Piemontite-bearing Chert Complex In Central Jilin:Characterizations And Tectonic Implications

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 ages of the piemontite-bearing chert, combined with the metamorphic p-t condition and Paleontology data to discuss the final collision of the Jiamusi-Khanka Block and the North China Craton. Through the field geological survey, the intense deformation of the rocks and resulting destruction of the original sedimentary sequence indicate that most of the Paleozoic strata in theis region might be tectonic mélange without any significance for the traditional stratigraphic division and correlation. The typical metamorphic mineral assemblages : Quartz(75%-80%)+Muscovite(10%)+ Piedmontite(<10%)+Garnet(<5%)+ Feldspar(<5%). Which muscovite can indicate the rock pressure environment, the electron microprobe analyzes show that the muscovite is Phengite, it should be the product of high-pressure metamorphism. The white mica can indicate the rock pressure environment, electron probe analysis shows that the white mica Si≥3.305(based on the oxygen = 11), which white mica as silica white mica, as a product of high pressure metamorphism, to prove that the experience of high pressure reaction. The piedmontite schist LA- ICP-MS U-Pb datas result shows that the minimum age is for the 233 Ma, according to the protolith age is formed on indo-chinese epoch; The maximum age is for the 1800 Ma, the existence of the ancient basement of information for the northeast region. It needs to be pointed out that the peak age at 233 Ma limited proof of the high-pressure metamorphic belt formed on indo-chinese epoch, is the Triassic, not previously considered Permian or older. Otherwise, the weighted average of the felsic vein age is 217±3 Ma(MSWD=0.37), according to this age, we can limit the age of Changchun-Yanji suture. 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.