Progressive accretionary tectonics of the Beishan orogenic collage, southern Altaids: Insights from zircon U-Pb and Hf isotopic data of high-grade complexes

Abstract High-grade gneissic rocks of the southern Altaids in Central Asia bear one of the key elements needed to solve a basic architectural controversy in accretionary orogenesis. High-grade gneissic rocks widely crop out across the Beishan orogenic collage, southern Altaids, mapped as the Beishan Complex or Dunhuang Group, which was previously regarded as Neoarchean to Paleoproterozoic crystalline basement, but without systematic field, geochronological and geochemical data. This study provides field, geochemical and zircon U–Pb and Hf isotopic data for gneissic rocks from the Beishan Complex. Geochemical data show that the gneissic rocks were probably formed in a subduction environment. Most zircons from the Beishan Complex are magmatic origins because their Th/U ratios are mostly in the range of 1.0–0.1; they show age peaks at 494 Ma, 464 Ma, and 375 Ma, with a few Precambrian ages. Zircon Hf isotopic compositions indicate that both old and juvenile materials were involved in the formation of the Beishan Complex, which is different from the basement rocks of the Tarim Craton both in radiometric age and Hf isotopic composition. All these data thus do not support a uniform Precambrian basement in the Beishan area as previously proposed, but instead the Beishan Complex probably formed in a subduction–accretion environment. Combined with regional geology and geochemical data, we conclude that the Beishan orogenic collage was characterized by progressive long-lived accretionary tectonics during Late Precambrian to Paleozoic time, therefore shedding light on the controversy regarding the architecture of accretionary orogens.