Assessing Physicochemical Conditions of Paleocene- Eocene Syenitic Rocks in the Central Sanandaj-Sirjan Zone, North of Golpayegan, Iran

Abstract

The Sanandaj-Sirjan Zone (SSZ) records protracted subduction of the Arabian plate beneath Eurasia. This research examines syenitic plutonic rocks from the northern Golpayegan region, central SSZ, aiming to determine their crystallization conditions and tectonic setting. The rocks consist mainly of orthoclase, calcic amphibole (ferro-tschermakite), and biotite (lepidomelane), with biotite chemistry indicating crystallization from a potassic alkaline series, supported by whole-rock geochemistry. Amphibole-only thermobarometry (Ridolfi et al., 2010) yields crystallization temperatures of 882–915 °C (avg. 902 °C) and pressures of 3.68–5.59 kbar (avg. 4.67 kbar), corresponding to depths of ~15–18 km. Oxygen fugacity estimates, derived from biotite Fe³⁺/Fe²⁺ ratios, yield a log fO₂ of approximately -13, indicating reducing conditions near the FMQ buffer. The alkaline affinity, mid-crustal emplacement, and reduced redox state, when combined with regional tectonic frameworks, point to a post-collisional origin linked to extensional fracturing after the Laramide orogenic pulse. These results provide quantitative P–T–fO₂ constraints on Paleocene-Eocene alkaline magmatism in this segment of the SSZ and are discussed within the broader context of regional tectonomagmatic evolution.