Tectonic affinity of the west Qinling terrane (central China): North China or Yangtze?

摘要

Our ignorance about the tectonic affinity of the western Qinling-Songpan-Ganzi tectonic region, which is strategically located between the northeastern corner of the Tibetan Plateau, the northwestern corner of the Yangtze block, and the southwestern corner of the north China block, limits our understanding of the tectonic evolution of east Asia. Basaltic volcanic rocks in the Duofutun area within the west Qinling terrane in Qinghai Province (China), the northernmost part of the Songpan-Ganzi region, contain coeval magmatic zircons that constrain the eruption age of the host basalts to ～14 Ma. More significantly, the basalts have entrained zircon xenocrysts from the deep crust that record the presence of unexposed Neoarchean (2.7-2.5 Ga) basement. U-Pb and Hf isotope data from the xenocrysts reveal that this basement has undergone a complex evolution that includes the addition of new mantle-derived material at ～2.7-2.4 and 1.1-0.8 Ga and crustal reworking events at ～1.8 and 1.4 Ga. Phanerozoic thermal events at 320-300, 230, and 160 Ma have also modified (reworked) the basement. Using these data, we interpret at least the western part of the west Qinling orogenic terrane as a microcontinental block that originally separated from the north China block, closed with the northern Yangtze block during the Meso-Neoproterozoic, and then redocked with the southern part of the north China block in the Phanerozoic (i.e., early Paleozoic). The west Qinling terrane was then affected by the northward subduction and collision of the Yangtze block in the Paleozoic and early Mesozoic and underwent litho-spheric extension in Jurassic time.