Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

Abstract

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.