Late Mesozoic topographic evolution of western Transbaikalia: evidence for rapid geodynamic changes from the Mongol-Okhotsk collision to widespread rifting

International audienceThe Mesozoic geodynamic evolution of Transbaikalia has been largely controlled by the scissors-like closure of the Mongol –Okhotsk Ocean that separated Siberia from Mongolia–North China continents. Following the oceanic closure, the tectonic evolution of that region was characterized by collisional uplift and subsequent extension that gave rise to the formation of metamorphic core complexes. This complex tectonic setting prevailed simultaneously between 150 Ma and 110 Ma both in Transbaikalia, North Mongolia, and within the North China Craton. Published paleobotanical and paleontological data show that the oldest Mesozoic basins had formed in western Transbaikalia before the estimated age of extension onset. However no precise geochronological age is available for the onset of extension in Transbaikalia. The Tugnuy Basin, as probably the oldest Mesozoic basin in western Transbaikalia, is a key object to date the onset of extension and following changes in tectonic setting. In this study, U–Pb (LA-ICP-MS) dating of detrital zircons from three key Jurassic sediment formations of the Tugnuy Basin are used to identify the potential source areas of the sediments, understand the changes in sediment routing and provide insights on the topographic evolution of western Transbaikalia. Our results show several significant changes in tectonic regime after the closure of the Mongol–Okhotsk Ocean. A wide uplifted plateau formed during the closure of the Mongol-Okhotsk Ocean, determining the Early Jurassic drainage system reaching the Angara-Vitim batholith to the north and shedding sediments to the continental margin to the South. The following collisional event at the end of the Early Jurassic led to the uplift of the collision zone, which partially inverted the drainage system toward the North. A strike-slip displacement induced by the oblique collision initiated some of the early Transbaikalian depressions, such as the Tugnuy Basin at about 168 Ma. A phase of basin inversion, marked by folding and erosion of the Upper Jurassic sediments, could correspond to the short-term collision event that took place during the latest Jurassic–earliest Cretaceous in the eastern Central Asian Orogenic Belt. The following inversion in tectonic regime from compression to extension is consistent with the mid-lower-crustal extension that led to the formation of the numerous metamorphic core complexes throughout northeastern continental Asia during the Early Cretaceous

Early Palaeoproterozoic granulite-facies metamorphism and partial melting of eclogite-facies rocks in the Salma association, eastern Fennoscandian Shield, Russia

The Salma-type Archaean eclogites exposed along the northwestern boundary of the Belomorian Eclogite Province in the eastern Fennoscandian Shield formed as a result of the Mesoarchaean–Neoarchaean subduction and collision. The common protoliths of the Salma-type subduction-related eclogites were oceanic layered gabbro and volcanic-sedimentary assemblage. The eclogite-facies pillow lavas and associated alumina-siliceous sediments that fill interpillow space and intercalate with lava flows are the main objects of our work. The kyanite-garnet–phengite–quartz rocks formed after alumina-siliceous sediments contain fluid inclusions trapped in large relic quartz grains. The fluid inclusions yielded an isochore that corresponds to PT-conditions of a beginning of the Salma oceanic rock subduction from the seafloor level that generally confirms the sedimentary provenance of these rocks. The alumina-siliceous sediments underwent the eclogite-facies metamorphism at pressure no lower than 21 kbar and temperatures of 650–750 °C and transformed into kyanite-garnet–phengite–quartz rocks. During exhumation under granulite-facies conditions at temperatures up to 900 °C and pressure down to 9 kbar, eclogite facies metasediments underwent partial melting accompanied by disequilibrium breakdown of phengite + quartz association with formation complex polymineralic pseudomorphs consisting of feldspars, biotite, muscovite, kyanite, corundum, and dumortierite. U-Pb dating of Th-rich igneous zircon from melted metasedimentary and mafic rocks using the LA-ICP-MS and TIMS methods yielded the time of granulite facies event accompanied by partial melting processes at ~2.45 Ga. After this, zircon underwent fluid-induced alteration, causing partial dissolution followed by precipitation of new Th–poor zircon and zircon rims around ancient grains at ~1.9 Ga ago © 2021 Elsevier B.V