Comparative study of Low-grade metamorphic Precambrian supracrustal rocks and HP–UHP Rocks in the South Altyn Tagh: Insights into subduction-exhumation

Abstract. Low-grade metamorphic (LGM) rocks are widespread in high- to ultrahigh-pressure (HP–UHP) subduction zone yet frequently neglected in orogenic evolution. Establishing their spatiotemporal relationship with HP–UHP rocks and comparing protolith affinities are key to deciphering subduction zone architecture and exhumation dynamics. Here we investigate LGM Precambrian supracrustal rocks in the South Altyn Tagh (SAT) through field investigations, chronology and geochemical analysis, and comparison with HP–UHP rocks. Granites emplaced at 933–898 Ma, exhibiting crustal melting and syncollisional granite affinities, serving as robust markers for Rodinia convergence, consistent with protolith of regional HP–UHP granitic gneiss. Mafic dyke emplaced at ~806 Ma, exhibiting within-plate basalt (WPB) affinities, serving as markers for regime transition from collision to extension, consistent with protolith of regional eclogite and garnet pyroxenite. (Meta-)sedimentary rocks deposited during 939–932 Ma, exhibiting Taxidaban Group (Central Altyn block, CAB) affinities. Results reveal these LGM rocks lack significant Cambrian metamorphic (HP–UHP) overprinting but share protolith ages and characteristics with HP–UHP units, indicating shared formation origins yet distinct pre-subduction tectonic affiliations. This comparison implies that these supracrustal rocks may represent the non-subducted overlying plate of the SAT Early Paleozoic subduction zone. Synthesizing our data with existing metamorphic records, we propose that the current spatiotemporal distribution of LGM and HP–UHP rocks in the SAT resulted from: (1) Early Paleozoic whole-slab continental subduction, followed by (2) differential exhumation and late-stage modification.