Archean-Proterozoic Metamorphic Rocks And Tectonic Evolution Of The Western Langshan Mt., Northern Margin Of North China Craton

Craton, occupied by about 50% of the earth’s land area, refers to the long-term steady tectonic units in the continental crust. It plays an important role in tectonic and environmental evolution of the Earth. The North China Craton (NCC), located in northern north China, is a relatively larger, earlier craton in the Earth. Study on the NCC is important to understanding the early continental crust evolution, current tectonic-environment pattern formation and mineral resources exploration of China.This study focuses on the western Langshan Mt., where the researcher’s attention was poor in the northern margin of the NCC. Based on the anlaysis of metamorphic rocks for petrology, geochemistry and their metamorphism in the Dongshengmiao area, this paper discusses the Archean-Proterozoic tectonic evolution of west section of the north NCC.The metamorphosed strata in the Dongshengmiao area, western Langshan Mt., consists of the Middle Archaeozoic Wulashan Group, Upper Archaeozoic Seertengshan Group, Middle Proterozoic Zhaertaishan Group. The Wulashan Group, a high-grade metamorphosed neutral-basic volcanic rocks and sedimentary rock, comprises Qinggele Formation and Atugui Formation, the former consists of the three subunits of biotite-plagioclase gneiss, hornblende biotite plagioclase gneiss and chorismite, the latter consists of graphite-bearing metamorphic rock. The Zircon U-Pb age of the granite intrused in the gneiss is 2519±14-2563±21Ma. The Seertengshan Group can be divided into quartzite unit, schist unit, marble unit, and green schist assemblage in the upper part was discovered in this study. The Zhaertaishan Group is mainly comprises slab and phyllite.The measurement of lithogeochemistry shows that, the averaged mainly elements of the hornblende biotite plagioclase gneiss of Wulashan Group is consistent with the Chinese mafic rock, with character of sodium-rich, potassium-poor; The biotite plagioclase gneiss is rich in SiO2 (an average of 82.28%)and K2 O (an average of 2.48%) and poor in Na20 (1.41%). The wulashan group is rich in light REE, with ΣREE of 54.63-303.76×10-6(an average of 139.57×10-6),ΣCe/ΣY=4.01-15.97, (La/Yb)N=11.32-56.63, and slightly poor in Eu anomaly; and their trace elements are ten to hundreds of times with comparison to the chondrific, especially rich in incompatible elements Rb, Th, La, Ce, Nd. The albite-actinolit e-schist of Seertengshan group is rich in sodium and poor in potassium. The mica-quartzose schist is rich in sodium and poor in potassium, with the ΣREE of 90.41-178.75× 10-6(an average of 117.99×10-6) and ΣCe/ΣY=0.88-3.06, (La/Yb) N=1.9-9.2,δ Eu =0.73-0.89. The Seertengshan Group is rich in light REE and trace elements (except Ba), more than 10 times relative to the chondritic, particular in the incompatible elements of Rb, Th, La, Ce, Nd.Combination of the remanent fabric and lithogeochemistry features, the original rocks of the hornblende biotite plagioclase gneiss is mafic volcanic rocks formed in the island and the mid-ocean ridge, while the biotite-plagioclase gneiss metamor-phosed from epicontinental clastic rocks. The original rock of the albite-actinolite-schist, the quartz schists and mica schist quartz of the Seertengshan Group are volcanic rocks in the island-arc, oceanic basin clastic rock and carbonate, respectively. The original rock of Zhaertaishan group is passive epicontinental clastic rocks.The rocks of Wulashan Group occupied the metamorphic mineral assemblage of Hb+Bi+Pl+Q, with metamorphic temperature of 515-589℃ and pressure of 0.35-0.72 GPa, which indicates the rocks of Wulashan Group undertook regional Amphibolite facies metamorphism. The rocks of Seertengshan Group has three kinds of metamorphic mineral assemblage:Ms+Bi+Pl+Q, Bi(Hb)+Pl+Q and Act+Ab+Q, with the metamorphic temperature of 593℃ and pressure of 0.85 GPa, which indicates the metamorphism of rocks of Seertengshan Group was regional Epidote-Amphibolite facies. The rocks of the Zhaertai Group undertood low regional Greenschist metamorphism. Meanwhile, the contact metamorphism are found in the rocks of Seertengshan Group and Zhaertaishan Group near the Permian-Triassic granitic intrusions, with metamorphic temperature of 509-559℃ and pressure of 0.72-0.73 GPa, which indicates the contact metamorphic grade reaches to Amphibolite facies. In addition, it is found that the biotite is a common and high idioblastic metamorphic mineral in the western Langshan Mt., but slightly varied in content, grainsize and color. It is lesser content, bigger and in brown in the rocks of Wulanshan group, and larger content, smaller and redbrown in rocks of Seertengshan Group and the least, smallest and green, brown in the rocks of Zhaertaishan Group, all this also indicates the difference of the metamorphic grade.In combination with regional geology, this paper proposes that the Wulashan Group and Seertengshan Group form the basement of west section of the northern margin of NCC. And four geotectonic evolution phases are recongised in this region during Archeozoic-Proterozoic era:1) the envolution of ocean basin during the Middle Archeozoic; 2) the closure of ocean during the early Late Archeozoic; 3) collision and formation of mountain during the late Archeozoic-early Proterozoic, with the amphibolite facies metamorphism of wulashan group; 4) intracontinental rift evolution during the middle Proterozoic.