Zircon Geochronology and Tectonic Evolution of Eclogites from the Beishan and Qinling Orogens, China

Central Asia is host to some of the best examples of ultra-high pressure metamorphic rocks in the world. The study sites located in Beishan and Qinling Orogens contain examples of these high pressure rocks, called eclogites. They are an evidence of ancient continental collisions and their age and composition provide invaluable information about tectonic mechanics during collisions. This study focuses on geochronology of eclogites sampled from Beishan and Qinling Orogens. The Beishan eclogite, occurs as small mafic pods within highly deformed gneisses. The geological context and geochemistry of these eclogites determined in this study suggests they were originally rift-related MORB-type basalts rather than discrete fragments of oceanic crust, as previously thought. Zircon dates obtained from the Beishan eclogite reveal peak metamorphism at 466 ± 27 Ma, which is concordant with the literature. However, U/Th ratios in zircon cores (ca. 880 Ma) are too low to have crystallized from mafic magma and therefore represent xenocrystic cores or previous metamorphic event rather than age of the protolith. The maximum age of the eclogite protolith is constrained by the crystallization age of the country rock orthogneiss at 920 ± 14 Ma. The end of retrogression metamorphism in the studied terrane was constrained by granitic vein that intruded the eclogite at 424 ± 8.6 Ma. This study provides the age for this rock for the first time. From evidence gathered for Beishan Orogen, this study proposes new tectonic model for initial stage of orogen development. The Qinling eclogites did not yield significant amount of zircons, so they could not be dated with certainty. One zircon was dated at 490 ± 83 Ma which is consistent with metamorphic age established in literature, but statistically they cannot be used as a reliable age. However, its remarkably well-preserved eclogite facies mineral assemblage, that is contrary to Beishan eclogite, suggest diversity of dynamics during high pressure terrane exhumation. Lastly, this study has found possible connection between extrusive tectonics of India- Eurasia collision and its long distance (~600 km) effects on Beishan terrane. Here named, “Cenozoic Thermal Event”, led to reset of metamict zircons in the eclogite, where new ages correspond to major tectonic developments in Tibetan Plateau. This finding provides a new, previously undocumented relationship between tectonism and reset of the zircon geochronometers, which may have significant implications for future geochronology of crustal-scale fault systems