Protracted subduction and slab rollback of the Paleo-Asian Ocean during the late Paleozoic: Insight from polyphase metamorphism in the Baoyintu Group, central-western Inner Mongolia

Understanding the relationship between tectonic patterns and metamorphic processes generated by subduction is essential for reconstructing the evolutionary history of orogenic systems. However, these connections can be obscured by poly-metamorphism that overprints or erases evidence of earlier metamorphism. To evaluate the role of continuous subduction on overprinting previous metamorphic events, we examine the subduction dynamics of the Paleo-Asian Ocean during the late Paleozoic. This evaluation is based on evidence of multi-stage metamorphism, including high-, medium-, and low-pressure type metamorphism identified from the metasedimentary rocks in the Baoyintu Group terrane of central-western Inner Mongolia, China. We use petrographic observations, phase equilibrium modeling, and U-Pb and Rb-Sr geochronology. High-pressure metamorphic mineral assemblages from the Baoyintu Group include chlorite, garnet, phengite, rutile, and quartz. Medium-pressure metamorphism is characterized by chlorite, biotite, garnet, muscovite, staurolite, kyanite, plagioclase, rutile, ilmenite, and quartz. Low-pressure metamorphism is characterized by chlorite, garnet, biotite, muscovite, andalusite, sillimanite, rutile, ilmenite, and quartz. Phase equilibrium modeling suggests apparent pressure-temperature conditions of ∼14−19 kbar and 510−580 °C for the high-pressure metamorphism, ∼5−10 kbar and 575−770 °C for the medium-pressure metamorphism, and ∼3−7 kbar and 565−750 °C for the low-pressure metamorphism. In situ mica Rb-Sr geochronology yields a metamorphic age for the high-pressure phase of ca. 369 Ma; rutile U-Pb geochronology of medium-pressure metamorphism yields a metamorphic age of ca. 399 Ma. Monazite and rutile U-Pb geochronology of the low-pressure assemblage indicates a metamorphic age of ca. 267−252 Ma. Devonian high- and medium-pressure belts are distributed in a northeast-southwest direction, consistent with the timing of magmatic events in the Baoyintu Group during subduction. The Permian low-pressure belt is oriented east-west, partially overprinting the high- and medium-temperature belts, and is contemporaneous with magmatic events related to extension. These results suggest that the subduction boundary of the Paleo-Asian Ocean in this region was northeast-southwest during the Devonian and shifted to east-west during the late-stage subduction in the Permian. This study reveals that irregularities along subduction boundaries can lead to multiple orogenic events, prompting a re-evaluation of inferring complex tectonic processes from polymetamorphosed rocks.