Devonian Magmatism Along The Northern Margin Of The North China Block And Its Tectonic Setting

The northern margin of the North China Block(NCB)is located to the southeast of the Central Asian Orogenic Belt,it was strongly influenced by the Paleo-Asian tectonic system and experienced complex tectonic evolution.The northern North China Craton was a passive continental margin during the early Paleozoic,from the late Paleozoic,the northern margin of NCB transformed to an Andean-style active continental margin.Devonian is a period for continental margin transition of the northern margin of the NCB,therefore,the petrogenesis and origin,emplacement machine and tectonic setting of the Devonian magmatic rocks is the key to understand the evolution of the northern NCB during the late Paleozoic.In this thesis,geochronological,whole-rock geochemical,Sr-Nd-Hf isotopic and deformation analysis were performed on the Devonian alkaline magmatic belt and the age,rock association and deformation characteristic were performed the Devonian volcanic rocks in Jiefangyingzi area of Chifeng along the northern margin of the NCB.The main advancements obtained from this thesis are as follows:1.Devonian alkaline rocks emplaced during the early-middle Devonian at around400–380 Ma and constitute an E–W-trending alkaline magmatic belt that extend ca.900 km long along the northern margin of the NCB.2.New zircon LA-ICP-MS U-Pb dating identified a new Devonian alkaline intrusion(Baicaigou intrusion)and geochemical and Sr-Nd-Hf isotopic compositions show that the Devonian alkaline rocks in the northern NCB were mainly originated from partial melting of an enrichment lithospheric mantle with different involvement of ancient lower crustal component and fractional crystallization.3.The conspicuous difference between the Sr-Nd-Hf isotopic dates of the coeval magmatic rocks indicates the northern NCB has an inhomogeneous,variably enriched subcontinental lithospheric mantle during the Devonian period.It was characterized by significant vertical crustal growth during the Devonian period due to partial melting of subcontinental lithospheric mantle to produce alkaline magmatic rocks.4.Combined with the Early Paleozoic continental nature of the northern margin ofthe NCB,the tectonic history of the Bainaimiao island arc belt and no or very weak deformations of the Devonian intrusions,formation of the Devonian alkaline magmatic belt rocks in the northern margin of the NCB were related to post-collision extension after arc-continent collision between the Bainaimiao island arc and the northern margin of North China Craton during the latest Silurian.5.The Devonian volcanic rocks in Jiefangyingzi area of Chifeng along the northern margin of the NCB are composed mainly of metamorphic rhyolitic rocks and metamorphic basaltic andesite,metabasalt(plagioclase amphibolite)and exhibit characteristics of bimodal volcanic rocks.6.There is a NE-SW striking ductile shear belt in Jiefangyingzi area of Chifeng along the northern NCB,all of metavolcanics and sedimentary rocks with strongly derformation are characterized by NEE-SWW subhorizontal lineations and L-type tectonite.The ductile deformation formed during the Middle-Late Triassic.