Metamorphic evolution of amphibolite from Proto-Tethys South Altyn orogen and its geological significance

Abstract Medium- to low-grade metamorphic (LP–MP) rocks, though major constituents of collisional orogens, remain less studied than high-to ultrahigh-pressure (HP–UHP) counterparts. Their peak conditions and P–T–t evolutions are poorly constrained due to lack of diagnostic assemblages. This study investigates amphibolites enclosed in HP pelitic granulites in Munabulake in South Altyn (SA) HP–UHP belt, unveiling a two-stage metamorphism through inclusion analysis and thermodynamic simulation. The eclogite-facies metamorphism (firstly identified in the westernmost SA) is evidenced by zircon-hosted garnet + omphacite + rutile inclusions and flat heavy rare earth elements (HREEs) patterns without negative Eu anomalies, yielding a peak age of 501.6 ± 2.7 Ma. The amphibolite-facies metamorphism is recorded by titanite-hosted amphibole + plagioclase inclusions, yielding a retrograde age of 437 ± 6.2 Ma and P–T conditions of 3.7–6.1 kbar/640–725 °C. These findings demonstrate that the amphibolite originated from retrograde metamorphism of eclogite. Integrated with previous studies, our results reveal potentially extensive HP–UHP exposures across the SA, with most rocks attaining eclogite-facies conditions at ~ 500 Ma, documenting an entire continental deep subduction of SA during Early Paleozoic. And the heterogeneous spatial-temporal distribution of metamorphic rocks across different grades in SA reflects differential exhumation processes or variable intensities of retrograde overprinting.