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Early Paleozoic Island arc accretion to the North China craton and the Shang Dan fault zone: A major paleoplate boundary in eastern Asia

Abstract:

The early Paleozoic tectonic evolution of part of the Qinling-Dabie-Shandong orogenic belt is reconstructed on the basis of the results of field mapping, geochemical, and geochronological studies. The Qinling orogenic belt separates the Archean-Proterozoic North China craton (NCC) from the South China craton (SCC, also Yangtze craton). The study area is located about 90 km southwest of the city of Xi'an. Three lithologically distinctive zones, separated from each other by fault zones, can be distinguished. Passive continental margin deposits of the NCC (zone 1) have εNd(t) values between -6.16 and -12.54 and TDMages between 1.29 and 2.20 Ga. They are separated from an early Paleozoic island arc sequence (zone 2) by the sinistral Chenche fault. Mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late Proterozoic time. Similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc. Accretion was accompanied by a strong deformation event between early Ordovician and late Silurian time. Magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short-lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the NCC in late Silurian to early Devonian time. The εNd(t) values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1.78 to -1.05. Mafic to intermediate intrusions in the southern part of zone 2 have positive eNd(t) values from 1.28 to 2.04 as well as low 87Sr/86Sr(i) values between 0.705 and 0.706 indicating an increasing mantle component toward the south. Low-grade metamorphic pelites are separated from the island arc sequence of zone 2 by a ≤ 2 km wide Shang Dan shear zone. Isotopic studies show strong similarities between a granite sample from zone 3 and samples from the SCC. 87Sr/86Sr(i) values increase from 0.709 to 0.713, whereas εNd(t) values decrease from -1.90 to -5.34 in a southern direction. The shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the NCC and the SCC. The results presented in this paper document a previously unknown early Paleozoic tectonic evolution of the Qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern Asia.