Mineral chemistry and P-T conditions of the adakitic rocks from Torud-Ahmad Abad magmatic belt, S-SE Shahrood, NE Iran (original) (raw)

Torud-Ahmad Abad magmatic belt is located 175 km east and southeast of Shahrood in the northern part of the Central Iran Structural Zone and includes a thick sequence of Paleocene to middle Eocene volcanic and volca-nosedimentary rocks. This magmatic belt was formed by numerous hypabyssal igneous adakitic domes constituting basaltic andesite, andesite, trachyandesite, dacite, trachydacite, and dacite. The investigated rocks are mainly composed of pyroxene, amphibole, and plagioclase, with minor biotite and opaque minerals. Mineral chemical analysis reveals that plagioclase composition varies from albite to labradorite, clinopyroxene varies from diopside to augite, and amphibole varies from Mg-hastingsite to Mg-hornblende. Amphibole geothermobarometry suggests crystallization temperatures of 850-1050 °C, at 2-6 kbar and the temperature of 920-970 °C, at a pressure of 3-4.5 kbar, which are conditions in agreement with andesite and dacite formation. Clinopyroxene crystallized at temperatures of 1020-1170 °C, at 2-10 kbar, indicating crys-tallization at crustal depths of maximum 30 km for the studied intrusive rocks in the Torud-Ahmad Abad magmatic belt.