The evolution of Permian continental crust in northern part of North China

At present there exist different understandings about the structural settings of northern part of North China( including the northern margin of North China Craton and Hinggan-Mongolia Orogenic Belt) in the Permian. According to an influential viewpoint,evolution of the Paleo-Asian Ocean has continued until the end of Permian or the beginning of Triassic,with the northern margin of North China Craton becoming anAndes-type continental marginas a result of oceanic plate subducting toward underneath the North China Craton. And the active continental margin has correspondingly changed from an early state of subdaction-convergence( C2) to a late state of detachment of subducting plate and extention of continental margin( P2-T1-2). Such an understanding is largely based on evidences of magmatism. This viewpoint will be discussed in this paper in terms of sedimentary formation,deformation,metamorphism and regional tectonic settings with respect to the continental crust evolution of the concerned area in the Late Paleozoic,and the whole process of evolution will be divided into three stages according to our understanding:( 1) the climactic orogeny of Hingga-Mongolia Orogenic Belt in the Middle Devonian before Famennian;( 2) the post-collision in the Late Devonian-Early Carboniferous; and( 3) the non-orogenic extension in the Late Carboniferous-Early Permian,when the young continental crust of Hingga-Mongolia Orogenic Belt under the extensional settings produces newly-born rifts,which-instead of any oceanic plate-controls the concurrent sedimentation and magmatism. All the geological events of the three stages have their expressions in the northern margin of North China Craton. The isotopic ages of plutonic rocks from the area show three peak values: Middle-Late Devonian,Early Permian,and Late Permian. The rock assemblages and Sr-Nd isotopic characteristics of plutonic rocks of Early Permian( 280Ma) and Late Permian( 260Ma) are particularly studied to reveal magma generation. It is seen from the R1-R2 diagram that the Permian plutonic rock assemblages in this area are different from the Andes-type orogenic zones which are mainly consisted of tonalite and granodiorite. The Early Permian plutonic rocks have bimodal character and closer relations with EMI-type mantle sources as shown by their Sr-Nd isotopic features.Inspired by a previous study on the primary magma in Gaositai,it is further supposed that there has been an upwelling process of intraplate OIB-type magma along the lithospheric faults in northern margin of North China Craton in the Early Permian. In contrast,the Late Permian plutonic rocks are mainly syenogranite and monzonitic granite with higher alkaline degree,these intermediate-acidic rocks possess the features of high-alkaline or high-potassium,and their Sr-Nd isotopic characteristics show a trend of the magmatic sources evolving toward EMII-type,with the increased melting materials of the middle-upper crust suggesting uprising of the thermal belt. In summary,such evidences as from magma generation,magma-reservoir developmental,and space-time distribution of the magma all prove that formation of the Permian plutonic rocks is related more with the intraplate thermal evolution,different from that of Andes-type orogenic zones. Hinggan-Mongolia Orogenic Belt belongs to Central Asian Orogenic Belt and can be examined within broader tectonic settings. There are 8 belts of Late Paleozoic and Mesozoic peralkaline and alkaline( alkali-feldspar) granitoids and genetically related bimodal volcanics in the Central Asian Orogenic Belt. These tectonomagmatic belts stretch over thousands kilometers,their NE strike represents a new tectonic framework,and their isotopic characteristics of high eNd( t) and low eSr( t) reflect repeated activities of rifts under extensional settings. The Central Asian Orogenic Belt on the whole as a young continental crust,together with North China Craton and Tarim block,has entered a new evolutional stage since the beginning of Early Permian. If cratonization of forming progress of the continental lithosphere is studied, the Late Devonian-Early Carboniferous of Central Asian Orogenic Belt is the first stage of cratonization after collision,and the second stage of cratonization begins in the Late Carboniferous-Early Permian,when new zonal extensional structures emerge extensively,marking the importance of plutonic activities becoming increasingly obvious.