Geochemistry And Tectonic Implications Of The Late Palaeozoic Mafic-ultramafic Rocks Belt On The Middle Segment Of The Northern Margin Of The North China Plate

A mafic-ultramafic rocks belt is discontinuously exposed from Wulatezhongqi, Damaoqi to Bayan Obo Region, Inner Mongolia, NEE extending over 200km long and 10km wide. The belt belongs to the mid-segment of the northern margin of the North China Plate and it is 100km away from Solonker suture, so it is important and representative in the tectonic evolution of the northern margin of the North China Plate. According to SHRIMP U-Pb zircon age and K-Ar age, these rocks formed in the Permian, so the belt can reflect the tectonic history of the North China Plate and the Siberia Plate during the Permian.Two types of cumulate gabbros are exposed in Wulatezhongqi area, including amphibole gabbros and olive gabbros. The major petrogenetic minerals in the Wengeng gabbros are olive, pyroxene, plagioclase and hornblend. The amphibole gabbros show geochemical characteristics of enriched in LILE(K,Rb,Sr,Ba), and depleted in HFSE (Nb,Ta,Zr). The geochemistry of the olive gabbros is characterized by low SiO2 and high MgO contents, enriched in LILE(K,Rb,Sr,Ba) and LREE, and unfractionation in HFSE. According to the diagrams of AlZ-TiO2, Wo-En-Fs and An-Mg′, the amphibole gabbros are suggested to be generated at an island arc and the olive gabbros are formed in a back-arc basin setting.Huheengeer Complex shows a concentric zone in rock distribution: hornblende-peridotite, hornblende-gabbro and diorite from the core to margin successively in Bayan Obo Region, Inner Mongolia. The Complex also shows geochemical characteristics of enriched in LILE(K,Rb,Sr,Ba), and depleted in HFSE (Nb,Ta,Zr,Hf). According to the diagrams of AlZ-TiO2, An-Mg′, Hf/3-Th-Ta and Hf/3-Th-Nb/16, the Huheengeer Complex is suggested to be generated at an active continental margin, and it should be a record of Paleoasian ocean plate subduction to northern side of north China plate.An ophiolitic melange belt is discontinuously exposed in Harihada- Chegendalai area to the south of Tuilamamiao in NE of Damaoqi. The ophiolitic mélange in the belt are mainly composed of metamorphic peridotites, metamorphic gabbros, metamorphic basic lavas and high-Mg diorites. The metamorphic peridotites display SiO2 (39.08%~40.45%),TiO2 (0.02%~0.03%), Mg′(87.10~90.33). The metamorphic gabbros show geochemical characteristics of enriched in LILE(K,Rb,Sr,Ba), and depleted in HFSE (Nb,Ta,Zr). Furthermore, there are some high-Mg diorites exposed. In summary, the Daomaoqi ophiolitic mélange formed in a supra-subduction zone (SSZ).Combining with contrast of mineral geochemistry, element geochemistry, geochronology and tectonic setting, it shows clearly that the mafic-ultramafic rocks belt be generated at an island arc or active continental margin in Early Permian, and it should be a record of Paleoasian ocean plate subduction to northern side of north China plate.