The Composition And Evolution Of The Sushui Complex In The Zhongtiao Mountains, The South Of North China Craton

The Zhongtiao Mountains of Shanxi Province is located in the south of North China Craton, and it’s one of the important component of the early Precambrian basement in China. The early Precambrian rock is complete in this region, which is composed principally of the Archean to Paleoproterozoic Sushui complex, Paleoproterozoic Jiangxian, Zhongtiao and Danshanshi Groups and Mesoproterozoic Xiyanghe and Ruyang Groups. The Sushui complex, located in the northwest of the Zhongtiao Mountains, is the lowest lithologic-structural unit of Precambrian complex in the region, and it is unconformably overlain by the Paleoproterozoic-Mesoproterozoic metamorphic layered sequences. The Sushui complex is consisted chiefly of tonalite-trondhjemite-granodiorite(TTG) gneisses and metagranitoids along with amphiboltes, metamorphosed supracrustal rock xenoliths and mafic dikes,which have all been influenced by metamorphism. In recent decades, the geometry and timing of amalgamation of the NCC has become one of the hot topics on Precambrian geology research in North China Craton. The TTG gneisses of Sushui complex can be as the research object to study the tectono-magmatic thermal event of North China Craton during late Archaean and the formation and evolution of the early Precambrian continental crust. Moreover, The research on the Precambrian granitic rocks in the Sushui complex can provide important information for early Precambrian geological evolution of NCC. Therefore, the TTG gneisses, Precambrian granitic rocks and amphibolite of the Sushui complex have been selected to study in this paper. Combined with the petrological and experimental data, LA-ICP MS zircon U-Pb geochronology and zircon Hf isotopic compositions, we obtained their formation ages, petrogenesis and tectonic setting. And then the composition and evolution of the Sushui complex was discussed in this article. In addition, based on the former research and our acquired data, the geochronological framework in the Zhongtiao Mountains has been built, and the geological significance of each phase of tectonic thermal event was discussed. Finally, this article discussed the structural framework of the south NCC in the Precambrian period by comparing the Sushui complex, the Taihua complex and Dengfeng complex, including their rock association character, formation ages and distribution.In this paper, the main achievements are as follows:1. Detailed geochronological study shows that the TTG rocks in the Sushui complex could be divided into two phases and respectively formed in ～2.71Ga and ～2.55Ga. The former is mainly outcropped in the southwest part of the Zhongtiao Mountains, and located in the temple of GuanYu in Xiezhou. The LA-ICP MS U-Pb zircon age of this TTG rocks is 2716±42Ma, indicating they were the oldest rock units in this area. Their petrographical and geochemical characteristics show that these rocks belong to high-Al TTGs, which was generated by partial melting of newly basaltic rocks. Their magmatic origin was mainly related to the partial melting of 2.7-2.85Ga juvenile crust, which represents the major crustal growth stage in the Zhongtiao Mountains. However, the ～2.55Ga TTGs is distributed mainly in the area of Yuanqu-Jiangxian, their LA-ICP MS U-Pb zircon age is 2543～2561Ma. Zircon Hf isotopic compositions indicated that magmatic source were from the partial melting of 2.7～2.8Ga basaltic lower crust and contaminated by a few mantle sourced materials at the end of Neoarchaean, so this tectonic thermal event represents a major event of crustal reworking.2. The Henglingguan and Xiezhou granitic rocks are similar in the petrography and geochemistry feature, and have almost same forming age(with a zircon age of 2609±31 Ma and 2620±14Ma), suggesting that the two granitoids instrusion are the product of the same magmatism. Based on their petrographical, geochemical characteristics and zircon Hf isotopic compositions, we concluded these two Neoarchean granitic rocks belongs to high-K cala-alkaline granitoids, and probably the result from partial melt of -2.7Ga TTG rocks and mafic lower crust. Besides the tectonic thermal event in -2200Ma, recorded by the edge of zircon, may reflect a geological event in paleoproterozoic.3. The Yanzhuang K-feldspar granite was formed in Paleoproterozoic, with an age of 2351±37Ma. They are typically characterized by low Sr and Yb and similar to Himalayan-type granitic rocks, which related to the partial melt of continental crust caused by crustal thickening.4. The amphibolite of the Lengkou group is formed in ～2561Ma. The geochemical characteristics indicate that they are similar to the intracontinental tholeiite, and the Hf isotopic compositions reflected that these rocks were chiefly derived from the depleted mantle source, but also with some crustal contamination. So we considered that the amphibolite was formed in a continental intraplate setting, the fluid induced the partial melting of the depleted mantle along with the TTG magma in the process of rising, then produced these amphibolite. However, the amphibolite as enclaves in TTG gneiss were mainly derived from the depleted mantle source, and then they were brought out as xenolith.5. In summary, it can be found that the Sushui complex is comprised of -2.7Ga TTG gneiss,2.6～2.55Ga TTG gneiss and calc-alkaline granitoids,～2.56Ga amphibolite and-2.35Ga Yanzhuang K-rich granite. The ～2.7Ga TTGs are formed by the partial melting of newly basaltic rocks.2.6-2.55Ga rocks are most likely to be derived from the mantle plume activity and this magmatic event caused the basic framework of the Sushui complex formed, indicating that the North China Craton was formed by the microcontinents in the Neo-Archaean. The K-rich granite indicated that paleoproterozoic tectonic evolution has begun in ～2.35Ga. In general, it showed the evolutionary process from TTGs to calc-alkali series and then to alkaline rocks.6. Early Precambrian geological events in the Zhongtiao Mountains can be divided into five phases. According to their geological significance, we can draw a conclusion that there is no obvious episodic character of crustal growth of the central North China Craton in the long period of 1.0Ga between～2.8Ga and ～1.8Ga, but displays a feature of small frequency persistent pulsing growth.7. By comparing the Sushui complex in the Zhongtiao Mountains with the Taihua complex in Xiaoqinling and Lushan area, including their rock association character, formation ages and distribution, it can be perceived that they have obvious similarities, and should be the same geological body. The Sushui complex and Taihua complex together made up the Precambrian basement in the south margin of North China Craton, which formed a large plunging syncline together with the Dengfeng complex. The hinge of the syncline dip toward the east. The Sushui complex and Taihua complex constitute the limbs respectively, and the syncline core was the Dengfeng complex. The structural trend of them is northeast to southwest in space, which does not support the view that the eastern block of the North China Craton collided with its western block along the central orogenic belt.