Early Paleozoic Tectonic Evolutions In The Middle Part Of Northern North China Plate And The Time Constraints

As the largest Phanerozoic accretionary orogenic belt,the Central Asian Orogenic Belt(CAOB)has recorded complex orogenic processes including subduction and collision.The Xingmeng Orogenic Belt is located in the eastern part of the CAOB,which mainly covers the vast areas of Northeast China and west Inner Mongolia.The middle part of the northern margin of the North China Plate(NCP),as an important part of the Xingmeng Orogenic Belt,is a key area for understanding the subduction,collision and orogenic processes of the Paleo Asian Ocean(PAO)during the Early Paleozoic.In order to understand and clarify the tectonic evolution during the Early Paleozoic in this area,we carried out detailed zircon U-Pb geochronological,whole-rock geochemical and Lu-Hf isotopic analysis on magmatic rocks from several important tectonic units in the area,such as the Wenduermiao subduction accretion complex and Bainaimiao island arc belt.The Early Paleozoic evolution and key geological problems in the area are discussed and new understandings are put forward in this paper.(1)The formation of the Wenduermiao subduction accretion complex and the intra-ocean island arcThe Wenduermiao subduction accretion complex is a set of ophiolite mélange.The main tectonic units include Early Paleozoic Wenduermiao Group,Late Cambrian ultrabasic rocks,Late Cambrian-Early Ordovician plagioclase amphibolite and Ordovician trondhjemite.The LA-ICP-MS zircon U-Pb age of the amphibolite was470.5±1.1 Ma.Lu-Hf isotopic analysis results showed that the?_Hf(t)values of the amphibolite ranged from 3.8 to 9.1,and the T_DM1 ranged from 716 to 923 Ma.Combined with the geochemical analysis results,the plagioclase amphibolite was formed in an island arc environment under the subduction background,and the magmatic source was the partial melting of the depleted mantle wedge which was transformed by the melts and fluids derived from subduction plate.The trondhjemite yielded a weighted mean age of 464±11 Ma.Lu-Hf zircon analysis results showed that the?_Hf(t)values of the trondhjemite ranged from 10.0 to 13.8 Ma,and T_DM2 ranged from 601 to 945 Ma.Combined with the geochemical characteristics,the trondhjemite belongs to Xiwan type adakitic granite,and the magma source is the partial melting of subduction oceanic plate.The geochemical results of ultrabasic rocks indicate that they were transformed by subduction plate fluids and showed the characteristics of SSZ-type ophiolite.These results indicate that the Wenduermiao mélange contains components formed by intra-ocean subduction.Combined with previous researches,the intra-ocean subduction in the Wenduermiao ocean may have started at?490 Ma,and the subduction may be induced by the difference of material density in the oceanic crust.(2)Characteristics of Early Paleozoic magmatism in the Bainaimiao island arcThe Late Ordovician and Silurian magmatic rocks on the Bainaimiao island arc have been systematically sampled and analyzed.Among them,quartz diorite collected from the Guolaibanhuduge yielded age at 448±0.9 Ma,which was formed in the Late Ordovician.Its?_Hf(t)values ranged from 1.7 to 6.9,with T_DM2 ranging from 1215 to1685 Ma.Geochemical characteristics show that the rocks were derived from the partially melted juvenile crust under subduction setting and underwent plagioclase differentiation crystallization.The age of the Early Silurian tonalite was 434±4 Ma,with?_Hf(t)values ranging from-0.7 to 5.1 and T_DM2 ranging from 1368 to 1893 Ma.Geochemical analysis results show that they were adakitic rocks and formed by partial melting of the thickened juvenile crust under subduction setting.The ages of the Middle and Late Silurian tonalites and monzogranite ranged from 423 to 429 Ma,with?_Hf(t)values ranging from-0.9 to 8.2,and T_DM2 ranging from 1255 to 1741 Ma.Geochemical characteristics show that the Middle-Late Silurian magmatic rocks are adakites,and their magmatic sources include partial melting of the subducted ocean plate and thickened juvenile lower crust.Combined with previous studies,the PAO should have subducted under the Bainaimiao island arc at?520 Ma.The subduction zone is an advancing type,which thickens the base of Bainaimiao island arc continuously,resulting in the formation of adakitic magmatic rocks.(3)The subduction polarity of PAO and the nature as well as basement characteristics of the Bainaimiao island arcThe geochemical characteristics of the Silurian magmatic rocks in the Bainaimiao island arc from north to south show a transition from normal island arc to adakitic,suggesting that crustal thickness and oceanic subduction depth are increasing from north to south,which indicates that the PAO should subduct southward under Bainaimiao island arc.The Early Paleozoic magmatic rocks in the Bainaimiao island arc were formed by the partial melting of juvenile crust,and their T_DM2 clustered in the range of 1.0-1.8 Ga,indicating that the Bainaimiao island arc had Mesoproterozoic juvenile basement.The?1.4 Ga granodiorites and tonalites in the Huangling area of Siping provide direct evidences for the existence of Mesoproterozoic basement in the Bainaimiao island arc.The above granitic magmatism was formed under extensional background caused by rollback or break-off of the subducted ocean plate.The rocks may be related to Laurentian-Baltic landmass and serve as the exotic basement material for the Bainaimiao island arc.The NCC was mainly in an extensional state during the Mesoproterozoic(1.8-1.0 Ga),when the juvenile crust may have been formed in its northern margin.In the continuous rifting processes during the Neoproterozoic(?900Ma),these juvenile as well as some ancient crustal materials may drift away from the northern margin of the NCC to form relatively independent blocks,but this drifting process was not accompanied by the formation of oceanic crust.All in all,the Bainaimiao island arc is similar to the Japanese-type island arc,and its basement may be complex,including the juvenile and ancient crustal materials from the northern margin of the NCC and the addition of exotic juvenile crustal material.(4)The relationship between the NCP and Gondwana supercontinentThe detrital zircons from the sedimentary in the Early Paleozoic back-arc basins showed age peaks of 1.2-0.6 Ga,and the NCP lacked magmatic activity in the above time period.Therefore,previous studies suggested that the provenance may be outside the NCP.Based on previous studies of stratigraphy,paleontology and paleomagnetic,we also believe that the NCP was located at the margin of Northeast Gondwana during the Early Paleozoic,and that the northern India,the Australian,the Tarim plates and other microcontinents in the CAOB may have contributed 1.2-0.6 Ga detrital zircons to the back arc basin in the northern margin of the NCP.(5)Arc-continent collision in the late Early Paleozoic and post-orogenic extension during the DevonianAt?420 Ma,the arc-continent collision between Bainaimiao island arc and the northern margin of the NCC began,and might last until?410Ma.The intra-ocean Wenduermiao arc also collided with the Bainaimiao island arc at the same period.The muscovite in garnet muscovite schist of Jianshan Formation of Bayan Obo Group yielded ⁴⁰Ar-³⁹Ar plateau age at 412±3 Ma,indicating that this collisional event was also recorded within the NCC.In the late Early Devonian,the northern margin of the NCP was in the post-orogenic extension setting,the Jianshan Formation of Bayan Obo Group and the granites intruding into the Bayan Obo Group recorded this event and yielded muscovite ⁴⁰Ar-³⁹Ar plateau ages between 397-389 Ma.At the same time,there developed a Devonian alkaline magmatic belt stretching?900km from the west to east in the northern margin of the NCP.Based on the spatial distribution of Devonian alkaline rocks and tectono-thermochronology records,this paper proposes the break-off of subducted plate was the regime for the Devonian post-orogenic extension.