LA-ICPMS Zircon U-Pb Dating Of The Heilongjiang Complex In The Luobei Area:New Constrains For The Tectonic Evolution Of Jiamusi Block,NE China

The Heilongjiang Complex is a sequence of high-pressure(HP)metamorphic rocks that are distributed along the N-S suture zone between the Jiamusi and Songliao blocks in NE China,recording the entire process of subduction and collision between the Jiamusi and Songliao blocks.In this study,we select the Heilongjiang Complex in Luobei area as the research target.We investigated the rock types,protolith and metamorphic age of the Heilongjiang Complex in Luobei area by the methods of field geological investigation,petrology and zircon U-Pb dating.These results,together with abundant Permian arc-affinity igneous rocks in the eastern margin of the Jiamusi Block and Mesozoic accretionary complex along the eastern Asian continental margin,can reveal the formation mechanism of the Heilongjiang Complex and the tectonic evolution history of the Jiamusi Block.The Heilongjiang Complex at Luobei is characterized by the occurrence of serpentinite,blueschist,greenschist,amphibolite,and marble that form lenses or interlayers in the matrix of two-mica schist,muscovite-albite schist,quartz schist and granite gneiss.In this study,we select six typical samples for zircon U-Pb dating.LA-ICP-MS zircon U-Pb dating of all samples shows a range of protolith ages from 435 to 210 Ma.These datas,together with a 492 ± 6 Ma age from a granite gneiss sample in the Yilan area,indicate that the protoliths of the Heilongjiang Complex were fromed over a protracted time-span from ~490 to ~210 Ma.In addition,zircon grains contain metamorphic rims in biotite-plagioclase gneiss(PXMS-19)and amphibolite(PXMS-27)that yield ages of 187 ± 3 Ma and 185 ± 2 Ma,respectively.These results,when combined with field evidence that the Heilongjiang Complex is overlain by Late Jurassic strata,indicate that the Heilongjiang Complex was metamorphosed between ~210 and ~185 Ma,and not in the Cretaceous as previous considered.We found that the Jiamusi Block was connected to the Central Asian Orogenic Belt(CAOB)before ~350 Ma by comparing the detrital zircon ages from the Heilongjiang Complex with CAOB.Then,the Jiamusi Block separated from the CAOB,and reunited with it between late Triassic and early Jurassic.Based on abundant Early-Middle Permian arc-affinity igneous rocks,we consider that the separation between the Jiamusi Block and CAOB was the result of back-arc extension setting.The Late Triassic to Cretaceous accretionary complex related to the subduction of the Paleo-Pacific plate are widely distributed along the eastern Asian continental margin,this evidence indicate that this area has been dominated by Paleo-Pacific tectonic regime since the Late Triassic.Therefore,the collision between the Jiamusi and Songliao blocks from Late Triassic to Early Jurassic was most likely driven by westward subduction of the Paleo-Pacific plate.