Thermobarometry and origin of andesitic lavas in Zoolesk area based on mineral chemistry (northeast of Sarbisheh, east Iran)

In Zoolesk area, about 11km northeast of Sarbisheh city in South Khorasan Province, volcanic rocks including andesite, dacite and rhyolite have cropped out where andesitic lavas have the maximum distribution and have been studied in this research.The main textures of andesites are porphyry with microlitic-glassy groundmass, glomeroporphyry and vesicular. Minerals forming andesites include plagioclase, pyroxene, amphibole, biotite and rarely sanidine. Plagioclase phenocrysts have oscillatory zoning and sieve texture and so corrosion, rounding and resorption rims can be seen in the margin of some crystals. Based on microprobe analyses, the composition of plagioclases changes from Ab67,An33to Ab54, An46 and are andesine type. Clinopyroxene phenocrysts have augite with minor diopside composition (En39-41 Fs13-17 Wo43-47) and orthopyroxenes are enstatite (En41-74Fs13-43Wo2-3). The Mg# values in clinopyroxene and orthopyroxene are variable between 69-75 and 56-76 respectively. Thermometry studies showed that the crystallization temperature of clinopyroxenes is 1150 to 1200°C and for orthopyroxene is 1050 to 1100°C. The calculated pressure for genesis of clinopyroxene and orthopyroxene in the studied samples was less than 2 kbar.