The Oligocene Avaj volcanic – plutonic complex of Central Iran: A record of magma evolution and mineral equilibria

Abstract

• High-An plagioclase feldspar cores formed during early stage of evolution of the primary magma . • Mineral-melt equilibrium conditions are determined in clinopyroxenes and plagioclase cores. • Zoning in plagioclase and smooth CSD patterns invoke an unsteady-state magma system. • Parental mafic magmas were hydrous and contained <4 wt% H 2 O. • The residence time of plutonic and volcanic magmas was 117 and 13 years, respectively. The Avaj Oligocene volcanic – plutonic complex is part of extensive Cenozoic magmatic activity within the Urumieh-Dokhtar magmatic arc of Iran. We use whole rock geochemistry, mineral compositions and crystal size distributions (CSD) in a suite of co-genetic basalt, basaltic andesite and gabbro to determine their petrogenesis. Ca-rich cores in plagioclase (An 79-86 ) overlap empirically modelled compositions, indicating equilibrium crystallization from melts represented by the whole-rock compositions. Clinopyroxene compositions (Mg# 74–80) are compatible with mildly fractionated mantle-derived magmas in an arc setting. Mineral-melt equilibrium is inferred from high Al contents and close correspondence between the measured DiHd and predicted Kd Fe–Mg (0.23–0.32) in clinopyroxenes, and Kd (Plg/melt) (An–Ab) values of plagioclase cores (0.11–0.15). Clinopyroxene-melt thermometers indicate crystallization at 1119–1173 °C for volcanic and 1099–1134 °C for plutonic rocks. Plagioclase crystal core saturation temperatures range from 1088–1162 °C (volcanic) and 1121–1163 °C (plutonic); these values overlap calculated mineral-melt equilibrium temperatures. Plagioclase CSDs are nearly straight for both volcanic and plutonic samples, with higher nucleation density and steeper slopes for the plutonic samples. Major element variations suggest the Avaj rocks represent co-genetic magmas related by fractional crystallization of the observed mineral phases. We suggest minor crustal assimilation occurred during ascent from a deeper reservoir to a shallower one; CSD data indicate longer magma residence time for plagioclase in the plutonic samples (∼117 years) compared to the residence time of basaltic samples (∼13 years).