Provenance of eclogitic metasediments in the north Qilian HP/LT metamorphic terrane, western China: Geodynamic implications for early Paleozoic subduction-erosion

In this contribution, we present new petrological, geochemical, zircon U-Pb and Hf isotopic data for eclogitic metasediments from the north Qilian orogenic belt, in which early Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasediments contain eclogite facies assemblages reflecting P-T conditions of 450-560°C and 1.9-2.4GPa, consistent with those of adjacent eclogites. REE compositions of the eclogitic metasediments overlap those of average upper continental crust. The SHRIMP and LA-MC-ICPMS U-Pb data of zircons from metasediments demonstrate input from sources with major age components about 500Ma and 1600-1900Ma (peak at ~1800Ma), with a smaller population at 660-800Ma (peak at 750Ma), and minor >1.9Ga zircon ages. The youngest detrital zircons suggest a maximum depositional age of ca. 475Ma, and combined with P-T conditions and previously determined metamorphic age suggest a burial rate of >0.6-0.7cm/yr. Zircon Hf isotopic analyses show that 500Ma zircons have positive εHf(t) (mainly between +8 and +16). By contrast, Meso- to Paleoproterozoic detrital zircons show a broad spectrum of εHf(t) (mainly between -5 and +10) with T _DMHf of 1800-2500Ma. These data suggest that eclogitic metasediments are derived from a mixture of Proterozoic continent crust and juvenile early Paleozoic oceanic and/or island arc crust, and their protoliths likely were deposited in a terrigenous-dominated continental margin basin rather than a pelagic oceanic crust environment. The new results are consistent with subduction erosion of the active continental margin during south-dipping subduction, as these sediments, formed in a fore-arc environment close to the Qilian block to the south, were transported in the subduction zone to 60-70km depth prior to their exhumation.