A late Neoproterozoic to early Paleozoic accretionary orogenic belt in the eastern Central Asian orogenic Belt: Evidence from the Irshi ophiolite m<acute accent>elange in the middle segment of the Great Xing'an Range

The tectonic affinity of the middle segment of the Great Xing'an Range (GXR) in NE China has become one of the focus issues about the Paleozoic crustal division and tectonic evolution of the eastern segment of the Central Asian Orogenic Belt. In this paper, we identified some ophiolite fragments from the early Paleozoic strata in the Irshi area of the middle segment of the GXR, which was previously thought to be the interior of the Precambrian Xing'an block. These ophiolite fragments and the surrounding deformed clastic rocks together constitute the Irshi ophiolite me<acute accent>lange (IOM). Zircon U-Pb dating reveals that the ophiolites formed during 603-608 Ma, which indicates that there was an oceanic basin during the late Neoproterozoic. The geochemical composition suggests that the ophiolites were formed in a back-arc setting. The youngest group of detrital zircons in the matrix ranges from 435 Ma to 480 Ma. In addition, the IOM was covered unconformably by late Silurian and Devonian strata, and it was thus likely emplaced during the early-middle Silurian. The identification of the IOM eliminated the possibility that the middle segment of the GXR was an independent Precambrian continental block. The spatial-temporal distribution of Proterozoic-early Paleozoic ophiolite, metamorphic complexes, and magmatic and sedimentary rocks in the middle segments of the GXR indicates that the tectonic affinity of the late Neoproterozoic-early Paleozoic crust in this area is an accretionary orogenic belt. The continuous subduction of the Paleo-Asian Ocean was responsible for the formation of the orogenic belt.