U-Pb zircon ages and geochemistry of the Wuguan complex in the Qinling orogen, central China: Implications for the late Paleozoic tectonic evolution between the Sino-Korean and Yangtze cratons

摘要

The tectonic evolution of the Qinling orogen, central China, is the key to understanding the assembly of the Sino-Korean and Yangtze cratons. The Wuguan complex, between the early Paleozoic North Qinling and Mesozoic South Qinling tectonic belts, can provide important constraints on the late Paleozoic evolutionary processes in the Qinling orogen. U-Pb zircon analyses, using laser ablation-multicollector-inductively coupled plasma-mass spectrometry, reveal protolith ages of 446 ± 2 Ma for a garnet amphibolite, 368 ± 3 Ma for a meta-andesite, and 351 ± 2 Ma for a mylonitized granitic dike from the Wuguan complex. Elemental geochemistry indicates typical island arc affinities for all the above rocks, but some amphibolites of unknown age have E-MORB signatures. Detrital zircons from a metaquartzite have an age spectrum with a major peak at 462 Ma, two subordinate peals at 828 and 446 Ma, and a youngest weighted mean age of 423 ± 5 Ma This suggests that at least some of metasedimentary rocks from the Wuguan complex belong to the part of the Devonian turbidite sequence of the Liuling Group, which was deposited in a fore-arc basin along the southern accreted margin of the Sino-Korean craton, whereas the late Ordovician precursors of the amphibolite might be derived from the Danfeng Group. The occurrence of late Devonian-early Carboniferous arc-related rocks in the Wuguan complex implies penecontemporaneous oceanic subduction, and therefore the Paleo-Qinling Ocean was not finally closed until the early Carboniferous. On the other hand, metamorphic zircon grains from two amphibolites yielded ages of 321 ± 2 and 318 ± 3 Ma. Hence, the Wuguan complex in the Qinling orogen and the Guishan complex in the Tongbai orogen constitute a medium-pressure Carboniferous metamorphic belt that is more than 500 km long, and which was formed in the hanging wall of a subduction zone.