Early Precambrian Geological Evolution Of Several Typical Areas In Western North China Craton

Located in the central segment of the Khondalite Belt in the Western Block of the North China Craton, the early Precambrian metamorphic basement of the Daqingshan Area is composed of Sanggan "Group", Wulashan "Group", Meidaizhao "Group" and meta-plutonic rocks. The Wulashan "Group" could be further subdivided into the Upper Wulashan "Subgroup" and Lower Wulashan "Subgroup". The study areas of Xiwulanbulang, Wuchuan and Baiyunebo locate in the Yinshan Block. Sanggan "Group" and gneisses of charnockite series are the dominant rocks in the Xiwulanbulang area, along with minor Neoarchean gneissic quartz-diorite and granodiorite. In the Baiyunebo area, the early Precambrian basement rocks are beneath the Baiyunebo Group by unconformity, and composed of mylonitic granite, granodiorite, diorite, monzonite, biotite gneiss and garnet-kyanite-bearing granitic gneiss.In this thesis, a systematic study was carried out on the early Precambrian basement rocks in the Hadamengou area in Daqingshan, the Xiwulanbulang area in Wuchuan and Baiyunebo by integrating petrology, geochemistry, zircon SHRIMP U-Pb age dating and in-situ Hf analyses. The main conclusions include:1) The Sanggan "Group" in the Hadamengou area was formed at～2.5Ga with a protolith of mafic-felsic volcanic-sedimentary series and the "Lower Wulashan Subgroup" was formed at2.45-2.40Ga;2) the early Paleoproterozoic garnet granite in the Daqingshan area is formed by anatexis of the supracrustal rocks. The leucosome and the garnet grains was formed by peritectic reaction of the biotite, rather than cystallized directly from the magma. The garnetite was formed by migrating, differentiation and accumulation of garnet grains during continuous evolution of the garnet granitic magma;3) The syeongranitic pegmatite in the Daqingshan area was formed at750-800℃with the source materials of early Precambrian metamorphic basement rocks in the same region;4) the～2.7Ga gneissic trondhjemite distribute in the Xiwulanbulang area, Wuchuan and the Archean rocks was almost subjected to the high-grade regional metamorphism at the end of Neoarchean;5) Magmatism at～2.6Ga and1.9-2.2Ga was discovered in the Baiyunebo region. Two episodes of tectono-thermal events were also recorded in this regions at the transition of Neoarchean to Paleoproterozoic (2.47-2.51Ga) and late Paleoproterozoic (1.86-1.94Ga);6) The source materials of the Baiyunebo Group and Sailinhudong Group are similar and composed of late Neoarchean (2.4-2.7Ga) and late Paleoproterozoic (1.8-2.1Ga) magmatic and metamorphic rocks. Part of the rocks was reworked during the high-grade metamorphism at the end of Neoarchean and late Paleoproterozoic. Their detrital materials are all from the early Precambrian basement along the northern margin of the North China Craton;7) the western part of the North China Craton was characterized by major crustal formation in early Neoarchean whereas recycling of older crustal materials was the main character in the late Neoarchean;8) the Yinshan Block and the Khondalite Belt was both involved in the late Paleoproterozoic tectono-thermal event, which is a common feature of the North China Craton. They share a similar geological evolution in the early Precambrian;9) The basements of the Western and Eastern Block of the North China Craton were probably formed by similar geological process and the North China Craton has been a unified craton at ca.2.5Ga.