Mineral chemistry and geothermobarometry of adakitic rocks from Kashan, central Urumieh-Dokhtar magmatic arc, Iran: implications for magma evolution

Abstract

The Kashan region is located in the central part of the Urumieh-Dokhtar magmatic Arc (UDMA). In this area, Miocene adakitic rocks, including andesites and rhyodacites, occur as lava flows and domes. The primary mineral assemblages consist of amphibole, biotite and plagioclase, with minor alkali feldspar. Mineral chemistry analyses reveal that plagioclase compositions from andesine to oligoclase, amphiboles vary from Mg-hastingsite, tschermakite to Mg-hornblende, and biotites span from annite to phlogopite. Amphiboles show high Fe^³+/(Fe^³++Fe^²+) ratios, whereas magmatic biotites in rhyodacites exhibit low total Al and narrow Fe/(Fe+Mg) ratios, indicative of relatively oxidizing conditions. Oxygen fugacity (ƒO₂) values range from 10^-10.4 bar (ΔNNO+0.75, representing nickel-nickel oxide) in andesites to 10^-12.3 bar (ΔNNO+1.7) in rhyodacites. Application of the calcic amphibole geothermobarometer indicates that amphiboles crystallized at temperatures of 791-986 °C and pressures of 620-800 MPa in andesites, and at 636-850 °C and 250-450 MPa in rhyodacitic rocks, corresponding to maximum crustal depths of 28 km. Primary biotites in rhyodacites crystallized slightly later at temperatures of 690-784 °C (average 712 °C).