A Discussion On Magmatism And Tectonic Setting In The Paleoproterozoic Magmatic Quiescence In The North China Craton

The North China Craton(NCC)is the largest and oldest craton in China,its basement evolution during the Paleoproterozoic has always been the focus of debate among scholars.During 2.45?2.20 Ga,the global magmatic activity decreased and the plate tectonics slowed down,indicating an obvious magmatic "quiescence".However,the records of magmatic activity in this stage are preserved in some areas of the NCC,which is of great significance for understanding the early crustal evolution and tectonic regime transformation of the NCC.In this paper,the ?2.3 Ga geologic bodies such as Lengkou metavolcanic rocks,Zhaizi tonalitic gneiss and Henglingguan granitic gneiss in the Zhongtiaoshan area,south of the NCC were selected for research.Their petrogenesis and tectonic setting as well as the compositions and evolution of magma source are discussed.Based on the detailed field geological survey,petrology,petrography,wholerock geochemistry and Sm-Nd isotope,zircon U-Pb chronology and Hf-O isotope studies.Among them,the Lengkou metavolcanic rocks are dominated by metabasic volcanic rocks,intercalated with metamorphosed intermediate to acid volcanic rocks,which have a magmatic zircon U-Pb age of 2317±13 Ma,but also contain some 2508 Ma inherited zircons.The rock assemblage and geochemical characteristics indicate that the subduction triggered the basic magmatic invasion caused by the partial melting of the mantle and led to the tectonic-magmatic event of the addition of crustal materials,which formed in the continental margin arc environment under the convergent background.The Zhaizi gneiss is mainly granodioritic and tonalitic gneiss,intercalated with a small amount of diorite enclaves.The emplacement ages of the two are close to 2289?2316Ma,and show the characteristics of magma mixing.The geochemical characteristics of Zhaizi gneiss are similar to TTG and C-type adakite,and it is speculated that they were formed in the partial melting of the Neoarchean crust,and the dark enclaves in Zhaizi gneiss are derived from the basic magma of the mantle source region,which provides the heat source for the partial melting of the Neoarchean crust.The Henglingguan granitic gneiss is mainly monzogranitic gneiss,with an emplacement age of 2306?2324 Ma,and has the characteristics of type I granite.It is speculated that it is mainly derived from the partial melting of Neoarchean TTG rocks and formed in the crust thickening event under extrusion setting.On the whole,the ?2.3 Ga magmatic activity in the Zhongtiao mountains is mainly controlled by the tectonic setting of active continental margin arc,where the MASH process occurred.During the ascent of basic magma,the Lengkou volcanic rocks were formed through the mixing of continental crust.The Henglingguan granite was formed by the intrusion of acidic magma from the partial melting of the ancient crust.At the same time,basic magma and acidic magma are mixed,forming the complex composition of intermediate and acid magma,which is zhaizi gneiss.Based on the magmatic activity of the North China Craton over the same period,it is speculated that the paleo-ocean in the middle of the NCC subducted under the eastern block in the early Paleoproterozoic,and the ocean-continent subduction constitutes the continental marginal arc model.The ?2.3Ga magmatic rocks distributed from TaihuaZhongtiao-Luliang south to north in the central part of the NCC were formed by a large amount of arc magmatic activity above the eastern block,which may indicate the continental margin outline in the early Paleoproterozoic.