Amphibolite layers of the Hamedan area Sanandaj-Sirjan zone, west Iran) contain amphibole crystals with strong optical zoning. These amphibolites occur as interlayers in middle Jurassic, Buchan-type andalusite-garnetstaurolite and sillimanite-garnet-andalusite schist of the area that were intruded by late Jurassic magmatic bodies of the Alvand plutonic complex. Electron microprobe analyses results show that the zoned amphiboles have ferrohornblende cores that change to ferroedenite toward the rims. The core-to rim increase of Al, Fe, Na, and K and decrease of Si and Mg along with edenitic substitution in amphiboles is consistent with increase in metamorphic grade. Thermobarometry calculations based on amphibole composition and hornblendeplagioclase thermometry provided 492 to 508 oC and 4.3 to 4.9 kbar for the inner cores, 495 to 514oC and 4.5 to 5.1 kbar for the outer cores, 538 to 564C and 5.3 to 5.8 kbar for the inner rims and 552 to 573oC and 5.5 to 5.9 kbar for the outer rims, respectively. These results point to a nearly isobaric prograde P-T path for the Hamedan area amphibolites, compatible with a metamorphic evolution dominated by the thermal perturbation associated with the late Jurassic magmatism of the northern Sanandaj-Sirjan zone.
