The Early Mesozoic Magmatism And Deformation In The East Segment Of The Northern Margin Of The North China Craton

The North China Craton (NCC) has become a hotspot of geological research and attracted more and more researchers from both domestic and abroad. A lot of studies have been carried out on Precambrian cratonization and Phanerozoic decratonization processes, and much progress has been achieved. The northern margin of NCC has a long continental margin history from Mesoproterozoic to Late Paleozoic. Recent researches on the northern margin of the NCC have been mainly focused on the late Paleozoic tectonic events. However, the tectonic setting during the early Mesozoic is still in dispute.Early Mesozoic period is a key stage after closure of the Paleo-Asian Ocean and the northern margin of the NCC entered into an intra-plate tectonic setting. Therefore, studying the early Mesozoic magmatism and deformation in the northern NCC is of great importance to understanding the geodynamic processes after the final closure of the Paleo-Asian Ocean and amalgamation between the Mongolian arc terranes and the NCC along the Solonker suture.The main contents of this thesis include:(1) Zircon U-Pb dating, whole rock geochemical and isotopic analysis and in-situ zircon Lu-Hf analysis were performed on contrasting latest Permian-Triassic volcanic rock suites from the northern NCC;(2) Based on field profile observation and isotopic dating, Early Mesozoic deformation in the Panshan area and the Western Hills of Beijing were established;(3) Petrogenesis and emplacement deformation of the Dushan composite batholith was studied in order to better understand the geological setting of the northern NCC during the Early Mesozoic time.The main advancements achieved from the thesis are as follows:1. There are two contrasting volcanic rock suites in the northern NCC during the latest Permian-Triassic. The earlier volcanic rock suite erupted during the latest Permian to Early Triassic at ca.245～255Ma and was composed of rhyolitic rocks. The later suite erupted during the Middle to Late Triassic at ca.228～238Ma and composed of andesitic rocks and minor rhyolitic rocks. Geochemical and isotopic data revealed that the latest Permian to Early Triassic volcanic rocks were likely produced by fractional crystallization of ancient lower crustal-derived magmas. However, the Middle to Late Triassic suites is mainly derived from lithospheric mantle and ancient crust, with minor involvement of asthonospheric mantle.2. The intrusion of the Panshan composite pluton resulted in formation of the U-shaped synclines (e.g. the Fujunshan syncline, the Zhuangguoyu syncline and the Shuiyu syncline). On the map scale, the U-shaped synclines obviously superimpose on the earlier regional east-west trending Malanyu anticlinorium fold. There are two folded granitic porphyry sheets in both Hongshikan anticline and Xidayu syncline in the western plunging zone of the Malanyu anticlinorium. Zircon U-Pb dating for these granitic porphyry sheets yield214±3Ma and213±2 Ma, respectively. The Panshan composite pluton emplaced during the late Triassic, with peak at about206Ma. The Langjiayu unit emplaced earlier, at about210Ma. Therefore, regional folding time can be constrained between210～213Ma. The Jixian thrust fault truncates the Fujunshan syncline and the Zhuangguoyu syncline, and then intruded by later emplacement of the Panshan composite pluton. The Jixian thrust fault may activate simultaneously with the intrusion of the Panshan pluton, and may last to～206Ma.3. On the southern limb of the Qingbaikou anticline (dome) in the Western Hills of Beijing, there is an angular unconformity between the upper Nandaling Formation and the lower Ordovician limestone. By dating the bed parallel dioritic sills in the core of the Qingbaikou anticline and andesitic tuff from the Nandaling Formation, the initial deformation time of the Qingbaikou anticline can be constrained between170～177Ma. Considering that the angular unconformity beneath the Nandaling Formation is local and it is parallel unconformity between lower Jurassic strata and underlying formations at most area of the Western Hills of Beijing, we proposed that the Early Mesozoic deformation is relatively weak in the Western Hills of Beijing.4. The Dushan composite batholith consists of the southern deformed Dushan pluton and the northern undeformed Dashizhu pluton. New zircon LA-ICP-MS U-Pb data indicate that the Dushan pluton and the Dashizhu pluton have the same emplacement ages within the limit of error. Deformation analysis on the Dushan pluton show that it formed by stoping and expansion in a contraction tectonic stress field during its emplacement. However, petrogenesis studies the Dushan composite batholith and the alkaline complexes in the northern NCC reveals that they were formed in a lithospheric scale extension setting, with upwelling of asthonosphere mantle. Therefore, the deep and superficial process may have decoupled during the early Mesozoic time in the northern margin of the NCC, and contraction deformation may last for a long time after closure of the Paleo-Asian Ocean.