High-grade complexes record the Late Permian-Middle Triassic arc metamorphism in the southernmost Altaids: Implications for the final closure of the Paleo-Asian Ocean

Abstract

High-grade metamorphic complexes or terranes in orogens play a vital role in reconstructing orogenic dynamics and tectonic history of Earth's past owing to their multi-tectono-thermal recording capacity. The Chijin Complex, located in the southernmost Altaids and composed of intensely deformed medium- to high-grade meta-volcanic rocks including (garnet-bearing) mica-quartz schists, felsic gneisses, and amphibolitic schists and gneisses, imposes crucial constraints on the fiercely disputed timing and tectonic architecture of the Paleo-Asian Ocean's (PAO) terminal evolution. This Complex has long been interpreted as the Precambrian basement of a microcontinent in tectonic reconstructions. Our detailed field investigations, geochemical research, and detrital age spectrum analyses of the Chijin Complex reveal structural-lithological-geochemical affinities to volcanic-magmatic extrusions originating from a continental arc regime. Anti-clockwise P-T paths and high T/P geothermal gradients (~24–37 °C/km), reconstructed by thermodynamic modeling and geothermobarometry for the Complex, reveal magmatic underplating mechanism from prograde “hot” T/P metamorphism resulting from thermal intrusion and finite arc crustal thickening to in-situ retrograde isobaric cooling. Hornblende and biotite 40Ar/39Ar and zircon UPb dating demonstrates that the continental arc system was still active as during the Early to Middle Triassic (~252–237 Ma) as a result of continuing southward subduction of the PAO, thereby constraining the final amalgamation of the Altaids to the Late Triassic. This study proposes a new perspective for redefining high-grade rocks at the southernmost Altaids as arc-metamorphic complexes rather than “reputed” ancient microcontinent, as well as a potential template example for discerning tectonic nature of semblable metamorphic complexes or terranes with distinct origins in global orogens.