Paleozoic Subduction-Accretion-Closure Histories in the West Mongolian Segment of the Paleo-Asian Ocean: Evidence from Pressure-Temperature-Time-Protolith Evolution of High-Mg and -Al Gneisses in the Altai Mountains

High-Mg, high-Al metasedimentary gneisses from the Altai Mountains, Mongolia, belonging to a subduction-accretion complex within the Central Asian Orogenic Belt can be divided into five rock types on the basis of mineral assemblages. Most rock types have high MgO and Al2O3content and low CaO, Na2O, Rb, and Sr content. All rock types experienced a similar medium-pressure metamorphism characterized by a “hairpin”-shaped counterclockwise pressure-temperature path. U-Pb zircon and U-Th-Pb monazite ages indicated metamorphism at ca. 356 Ma and 277 Ma and inherited ages of 510–379 Ma, suggesting possible provenance to granitoids comparable to those in the Altai Mountains, China. The zircons that newly nucleated at ca. 356 Ma are characterized by high concentrations of light rare earth elements without a Ce anomaly—features common in zircons from hydrothermally altered rocks and a reducing environment. Petrological and geochronological results in this study suggest the following tectonic evolution: (1) continuous subduction and accretion of paleo-Asian oceanic crust during the Early Paleozoic, resulting in periodic granitoid magmatism in the period 510–380 Ma and a continuous supply of granite-derived sediments providing detrital zircon and monazite grains to the accretionary prism; (2) ridge subduction during the Late Devonian–Early Carboniferous (ca. 356 Ma), resulting in hydrothermal metamorphism of the accretionary prism and interaction with seawater that produced rocks with unusual whole-rock chemistry; and (3) closure of the ocean leading to continental collision in the Early Permian (ca. 277 Ma), with part of the accretionary prism squeezed into lower crustal levels to form medium-pressure metamorphic rocks.