Chemistry and thermobarometry of amphibolites of the Faryab complex, southern Iran.

Metamorphic rocks in the Faryab complex are part of the Bajgan metamorphic complex with Upper Cretaceous age that crops out in the southeast of Sanandaj-Sirjan zone, south Iran. The metamorphic rocks of Faryab Complex have been metamorphosed in greenschist and amphibolite facies include garnet mica schist, epidote schist, epidote amphibole schist, amphibole schist, epidote amphibolite, amphibolite and garnet amphibolite. Minerals composing amphibolites are garnet, amphibole, epidote, plagioclase, quartz, chlorite and sphene as well as titanite and magnetite as secondary minerals. The composition of amphiboles in amphibolite and epidote amphibolite are made of calcic types and their chemistry varies from magnesio-hornblende through ferro-tschermakite-hornblende, ferro paragasite-hornblende, ferro edenite-hornblende to ferroedenite. The composition of plagioclas ranges from albite to oligoclase. The protolith of most amphibolites and epidote amphibolites in the Faryab complex are defined by the occurrence of key minerals in metabasites and are considered as basalt and gabbro. Several thermobarometeric calculation methods indicate that the highest temperature and pressure for the amphibolites, which were appeared adjacent to peridotites and located in the northern part of the complex, are about 700 °C and 9.7 kbar. By moving away from the peridotites into the lower structural units in the southern part of the Faryab complex, the temperature and pressure range in the amphibolite and epidote amphibolite and decrease to 510°C and 4.34 kbar on average, which is beginning of amphibolite and middle amphibolite facies. Investigation of the P-T pathes in conjunction with close associations of isograde lines show that geothermal gradients were high even at the beginning of metamorphism. Mineral chemistry and thermobarometric calculations along with consideration of the structural position of the Faryab complex indicate that the tectonic position of the Faryab complex in the Sanandaj-Sirjan zone may remined us an accretion-subduction complex. This complex was constructed in the north-dipping Neo-Tethyan subduction zone during the Late Cretaceous time, which caused the higher degree metamorphic rocks were thrusted onto the shallower ones.