Provenance of Eclogitic Metasediments in the North Qilian HP/LT Metamorphic Terrane, Western China: Geodynamic Implications for Early Paleozoic Subduction-Erosion

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Geological Sciences

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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 1.9–2.4 GPa, 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 500 Ma and 1600–1900 Ma (peak at ~ 1800 Ma), with a smaller population at 660–800 Ma (peak at 750 Ma), and minor > 1.9 Ga zircon ages. The youngest detrital zircons suggest a maximum depositional age of ca. 475 Ma, and combined with P–T conditions and previously determined metamorphic age suggest a burial rate of > 0.6–0.7 cm/yr. Zircon Hf isotopic analyses show that 500 Ma 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 1800–2500 Ma. 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 60–70 km depth prior to their exhumation.


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