EARLY PALEOZOIC ISLAND-ARC ACCRETION TO THE NORTH CHINA CRATON AND THE SHANG-DAN FAULT ZONE - A MAJOR PALEOPLATE BOUNDARY IN EASTERN ASIA

The early Paleozoic tectonic evolution of part of the Qinling-Dabie-Sh andong orogenic belt is reconstructed on the basis of the results of f ield mapping, geochemical, and geochronological studies. The Qinling o rogenic belt separates the Archean-Proterozoic North China craton (NCC ) from the South China craton (SCC, also Yangtze craton). The study ar ea is located about 90 km southwest of the city of Xi'an, Three lithol ogically distinctive zones, separated from each other by fault zones, can be distinguished. Passive continental margin deposits of the NCC ( zone 1) have epsilon Nd-(t) values between -6.16 and -12.54 and T(DM)a ges between 1.29 and 2.20 Ga. They are separated from an early Paleozo ic island arc sequence (zone 2) by the sinistral Chenche fault. Mafic volcanic lithologies in the southern part of zone 1 are associated wit h the development of a passive continental margin in late Proterozoic time, Similiar lithologies of zone 2 have geochemical characteristics of an oceanic island are. Accretion was accompanied by a strong deform ation event between early Ordovician and late Silurian time. Magmatic intrusions in zone 2 and the southern part of zone 1 are associated wi th a short-lived magmatic are that developed above a north dipping sub duction zone on the modified continental margin of the NCC in late Sil urian to early Devonian time. The epsilon Nd-(t) values of felsic intr usions from the northern part of zone 2 decrease in a northern directi on from 1.78 to -1.05, Mafic to intermediate intrusions in the souther n part of zone 2 have positive eNd((t)) values from 1.28 to 2.04 as we ll as low Sr-87/Sr-86((i)) values between 0.705 and 0.706 indicating a n increasing mantle component toward the south. Low-grade metamorphic pelites are separated from the island are sequence of zone 2 by a less than or equal to 2 km wide Shang Dan shear zone. Isotopic studies sho w strong similarities between a granite sample from zone 3 and samples from the SCC. Sr-87/Sr-86((i)) values increase from 0.704 to 0.713, w hereas epsilon Nd-(t) values decrease from -1.90 to -5.34 in a souther n direction. The shear zone that separates zone 2 from 3 is of fundame ntal importance and marks the boundary between the NCC and the SCC. Th e results presented in this paper document a previously unknown early Paleozoic tectonic evolution of the Qinling orogenic belt and delineat e one of the fundamental paleoplate boundaries in eastern Asia.