Nature and physicochemical conditions of crystallization in the South Dehgolan intrusion, NW Iran: mineral-chemical evidence


Abstract: The South Dehgolan intrusion in the Sanandaj-Sirjan Zone ranges in composition from calc-alkalic monzogabbro to syenite to alkalic granite. This suite is composed mainly of variable proportions of quartz, K-feldspar, plagioclase, biotite, hornblende, and pyroxene. The plagioclase composition varies between albite to labradorite. The biotites are Mg- to Fe-rich in the monzogabbro to granite, respectively. The amphiboles are calcic and the composition varies from magnesiohornblende to actinolite. Clinopyroxene compositions fall in the diopside-augite field. The average calculated near-solidus crystallization temperatures are 774 °C, 655 °C, and 775 °C for the monzogabbro-monzodiorite, syenite, and granite, respectively. Calculated average pressures of emplacement are 1.7, 1.3, and 1.9 kbar for the monzogabbro-monzodiorite, syenite, and granite rocks, respectively, crystallizing at different respective depths of about 6.7, 5.0, and 7.3 km. The interpreted oxygen fugacities in the monzogabbro-monzodiorite and syenite rocks are typical of arc magmas, with oxygen fugacities above the Ni-NiO buffer. Water contents in the monzogabbro-monzodiorite and syenite were calculated to range from 4.7 to 4.4 wt.% and in the granite is 3.8 wt.%. High water and volatile contents in the monzogabbro to syenite may have allowed the magma to reach shallower crustal levels. During the evolution of this magmatic system to higher silica contents, there was an increase in the activity of oxygen and decrease in the temperature and the emplacement depth from the monzogabbro-monzodiorite to syenite rocks; this is consistent with the typical evolution of the granitoid rocks. However, the calculated higher temperature and pressure and low $f$O2 and H2O content in the later granite reflects notable differences in its origin and crystallization conditions. The range in mineral compositions is this intrusive suite is consistent with a relationship to subduction of the Neotethys oceanic crust beneath the Central Iranian microcontinent, although there was a change from calc-alkaline to alkaline magmatism.

Keywords: Thermobarometry, oxygen fugacity, South Dehgolan, Sanandaj-Sirjan Zone, Iran

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