Metamorphic Evolution of the Internal Zagros (Se Iran): Constraints From Titanite and Rutile Petrochronology, and Thermodynamic Modelling of Amphibolite‐Facies Rocks in the Southern Sanandaj‐Sirjan Zone

ABSTRACT

The high‐temperature medium‐pressure metamorphic core of the Zagros Orogen is exposed within the overthickened upper plate of the Eurasian‐Arabian continental collision, known as the Sanandaj‐Sirjan Zone (SSZ). An integrated study of metapelitic and metabasic rocks, combining petrographic observations, titanite and rutile U–Pb petrochronology, trace‐element geochemistry and thermobarometric estimations, revealed a complex polymetamorphic evolution of the southern SSZ. Consistent Zr‐in‐titanite and Ti‐in‐biotite thermometric results, supported by petrographic evidence such as titanite equilibrated with high‐temperature mineral assemblages and fabrics, indicate titanite growth during high‐temperature retrograde conditions. Titanite U–Pb ages, together with amphibolite petrography, delineated multiple high‐temperature late‐peak retrograde episodes affecting early Palaeozoic units during the Late Triassic (~204 Ma), Early to Mid‐Jurassic (~187–165 Ma) and Cretaceous (~120–80 Ma). Rutile inclusions within garnet, and the consistency between rutile crystallization temperatures and peak P–T conditions indicate that rutile formed as a part of the near‐peak to peak mineral assemblages. Accordingly, rutile U–Pb ages were interpreted to provide minimum age constraints on amphibolite‐facies metamorphism during the Early Jurassic (~184–176 Ma), Late Jurassic (~143 Ma), Early Cretaceous (~97 Ma) and early Eocene (~51 Ma). Thermobarometric results showed peak metamorphic conditions of ~500°C–700°C and ~0.6–0.9 GPa for the investigated amphibolite sample, aligned with previously reported P–T estimates for regional metapelitic rocks. Correlation of these age populations with metamorphic records from the northern SSZ and Central Iran supports a multiphase metamorphic history involving Cimmerian events followed by Neotethyan‐related tectonic cycles. The ~51 Ma rutile age assigns a minimum age constraint on Eocene metamorphism and may reflect tectono‐metamorphic processes associated with the early stages of Zagros convergence. Uniform peak P–T conditions, together with spatial and interunit variations in metamorphic ages, suggest differential tectonically controlled exhumation of a broadly coherent metamorphic unit of the southern SSZ, linked to the Neotethyan evolution.