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

Abstract

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 TDMHf 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.