Metamorphic evolution and tectonic implication of the Heilongjiang Complex: Unraveling a complete Wilson cycle, Mudanjiang Ocean, Northeast China

The Heilongjiang Complex in northeast China (NE China), located at the primary suture zone between the Jiamusi and Songliao Blocks, formed by the closure of the Mudanjiang Ocean. Understanding the opening and closing processes of the Mudanjiang Ocean is of fundamental importance deciphering the tectonic evolution of the Paleo-Asian Ocean's closure and the subsequent subduction of the Paleo-Pacific Ocean. In this paper, garnet amphibolite, epidote blueschist and garnet mica schist of the Heilongjiang complex are comprehensively studied in order to constrain their metamorphic evolution, protolith characteristics. P-T pseudosection modelling of the garnet amphibolite reveals peak metamorphic P–T conditions of 10.5–12.7 kbar and 696–728 °C, suggesting an intermediate geothermal gradient associated with the warm subduction of basaltic oceanic crust during the early stages of the Mudanjiang Ocean's subduction. The garnet amphibolite shows a N-MORB-like geochemical signature, with zircon U–Pb ages indicating a late Permian protolith age of the oceanic crust about 255–254 Ma. Subsequent amphibolite-facies metamorphism occurred around 197 Ma. The epidote blueschist records peak P–T conditions of 13.2–14.7 kbar and 488–505 °C with geochemical affinities similar to those of OIB. Zircon U–Pb ages of 263–260 Ma suggest their formation within a basaltic seamount setting within the Mudanjiang Ocean during the middle Permian. The modelled peak P–T conditions of garnet-mica schist are 13.3–14.6 kbar and 487–507 °C, indicating a low geothermal gradient similar to that of the epidote blueschist. The 40Ar/39Ar phengite dating (187–165 Ma) of the meta-sedimentary rocks from the Heilongjiang Complex reflects a tectonic development ranging from the subduction of the Mudanjiang Ocean crust to its involvement in an eventual collision between the Jiamusi and Songliao Blocks. Integrating these findings with previous research, we propose a new tectonic framework for the evolution of the Mudanjiang Ocean. The Heilongjiang Complex represents a distinct orogenic rock sequence that recorded a complete and continuous Wilson cycle. The initial opening of the Mudanjiang Ocean is attributed to a back-arc extensional environment in the western Jiamusi Block, driven by westward subduction of the Mongol-Okhotsk Ocean during the latest Carboniferous to Permian. The eventual closure of the Mudanjiang Ocean and the subsequent amalgamation of the Jiamusi and Songliao Blocks occurred during the late Triassic to middle Jurassic, resulting from the westward subduction and compression of the Paleo-Pacific Plate beneath the Jiamusi Block. The Mudanjiang Ocean existed as a branch of the Panthalassa or Paleo-Pacific Ocean between the early Permian and middle Jurassic, with a lifespan of approximately 116 Myr (ca. 288–172 Ma).

中国东北地区的黑龙江杂岩（Heilongjiang Complex）发育于佳木斯地块与松辽地块之间的主缝合带，系牡丹江洋闭合作用的产物。厘清牡丹江洋的张开与闭合过程，对于解析古亚洲洋（Paleo-Asian Ocean）闭合以及古太平洋（Paleo-Pacific）板块后续俯冲的构造演化均具有关键意义。本文对黑龙江杂岩中的石榴石角闪岩、绿帘蓝片岩及石榴云母片岩开展综合研究，以限定其变质演化与原岩特征。通过石榴石角闪岩的P-T假相模拟，得到其峰期变质温压条件为10.5~12.7千巴、696~728℃，指示牡丹江洋俯冲早期玄武质洋壳暖俯冲过程对应的中等地热梯度。该石榴石角闪岩具有类正常洋中脊玄武岩（N-MORB）的地球化学特征，锆石U-Pb定年显示其洋壳原岩形成于晚二叠世，年龄为255~254 Ma。后续的角闪岩相变质作用发生于约197 Ma。绿帘蓝片岩的峰期变质温压条件为13.2~14.7千巴、488~505℃，其地球化学属性与洋岛玄武岩（OIB）相似。锆石U-Pb年龄为263~260 Ma，指示其形成于中二叠世牡丹江洋内的玄武质海山环境。石榴云母片岩的模拟峰期温压条件为13.3~14.6千巴、487~507℃，指示其地热梯度较低，与绿帘蓝片岩的地热梯度相近。对黑龙江杂岩中变沉积岩开展的多硅白云母40Ar/39Ar定年结果为187~165 Ma，该年龄记录了从牡丹江洋壳俯冲直至其参与佳木斯地块与松辽地块最终碰撞的构造演化过程。结合本次研究结果与前人研究成果，我们提出了牡丹江洋演化的全新构造框架。黑龙江杂岩代表一套独特的造山岩系，记录了完整且连续的威尔逊旋回（Wilson cycle）。牡丹江洋的初始张开归因于佳木斯地块西部的弧后伸展环境，该环境由晚石炭世末期至二叠纪期间蒙古-鄂霍次克洋（Mongol-Okhotsk Ocean）的西向俯冲作用所驱动。牡丹江洋的最终闭合以及佳木斯地块与松辽地块的后续拼合发生于晚三叠世至中侏罗世，该过程由古太平洋（Paleo-Pacific）板块向佳木斯地块下方的西向俯冲与挤压作用所导致。牡丹江洋作为泛大洋（Panthalassa）或古太平洋的分支，存续于早二叠世至中侏罗世，其寿命约为1.16亿年（约288~172 Ma）。