Metamorphic rocks in the Khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of Precambrian–Paleozoic age to the northeast. The Qom formation (Oligocene–Miocene) with a basal conglomerate transgressively overlies all of these rocks. The metamorphic rocks consist of both metasediments and metabasites. The metasediments are micaschist, garnet– staurolite schist and garnet–staurolite sillimanite schist with some meta-arkose, marble and quartzite. The metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc-alkaline rocks. Geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 8C at 5.5 and 7.5 kb pressure. Rims of minerals reveal a considerable decrease of pressure (!2 kb) and insignificant decrease of temperature. The PTt path of this metamorphism is normal. The MFG line passes above the triple junction of Al2SiO5 polymorphs, and the average geothermal gradient during metamorphism was from 27 to 37 8C/km, which is more concordant with the temperature regime of collision zones. We infer that crustal thickening during post- Cretaceous (possibly Eocene) collision of the Arabian plate and the Azerbaijan–Albourz block was the main factor that caused the metamorphism in the studied area
