Pre‐Eruptive Magmatic Conditions and Amphibole Phase Transitions in Dacitic Volcanics of the Qarah‐Chay Caldera, NE Iran

ABSTRACT Pre‐eruptive conditions of pressure, temperature, oxygen fugacity, and water content of the volcanic rocks from the Qarah‐Chay Caldera were reconstructed using the chemical composition of amphiboles. The caldera, located in the Binalud Zone, northeastern Iran, consists of dacitic volcanic rocks with three facies: volcanic breccia, ignimbrite sheets, and columnar dacite. Amphiboles in the volcanic breccia are predominantly calcic (magnesio‐hornblende and pargasite), whereas those in the ignimbrite sheets and columnar dacite are Fe‐Mg ± Mn amphiboles (cummingtonite). Magnesio‐hornblende and pargasite crystallised under pressures of 260–360 MPa, temperatures of 835°C–885°C, with water content of 5–8 wt.%. Experimental petrology reveals that cummingtonite is stable at temperatures below 810°C and pressures of 200–300 MPa. The eruption of dacitic magmas forming the volcanic breccias occurred at 835°C–885°C, 260–360 MPa, 5–8 wt.% H 2 O, and log f O 2 values between −14 and −8 according to histogram frequency and petrological constraints. Then, the subsequent eruptions occurred to form the ignimbrite sheets and cummingtonite‐bearing columnar dacites at lower temperatures (< 810°C), with higher water contents and in more oxidising conditions (log f O 2 between −13 and −3). These changes led to the transformation of calcic amphiboles into Fe‐Mg‐Mn type during late‐stage magmatic evolution and eruption.