The Timing Of Final Closure Along The Changchun-Yanji Suture Zone: Constraints From Detrital Zircon U-Pb Dating Of The Triassic Dajianggang Molasse Formation, NE China

The northeast China located among the North China Craton, the Siberia Craton and the Pacific plate. As this region was not only influenced by the collision of Paleo-Asian Ocean during the Paleozoic, but also influenced by the subduction of Pacific plate along the eastern margin of Asian Continent. Jiamusi-Khanka block is located in the eastern NE China and adjacent with Russian Far-East and North Korea that an ideal area for geologists to study the evolution and transformation of Paleo-Asian Ocean and Paleo-Pacific tectonic domains. In this paper, we focus on the provenance characteristics and U-Pb detrital ages of the Dajianggang Formation, combined with the current age data and Paleontology data to discuss the final collision of the Jiamusi-Khanka Block and the North China Craton. The Dajianggang Formation is located in the Changchun-Yanji suture zone of central Jilin Province and unconformably overlies the Changchun-Yanji Accretionary Complex, which is a mélange resulting from subduction of the Jiamusi-Khanka Block beneath the North China Craton. The samples were collected from the bottom part of the standard section- “the Xiaoshipeng-Paodingzi section”. As shown by cathodoluminescence(CL) images, the detrital zircons are mostly the stubby to elongate in shape with magmatogenic oscillatory zonings. Their chondrite-normalized patterns mostly share similar characteristics, with enriched LREE and flat HREE and the higher Th/U ratios, indicating the magmatic origin as well. LA-MC-ICP-MS U-Pb dating of detrital zircon from four samples of the formation yields ages of 2516 to 216 Ma. Zircons with U-Pb ages at 2516-2501 Ma and 1897-1832 Ma indicate a provenance from Precambrian basement rocks of the North China Craton. The 525-482 Ma ages indicate a provenance from metamorphic rocks of Late Pan-African age in the JKB that have a tectonic affinity to the Central Asian Orogenic Belt(CAOB). Zircon grains with ages of 383-314 Ma and 275-250 Ma were likely derived from the underlying Changchun-Yanji Accretionary Complex. The youngest population has a peak age of ca. 225 Ma, which together with Late Triassic fossils, suggests that deposition of the Dajianggang Formation was Late Triassic or younger. This result supports the view that the final collision of the JKB and NCC along the Changchun-Yanji suture took place before the Late Triassic. Furthermore, this closure time is at least 10-20 Ma later than closure along the Solonker-Xar Moron-Changchun suture in the Late Permian. We thus establish that the Changchun-Yanji suture is not related to the collision between the Siberia Craton and the North China Craton but was instead related to Paleo-Pacific plate subduction. Consequently, the Changchun-Yanji suture is not the eastward extension of the Solonker-Xar Moron-Changchun suture as previously considered, but the southern margin of the Jilin-Heilongjiang high-pressure metamorphic belt(Ji-Hei HP belt), and resulted from westward subduction of the Paleo-Pacific Ocean. Thus, the Late Triassic marked the switch in subduction from the Paleo-Asian Ocean to the Paleo-Pacific Ocean in NE China.