Early Mesozoic accretionary complex scattered throughout NE China and its tectonic implications: Insights from the mylonitic meta-sedimentary rocks in Huanan, Heilongjiang Province

Abstract

In NE Asia, the early stage (T-J) subduction history of the Paleo-Pacific oceanic plate was largely reconstructed by studies on the Heilongjiang complex, a blueschist-containing N-S trending high-pressure metamorphic belt, which is suggested to represent the “suture zone” between the Jiamusi Block and the rest of the Central Asian Orogenic Belt. However, recently more accretionary complexes with similar geochronological data were reported on both sides of the “suture zone”, making the tectonic affinities of the “suture zone” unclear. Here, we select the mylonitic meta-sedimentary rocks in Huanan, one of the potential accretionary complexes near the central Jiamusi Block to reveal its depositional and metamorphic history and discuss the tectonic implications. Our results show that these rocks were deposited in/after the Late Triassic (defined by the youngest detrital zircon group) and accepted clastics from both the Jiamusi and Songliao blocks. Before the Early Jurassic (c. 178 Ma by apatite UPb dating results in this study), they experienced high-P/low-T metamorphism (510–520 °C and 10.5–11 kbar), and then were heated to about 610 °C at 8.5–9.2 kbar during exhumation. The meta-sedimentary rocks in Huanan are components of the early Mesozoic accretionary complex in NE China. However, their present location is c. 85 km to the east of the previous proposed “suture zone” from Luobei to Mudanjiang. Considering the spatially scattering distribution of other similar accretionary complexes, such as the Zhangguangcai complex (c. 40 km to the west of the “suture zone”), we suggest that these complexes dispersedly exposed throughout NE China were either nappes transported by post-exhumation thrusting, or remnants of archipelagic oceanic crusts in the NW Paleo-Pacific region during the early Mesozoic.