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.
The text points out that the widely distributed LGM rocks in the SAT lack reliable Cambrian HP metamorphic records. To distinguish between the two competing hypotheses of "deeply subducted but overprinted" versus "never deeply subducted and tectonically interlard", what specific geological, petrological, or geochemical criteria (e.g., field contact relationships, mineral inclusions, trace element geochemistry, geochronology and trace element analysis of zircon/monazite) did this study employ to effectively identify potential early deep subduction signals in the LGM rocks that may have been obscured by later overprinting? How does this evidence rule out the possibility that the LGM rocks never underwent deep subduction?