Thickening and partial melting of the Northern Qinling Orogen, China: Insights from zircon U-Pb geochronology and Hf isotopic compositions of migmatite

The widespread migmatites in the Precambrian basement of the North Qinling Belt are products of partial melting of the Earth's crust and record the tectono-thermal response to Late Paleozoic collision along the Shangdan suture. The morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting. The zircons from leucosome, melanosome and banded migmatites yield U-Pb weighted mean ages of 408 +/- 2, 403 +/- 2 and 400 +/- 2 Ma, respectively, constraining the timing of migmatization. The granitic dyke intruded into the migmatites gives a zircon U-Pb weighted mean age of 396 +/- 2 Ma, constraining the minimum time of migmatization. Zircons from the melanosome and banded migmatite yield upper intercept ages of 1088-1061 Ma, representing the protolith ages of the basement of the North Qinling Belt. The zircon rims of the melanosome and banded migmatite predominantly display negative and positive epsilon(Hf)(t) values, respectively, suggesting the variable input of mantle material. Together with the regional geology and previous geochronological data, we propose that the thickening and partial melting in the North Qinling Belt was induced by the collision along the Shangdan suture between the North and South China Blocks from c. 408 to 400 Ma.