Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite norm...Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite normalized REE patterns. Rocks in Group #1 are characterized by nearly flat REE patterns （Lan/Ybn=0.86-1.3）, the lowest total REEs （29-52 ppm）, and weak negative to positive Eu anomalies （Eun/Eun=0.84-1.02）, nearly flat primitive mantle normalized patterns and strong negative Zr（Hf） anomalies. Their geochemical characteristics in REEs and trace elements are similar to those of ocean plateau tholeiite, which imply that this group of rocks can represent remnants of Archean oceanic crust derived from a mantle plume. Rocks in Group #2 are characterized by moderate total REEs （34-116 ppm）, LREE-enriched （Lan/Ybn=1.76-4.34） chondrite normalized REE patterns with weak Eu anomalies （Eun/Eun=0.76-1.16）, and negative Nb, Ta, Zr（Hf）, Ti anomalies in the primitive mantle normalized spider diagram. The REE and trace element characteristics indicate that they represent arc magmas originating from a sub-arc mantle wedge metasomatized by slab-derived fluids. Rocks in Group #3 are characterized by the highest total REEs （61-192 ppm）, the strongest LREEs enrichment （Lan/Ybn=7.12-16） with slightly negative Eu anomalies （Eun/Eun=0.81-0.95） in the chondrite normalized diagram. In the primitive mantle normalized diagram, these rocks are characterized by large negative anomalies in Nb, Ta, Ti, negative to no Zr anomalies. They represent arc magmas originating from a sub-arc mantle wedge enriched in slab-derived melts. The three groups of rocks imply that the formation of the Neoarchean Wutai Complex is related to mantle plumes and island-arc interaction.展开更多
The Wutai Complex associated with the adjacent Fuping and Hengshan Complexes represents the best and dassical cross-section in the middle segment of the Trans-North China Orogen, generally divided into Eastern and Wes...The Wutai Complex associated with the adjacent Fuping and Hengshan Complexes represents the best and dassical cross-section in the middle segment of the Trans-North China Orogen, generally divided into Eastern and Western Blocks. Unconformably overlying the Wutai and Fuping Complexes is the Hutuo Group considered as the youngest lithostratigraphic unit in the region and important both for interpreting Precambrian history as well as the overall evolution of the Trans-North China Orogen. Lack of knowledge about provenance of the sedimentary rocks in this group has hindered understanding of the depositional environments and tectonic significance. LA-ICP-MS was applied to obtain U-Pb zircon ages for the granitic pebbles, the lowest lithostratigraphic rock of the Hutuo Group, which, combined with previous lithostratigraphic, geochronological, structural and metamorphic data, provides new constraints on the sedimentary provenance and tectonic evolution of the region. The sequence of the Hutuo Group ranges upward from lower basal conglomerates and volcaniclastic rocks （Doucun Subgroup）, through clastic sediments, slates, dolomites and marbles （Dongye Subgroup）, to sandstones and conglomerates at the top （Guojiazhai Subgroup）. Zircons from granitic pebbles preserved in the Doucun Subgroup basal conglomerates give weighted mean ^207pb/^206pb ages between 2517 Ma and 2566 Ma, which are the same as those for the late Archean Wutai Granitoids, indicating that the pebbles were derived from the Wutai granitic intrusions （2566-2515 Ma）. Based on the new data and previous studies, combined with an igneous zircon crystallization age of 2087±9 Ma obtained for volcanics in the Hutuo Group, the Hutuo Group was deposited in a subduction-related retro-arc foreland basin environment that developed behind the Wutai arc during the eastward-directed subduction of the Western Block beneath the western margin of the Eastern Block. This basin underwent long-lived sedimentation and finally closed during the -1850 Ma collisional event that resulted in the final amalgamation of the North China Craton.展开更多
基金The National Natural Science Foundation of. China （Grant No.40420120135 and 40472096） are thankefl for the fthancial support.
文摘Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite normalized REE patterns. Rocks in Group #1 are characterized by nearly flat REE patterns （Lan/Ybn=0.86-1.3）, the lowest total REEs （29-52 ppm）, and weak negative to positive Eu anomalies （Eun/Eun=0.84-1.02）, nearly flat primitive mantle normalized patterns and strong negative Zr（Hf） anomalies. Their geochemical characteristics in REEs and trace elements are similar to those of ocean plateau tholeiite, which imply that this group of rocks can represent remnants of Archean oceanic crust derived from a mantle plume. Rocks in Group #2 are characterized by moderate total REEs （34-116 ppm）, LREE-enriched （Lan/Ybn=1.76-4.34） chondrite normalized REE patterns with weak Eu anomalies （Eun/Eun=0.76-1.16）, and negative Nb, Ta, Zr（Hf）, Ti anomalies in the primitive mantle normalized spider diagram. The REE and trace element characteristics indicate that they represent arc magmas originating from a sub-arc mantle wedge metasomatized by slab-derived fluids. Rocks in Group #3 are characterized by the highest total REEs （61-192 ppm）, the strongest LREEs enrichment （Lan/Ybn=7.12-16） with slightly negative Eu anomalies （Eun/Eun=0.81-0.95） in the chondrite normalized diagram. In the primitive mantle normalized diagram, these rocks are characterized by large negative anomalies in Nb, Ta, Ti, negative to no Zr anomalies. They represent arc magmas originating from a sub-arc mantle wedge enriched in slab-derived melts. The three groups of rocks imply that the formation of the Neoarchean Wutai Complex is related to mantle plumes and island-arc interaction.
基金This research was financially supported by the National Natural Science Foundation of China （Grants 40429001, 40420120135 and 40472098）Hong Kong RGC （Grants 7055/05P, 7058/04P, 7055/03P and 7048/03P）.
文摘The Wutai Complex associated with the adjacent Fuping and Hengshan Complexes represents the best and dassical cross-section in the middle segment of the Trans-North China Orogen, generally divided into Eastern and Western Blocks. Unconformably overlying the Wutai and Fuping Complexes is the Hutuo Group considered as the youngest lithostratigraphic unit in the region and important both for interpreting Precambrian history as well as the overall evolution of the Trans-North China Orogen. Lack of knowledge about provenance of the sedimentary rocks in this group has hindered understanding of the depositional environments and tectonic significance. LA-ICP-MS was applied to obtain U-Pb zircon ages for the granitic pebbles, the lowest lithostratigraphic rock of the Hutuo Group, which, combined with previous lithostratigraphic, geochronological, structural and metamorphic data, provides new constraints on the sedimentary provenance and tectonic evolution of the region. The sequence of the Hutuo Group ranges upward from lower basal conglomerates and volcaniclastic rocks （Doucun Subgroup）, through clastic sediments, slates, dolomites and marbles （Dongye Subgroup）, to sandstones and conglomerates at the top （Guojiazhai Subgroup）. Zircons from granitic pebbles preserved in the Doucun Subgroup basal conglomerates give weighted mean ^207pb/^206pb ages between 2517 Ma and 2566 Ma, which are the same as those for the late Archean Wutai Granitoids, indicating that the pebbles were derived from the Wutai granitic intrusions （2566-2515 Ma）. Based on the new data and previous studies, combined with an igneous zircon crystallization age of 2087±9 Ma obtained for volcanics in the Hutuo Group, the Hutuo Group was deposited in a subduction-related retro-arc foreland basin environment that developed behind the Wutai arc during the eastward-directed subduction of the Western Block beneath the western margin of the Eastern Block. This basin underwent long-lived sedimentation and finally closed during the -1850 Ma collisional event that resulted in the final amalgamation of the North China Craton.
