Tectonic Evolution Of The Precambrian Metamorphic Rock In East Kunlun Orogenic Belt

The East Kunlun Orogenic Belt (EKOB), located in the north portion of the Tibetan Plateau and west part of the Central China Orogenic Belt (CCOB), can be considered as a key to understanding the tectonic evolution of the CCOB. The widely distributed Precambrian metamorphic rocks, including the Jinshuikou and Binggou Groups in the central belt of the EKOB, the Kuhai and Wanbaogou Groups in the southern belt, provide vital evidences to study the tectonic evolution of the EKOB. In this thesis, a series of paragneisses and newly discriminated gneissic granites were studied after our detailed geological mapping. Combined with the petrological, LA-ICP-MS zircon U-Pb geochronological, MC-ICP-MS zircon Hf isotopic and geochemical studies, we confirm the rock assemblage, depositional and metamorphic age of the paragneisses, the formation age, petrogneisses and tectonic setting of the gneissic granites. Finally, the geochronological framework and tectonic evolution of the Precambrian metamorphic rocks in the EKOB was established based on this research and previous studies. The main achievements of this thesis are as follows.1. The Jinshuikou Group of the central belt is a set of middle-high grade metamorphic rocks, comprising mainly of paragneisses, migmatized paragneisses, schists, quartzite and marble. Its protolith is terrigenous clast interbedded with carbonate sediments, which deposits in a continental margin setting of an Eoarchean-Paleoproterozoic continental nucleus in the Paleoproterozoic time. Geochronological studies indicate that the Jinshuikou Group has experienced Neoproterozoic tectono-thermal event with age of ca.1.0-0.9 Ga, and Paleozoic tectono-thermal event.2. The Binggou Group of the central belt is predominant by the meta-carbonate and clastic rocks, including limestone, dolomite limestone, marble, silicious rocks, quartzite, meta-sandstone and schist. Zircon U-Pb ages of the quartzite and schist indicate that the depositional time of the Binggou Group is Mesoproterozoic with age of ca.1.3-1.0 Ga and that the metamorphic age is ca.400 Ma. The ca.400 Ma metamorphic age indicates that the Binggou Group has experienced the Paleozoic tectono-thermal event.3. The southern belt of the EKOB is an accretion complex belt with a set of accretion complex, including the Kuhai Group, the Wanbaogou Group and the Aqikekule Group. The Kuhai Group of the southern belt, comprising mainly of paragneisses, amphibolites and schist, deposits in late period of the Paleoproterozoic to Mesoproterozoic and has experienced a middle Neoproterozoic metamorphism with age of ca.770 Ma.4. The Aqikekule Group in western part of the southern belt, which comprises mainly of schist, is discriminated from the Kuhai Group based on the rock assemblages and geochronological results. The depositional time of the Aqikekule Group is later than 555 Ma and the peak age is 647 Ma after the geochronological studies of the mica schist.5. The Wanbaogou Group of the southern belt is dominated by the low grade metamorphic basic volcanic rocks, carbonate and clastic rocks, including basalt, limestone, dolomitc limestone, marble and quartzite. LA-ICP-MS zircon U-Pb geochronoligical studies on the quartzite indicate that the depositional time of the Wanbaogou Group is middle-late period of Mesoproterozoic with age of ca.1.4-1.0 Ga.6. A widely distributed Meso-Neoproterozoic magmatism is indicated in the EKOB. The Qushiang gneissic monzonitic granite with age of 1172 ± 20 Ma in the sourthern belt suggests that the Kuhai Group has been involved in the assembly of the Rodinia supercontinent. The Wulong orthogneiss with age of 1363 ± 18 Ma is the early magmatism in the central belt which is related to the formation of the Rodinia. While the most widely distributed Meso-Neoproterozoic magmatic rocks are the S-type granites having crystallization age of 1.0 to 0.9 Ga. They show typical S-type granite affinity with abundant Al-rich minerals like tourmaline, garnet and muscovite, and A/CNK ratios of>1.1. The Meso-Neoproterozoic S-type granites are suggested from the partial melting of the paragneisses and schist of the Jinshuikou Group after the whole rock geochemical and zircon Lu-Hf isotopic studies. Intergrated with regional geology, this Meso-Neoproterozoic magmatic event was caused by collision which is related to the assembly of the Rodinia supercontinent.7. Further studies indicate that the Precambrian metamorphic rocks and early Paleozoic vocanic-clastic rocks of the central belt of EKOB are comparable with that of the north margin of the Qaidam block. So the central belt of the EKOB is consisded to be part of the Qaidam block. It was splitting from the Qaidam block in the early period of early Paleozoic, and then subductting, exhumating and amalgamating to the Qaidam block. Furthermore, the sedimentary and metamorphic ages of the Kuhai, Wanbaogou and Aqikekule groups in the southern belt are also different with that of the Jinshuikou and Binggou Group in the central belt. Accordingly, the southern belt of the EKOB is suggested an accretion complex belt containing a series of metamorphic rocks with different ages and tectonic affinity, including the Kuhai, Wanbaogou and Aqikekule groups.