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）地处佳木斯地块（Jiamusi Block）与松辽地块（Songliao Block）之间的主缝合带，其形成与牡丹江洋（Mudanjiang Ocean）的闭合作用密切相关。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）闭合及古太平洋（Paleo-Pacific Ocean）后续俯冲作用的构造演化具有核心意义。本文对黑龙江杂岩中的石榴角闪岩（garnet amphibolite）、绿帘蓝片岩（epidote blueschist）及石榴云母片岩（garnet mica schist）开展了综合研究，以限定其变质演化与原岩特征。对石榴角闪岩开展的P-T假相图（P-T pseudosection）模拟结果显示，其峰期变质P-T条件为10.5~12.7 kbar、696~728 ℃，指示牡丹江洋俯冲早期玄武洋壳暖俯冲过程对应的中等地热梯度。该石榴角闪岩具有类似N-MORB的地球化学特征，锆石U-Pb定年结果显示其洋壳原岩形成于晚二叠世，年龄约为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P-T条件为13.2~14.7 kbar、488~505 ℃，其地球化学属性与洋岛玄武岩（Ocean Island Basalt，OIB）相似。锆石U-Pb定年结果（263~260 Ma）表明，该类岩石形成于中二叠世牡丹江洋内部的玄武海山环境中。模拟得到的石榴云母片岩峰期P-T条件为13.3~14.6 kbar、487~507 ℃，指示其地热梯度与绿帘蓝片岩类似，属于低地热梯度类型。对黑龙江杂岩中变沉积岩的多硅白云母（phengite）40Ar/39Ar定年结果为187~165 Ma，该年龄记录了牡丹江洋壳俯冲直至佳木斯地块与松辽地块最终碰撞的构造演化过程。综合本次研究结果与前人成果，我们提出了牡丹江洋演化的全新构造框架。黑龙江杂岩是一套独特的造山岩石序列，完整且连续地记录了威尔逊旋回（Wilson cycle）的全过程。牡丹江洋的初始张开，发生于晚石炭世至二叠纪蒙古-鄂霍次克洋（Mongol-Okhotsk Ocean）西向俯冲作用驱动下的佳木斯地块西侧弧后伸展环境中。牡丹江洋的最终闭合及佳木斯地块与松辽地块的最终拼合，发生于晚三叠世至中侏罗世，这一过程由古太平洋板块（Paleo-Pacific Plate）向佳木斯地块下方的西向俯冲挤压作用所驱动。牡丹江洋作为泛大洋（Panthalassa）或古太平洋的分支，存在于早二叠世至中侏罗世，存续时长约为1.16亿年（约288~172 Ma）。