مهندس اکبر احمدوند

The Rizu Formation of the Zarand area consists of felsic (rhyolite-trachyte) and mafic (microgabbro-dolerite) magmatic suites, overlain by dolomites of the Desu Formation. Felsic magmatic units are investigated in this research. The rhyolite-trachytes of the Rizu Formation are composed of alkali feldspar microphenocrysts that have been enclosed by fine grained and micro granular groundmass consisting of quartz, alkali feldspar and plagioclase. Whole rock geochemical data indicate that the samples are metaluminous with high FeO/MgO ratio, Na2O+K2O and HFSE, and low CaO, Ba and Eu contents that are in line with geochemical characteristics of anorogenic granites. According to the tectonic discrimination diagrams, the felsic rocks of the Rizu Formation were formed in within plate to continental rift settings. They show the same geochemical signatures as those of anorogenic rhyolites of the East African rift and Basin and Range province of the west coast of North America. In addition, the rhyolite-trachytes of the Rizu Formation suggest an A1 subtype on the basis of anorogenic granite discrimination diagrams, derived from fractional crystallization of an OIB-type mantle melt in a within plate (failed rift) environment. Their association with microgabbros is also reminiscent of bimodal magmatism in a continental back arc rift formed by the subduction of Prototethys beneath the Iranian plate during the Late Neoproterozoic-Cambrian

The Seghin metamorphic complex of ophiolitic mélange from the north Soghan, composed of metamorphic suites varying in protolith and metamorphic grade. The rock units of this complex have been metamorphosed under PT conditions of pumpellyite- actinolite, greenschist- Ep amphibolite and blueschist facies enclosed by a serpentinitic mélange. The pumpellyite- actinolite and greenschist- Ep amphibolite facies rocks display overprinted metamorphic evidences of blueschist facies as glaucophane overgrowth around actinolite and magnesiohornblende crystals. The average temperature and pressure calculated by thermocalc software and petrogenetic grids is 6- 7, 290- 350 and 11 Kb, 350 ˚C for pumpellyite- actinolite schist and blueschist samples, respectively. The relations of minerals transformation, thermobarometry calculations and the lawsonite preservation are in accordance with counter- clockwise metamorphic path. Generally, the metamorphic blocks of serpentinitic mélange with counter- clockwise PT path like Seghin complex are characteristics of long- lived subduction systems. During the onset of subduction event and attaining to blueschis facies, metamorphic path of down- going slab has passed through greenschist- Ep amphibolite facies. Then, the prograde metamorphic path extending across Pmp- Act subfacies owing to decreasing of geothermal gradient during maturity of subduction setting. Consequently, hairpin- like PT path is governed that is characteristic of Franciscan- type accretionary prisms.