibolite or schist, (2) the original lepidoblastic texture which is defined by long amphiboles in some harzburgite samples is not detectable in others due to intense serpentinization, (3) alteration of hypersthene and clinopyroxene to tremolite and actinolite and the cataclastic texture of dunite which is defined by the serpentinized ferrochrome spinel, and (4) low concentration of Cr and Ni. The concentration of oxides of the major elements and their MgO/MgO+Fe ratios together with the amount of some of the minor and rare earth elements (REE) in harzburgites are very similar to the oceanic harzburgites that are studied from other ultramafic rocks around the world. The emplacement of ultramafics, diabase units, basalt flows and their associated carbonates can be modeled in the context of the subduction of the Neo-Tethys under the Iranian block and obduction of slivers of the serpentinized oceanic lithosphere during the Zagros orogeny in Cretaceous. Existence of foraminifera such as Globotruncana in red limestone and radiolarian in radiolarites which are mixed with lava flows suggest an upper Cretaceous age. Other microfossils such as the upper Paleocene and lower Eocence Nomolites in the youngest rock in the area which is a dark, micritic limestone are further evidence that the emplacement of the Khoy ophiolite started in late Maestrichtian and continued to the middle of Eocene.