Hualong Complex, South Qilian terrane: U-Pb and Lu-Hf constraints on Neoproterozoic micro-continental fragments accreted to the northern Proto-Tethyan margin

摘要

The Neoproterozoic Hualong Complex dominates the South Qilian terrane. It contains mafic to felsic magmatic rocks, cherts and turbiditic sediments, some of which have been metamorphosed to amphibolite grade and is traditionally regarded as a pre-Cambrian micro-continental block rifted from the South China plate. New U-Pb geochronology of morphologically complex zircons sheds light on the history of the complex. Garnet-bearing amphibolite and quartzite have detrital zircon U-Pb main peak ages of 1.47-1.78. Ga with youngest U-Pb ages of 967. Ma and 964. Ma. They are intruded by Neoproterozoic orthogneisses that crystallized between ca. 850 and 940. Ma. Psammitic paragneisses are dominated by 940-780. Ma detrital zircon populations with a youngest U-Pb weighted mean age of 721. ±. 3. Ma and a main peak at 906. Ma. Metamorphic overgrowths on older zircons record amphibolite facies metamorphism of the Hualong Complex and correspond to the emplacement of the younger plutonic rocks during the Early Paleozoic northward subduction of Proto-Tethyan oceanic lithosphere. 426. Ma and 455. Ma detrital zircon populations that originated from synkinematic granitoids dominate quartz-mica schist. Negative to positive εHf(t) values for both 940-850Ma granitic orthogneisses and intruding 460-410Ma granites suggest a crustal-mantle mixture source. Hf TDM1 model ages and inherited zircon U-Pb ages are the same as those for garnet-bearing amphibolites and psammitic gneiss indicating that late magmatic rocks were formed by melting of the accreted Neoproterozoic rocks.