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Discovery of ∼1.37 Ga Granite in the Eastern Part of the Northern Margin of the North China Craton and its Geological Significance
Acta Geologica Sinica (English Edition)
... Recently, 1.38-1.32 Ga granitoids were reported at this area Wen et al., 2020). In this paper, we carried out detailed field mapping, geochronological and geochemical study on the Chenjiatun metamorphic complex in eastern Bainaimiao micro-block. ...
Identifying tectonic positions and origins of Precambrian micro-blocks within the Central Asian Orogenic Belt (CAOB) is essential for revealing global paleogeography and evolution of the early Earth. The Bainaimiao micro-block in the southeastern CAOB preserves multistage geologic records which can better reveal Precambrian tectonic history of micro-blocks within the Phanerozoic accretionary orogenic belt. This study presents integrated research of geochronology, geochemistry, and zircon Hf isotopes on the Chenjiatun metamorphic complex in the eastern Bainaimiao micro-block, which consists of meta-sedimentary rocks (plagioclase gneisses and mica schists), amphibolites and two-mica granites. The protoliths of amphibolites, two-mica granites and meta-sedimentary rocks were formed at 1.75 Ga, 1.38 Ga and 1.39–1.34 Ga, respectively, suggesting that they represent Paleo-Mesoproterozoic basement of the Bainaimiao micro-block. The detrital zircon age spectra of meta-sedimentary rocks show two representative age peaks at 1.80 Ga and 1.40 Ga, with zircon Hf isotopes varying from −11.11 to +10.67. Characteristic mostly positive zircon Hf isotopes and geochemical features of the Chenjiatun complex indicates that the Bainaimiao micro-block probably has tectonic affinity with the Laurentia-Baltica continent. Paleoproterozoic amphibolites with positive εHf(t) (+5.67 to +15.42) were derived from metasomatized mantle wedge mixed with sediment melts. They are characterized by arc-like geochemistry such as high Th/Yb (0.73–2.77), enrichment in LILEs and depletion in HFSEs (eg. Nb and Ta), similar to coeval arc magmatism in the Laurentia-Baltica continent, which indicate the Bainaimiao micro-block was possibly in a subduction convergent setting in the late Paleoproterozoic. By contrast, Mesoproterozoic two-mica granites, with mostly positive εHf(t) (−0.29 to +11.53), belong to typical S-type granites and could be generated from partial melting of meta-pelites with participation peritectic garnets. These two-mica granites, together with ∼1.40 Ga juvenile A-type grantioids from nearby and other Precambrian micro-blocks in the southern CAOB, exhibit relatively high alkalis and HFSEs contents, young Hf model ages (1.50–1.40 Ga) and were generated at high-temperature environment. These findings reveals that the Bainaimiao micro-block was in an extensional setting at 1.40 Ga coeval with the breakup evolution of the Columbia supercontinent.
The study area is in the Northeastern Sichuan Basin. Regional geology has been well established with tremendous geological studies, which indicating the paleogeography in this area is characterized by shallow water carbonate platforms skirting with marine troughs. Detailed geological characterization at reservoir level varies from field to field however, due to data availability and level of research depth.
A map of major Precambrian mafic dyke swarms and related units in the North China Craton is compiled, and the features and geological implications of these swarms are demonstrated. The Archean dyke swarms are available to portray the early crustal growth and cratonization. The middle Paleoproterozoic (2200-1850 Ma) swarms and related magmatic series could constrain the tectonic evolution: They approve that the craton was amalgamated by two sub-cratons. The late Paleoproterozoic (1800-1600 Ma), Mesoproterozoic (1400-1200 Ma) and Neoproterozoic (1000-800 Ma) series swarms are important in paleogeographic reconstruction: they indicate that North China might have connected with some of the North European and North American cratons during Proterozoic. Dyke swarms are not only geological timescales and tectonic markers but also evolution indicators of lithospheric mantle: they imply a rejuvenation of the sub-continental lithospheric mantle of North China at 1780-1730 Ma. These swarms occurred with several rifts, including the Hengling (2200-1970 Ma), Xuwujia (1970-1880 Ma), Xiong’er (1800-1600 Ma), Yan-Liao (1730-1200 Ma), and Xu-Huai (1000-800 Ma). Among them, the Xuwujia rift was possibly continental arc associated; whereas the others were intra-continental. In addition, the Xiong’er and Xu-Huai rifts were possibly triple junctions along the present southern and southeastern margins of the Craton, respectively. Different tectonic settings of these rifts and dyke swarms would result in diversified series of ore deposits.
Large volumes of diabase sills occur within the late Paleoproterozoic to Mesoproterozoic sedimentary rocks in the northern part of the North China Craton (NCC) and previous geochronological studies suggest some of these were emplaced in the Mid-Mesoproterozoic (Ectasian). Here we present new zircon and baddeleyite SHRIMP, LA-ICP-MS and SIMS U–Pb analyses on representative samples of the diabase sills emplaced into the Wumishan, Tieling and Xiamaling Formations. The results show emplacement ages ranging from 1325±5Ma to 1316±37Ma, suggesting voluminous magmatism leading to the formation of the diabase sill swarms (the Yanliao mafic sill swarms) in the Mid-Mesoproterozoic. Zircon LA-ICP-MS U–Pb dating of the Shangdu-Huade granitic pluton in the northern NCC yielded similar emplacement ages ranging from 1331±11Ma to 1313±17Ma. These ages are comparable with the 1.4–1.2Ga rift-related anorogenic magmatism recorded elsewhere on the globe such as North America, Greenland, Baltica, and Siberia, associated with the breakup of the Columbia (Nuna) supercontinent. The Mid-Mesoproterozoic magmatic rocks in the northern NCC constitute a typical bimodal magmatic suite. The geochemical and Nd–Hf isotopic data suggest that the diabasic rocks were likely generated by partial melting of the depleted asthenosphere mantle coupled with slight crustal assimilation in a continental rift setting; however, the granitic rocks were generated mainly through partial melting of the ancient continental crust, probably induced by the upwelling of hot asthenosphere mantle during continent rifting processes. The recognition of these bimodal magmatic rocks indicates that the northern NCC experienced Mid-Mesoproterozoic rifting events that are considered to have led to the final breakup of the Columbia supercontinent. The breakup of the NCC from the Columbia supercontinent occurred during Mid-Mesoproterozoic time, probably at around 1.33Ga. The carbonatite rocks and rare earth element-niobium mineralization in the giant Bayan Obo ore deposit in the northern NCC were also probably related to the continental rifting event in the Mid-Mesoproterozoic.
Petrogenesis of the Mesoproterozoic Gneissic Granite in the Sonid Left Banner area, Inner Mongolia, and Its Tectonic Implications
- Sun Lixin
- Ren Baofang
- Wang Shuqing
- Xu Xinying
- Zhang Yun
Petrogenesis of the Mesoproterozoic Gneissic Granite in the Sonid Left Banner area, Inner Mongolia, and Its Tectonic Implications
- Sun L.X.