Petrography and mineral chemistry of volcanic rocks in Kuh-e-Siah (North of Gavkhuni lagoon, SE of Isfahan)

Kuh-e-Siah is located in the north of Gavkhuni lagoon and east of the Varzaneh (SE of Isfahan) and is a part of Urumieh-Dokhtar magmatic arc (UDMA). The volcanic rocks of this area are vesicular basalt and basaltic andesite with Late Miocene age. The most common textures of these rocks are porphyry، microlitic porphyritic، hyaloporphyric and vesicular. Plagioclase، olivine and clinopyroxene are the main rocks forming minerals of basalts. Orthopyroxene and Cr-spinel are the accessory phases. The major minerals of the basaltic andesites are plagioclase، olivine and orthopyroxene. Clinopyroxene and Cr-spinel are present as accessory minerals. The secondary minerals of all studied rocks are chlorite، calcite and magnetite. In these rocks plagioclases are labradorite and bytownite، olivines are chrysolite (mainly changed into chlorite)، orthopyroxenes are enstatite (some of them have a rim of pigeonite)، clinopyroxenes are augite and chlorites are corundophilite in composition. Olivine-spinel geothermometry yielded 1 to 10 Kbar، and the temperature varies from 914 to 1201 oC. Also، orthopyroxene thermometery presents temperature of 1248 to 1304 oC for pressures of 1 to 10 Kbar. The whole rocks geochemical data and the chemistry of minerals indicate that the studied rocks are calc-alkaline in composition and geotectonically belong to the volcanic arc setting.