Early Paleozoic (ca. 465 Ma) eclogites from Beishan (NW China) and their bearing on the tectonic evolution of the southern Central Asian Orogenic Belt

Abstract

The Beishan orogen is situated in the southern margin of the Central Asian Orogenic Belt (CAOB). It is connected with the Tianshan orogen to the west and the Mongolia-Xing’anling orogen to the east. This orogen exhibits a well-preserved Neoproterozoic to Late Paleozoic sequence containing ophiolites, accretionary complexes and eclogites. In this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the Beishan orogen. U–Pb zircon dating of eclogites yielded two concordant ages, 886 ± 4 Ma for the cores and 465 ± 10 Ma for the rims. The zircon cores display bright luminescence with homogeneous or planar/sector zoning, intermediate Th/U ratios (0.17–0.47), and HREE-enriched patterns with negative Eu anomalies (Eu/Eu * = 0.16–0.58). These features, coupled with their formation temperatures of 680–710 °C, suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode. By contrast, the zircon rims have dark luminescence with homogeneous or fir-tree zoning, low Th/U ratios (<0.02), and flat HREE distributions with small or no negative Eu anomalies (Eu/Eu * = 0.55–1.16). Thus, the rim age reflects the time of the eclogite facies metamorphism. The occurrence of tiny inclusions of garnet, omphacite and rutile in the zircon rims is consistent with the above age interpretation. In addition, the mineral inclusions and host zircons yielded P–T conditions of 660 °C and >12 kbar for the eclogite facies metamorphism. On the other hand, zircon dating of a paragneiss near the eclogite produced two major groups of ages, ca. 1450 Ma for a magmatic event and ca. 900 Ma for a metamorphic event. It seems that eclogites and their country rocks have suffered the same earlier metamorphic episode. Based on the available age data obtained for eclogites, ophiolites and granitoids from the Beishan area, a tectonic scenario can be constructed as follows. The ca. 1000 Ma oceanic crust representing by protoliths of the eclogites was involved in Early Neoproterozoic orogenesis to have formed a microcontinental block. During the Early Paleozoic, island arcs and microcontinental blocks were progressively accreted to the northern margin of the Tarim craton (Dunhuang block). This was followed by the closure of Paleoasian Ocean and final amalgamation between the Tarim craton and Kazakhstan block in the Late Paleozoic.