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

Abstract

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 1.47–1.78Ga with youngest U–Pb ages of 967Ma and 964Ma. They are intruded by Neoproterozoic orthogneisses that crystallized between ca. 850 and 940Ma. Psammitic paragneisses are dominated by 940–780Ma detrital zircon populations with a youngest U–Pb weighted mean age of 721±3Ma and a main peak at 906Ma.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. 426Ma and 455Ma detrital zircon populations that originated from synkinematic granitoids dominate quartz–mica schist.Negative to positive ɛHf(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.