The Paleozoic Tectonic Evolution Of The West Segment Of The Northern Margin Of The North China Craton

This work presents detailed field relations,geochronological,geochemical,and Lu-Hf isotopic data for a Paleozoic magmatic suite from the northern margin of the North China Craton?NCC?and the Bainaimiao arc belt in the Damao area of Inner Mongolian Autonomous Region to study the ages,tectonic settings of the magmatic rocks of the west segment of the Bainaimiao arc belt and the northern margin of the NCC.Together with previous work,we have discussed the nature and tectonic implications of the Bainaimiao arc belt and the tectonic evolution of the continental margin of the NCC,the existence and evolution of the southward subduction of the Paleo-Asian Ocean?PAO?,and furtherly its final closure in the study area.The following conclusions can be drawn:?1?The arc-related magmatism of the west segment of the Bainaimiao arc belt consists mainly of diorite with minor gabbroic diorite and tuff.Zircon U-Pb dating of the gabbroic diorite and diorite yielded crystallization ages at ca.458 Ma and 428 Ma respectively.These two rocks show typical continental signals such as Pb,K enrichment and arc signals including:the properties of calc-alkaline and metalumious to weak peraluminous,the enrichment of Large ion lithophile element?LILE??e.g.Rb,Th,U,etc.?and light rare earth element?LREE?and the depletion of High field strength element?HFSE??e.g.Nb,Ta,Ti,etc.?and Heavy rare earth element?HREE?.The above evidence indicate that they were formed in the contientnal margin arc setting.In addition,the younger middle Silurian diorite as the main part of arc magmatism crops out mainly in the central and southern parts of the Bainaimiao arc belt.The middle Ordovician gabbroic diorite crops out as xenoliths in the diorite or tectonic fragments mainly in its northern part with limited distribution.Thus,The arc-related magmatism thus shows a southward younging distributional pattern.In addition,these middle Ordovician to middle Silurian dioritic rocks have xenocrystic zircons with ages ranging from 450 to 530 Ma.This suggests that the arc-related magmatism possibly mainly occurred in the period between the middle Ordovician to middle Silurian and might initiated in the early Cambrian.The middle Silurian diorite has negative?Hf?t?values?-0.87^-6.11?and other continental signals?such as positive Pb,K and negative Nb,Ta,Ti anomalies?and relative high Mg#values?46.2⁵8.8?,indicative of the existence of both mantle and continental crust materials in the magma source.Thus,its Hf model ages?ca.1.47¹.79 Ga?represent the mixed times of juvenile and ancient continental crust,indicating that there were probably late Paleoproterozoic to early Mesoproterozoic and even older basement rocks beneath the Bainaimiao arc magmatism.The middle Ordovician gabbroic diorite has positive?Hf?t?values?2.15⁶.80?with Hf model ages ranging from ca.1.00 to 1.33 Ga indicative of a mainly mantle-derived magma source.The continental signals?such as positive Pb,K and negative Nb,Ta,Ti anomalies?imply the existence of continental crust materials in the magma source.Thus,the middle Ordovician gabbroic diorite are mainly the products of mantle input and reworking of minor young continental materials.In summary,the Bainaimiao arc-related magmatism commenced at middle Ordovician probably Cambrian and continued to middle Silurian.It recorded southward arc-accretion of the Bainaimiao arc.In addition,there were probably late Paleoproterozoic to early Mesoproterozoic and even older basement rocks beneath the Bainaimiao arc magmatism.Thus,the Bainaimiao arc belt was a continental fragment and reworked by the early Paleozoic arc-related magmatism.During the reworking process,there are some mantle materials inputted and we suggest it should be termed as Bainaimiao arc terrane.?2?The²70 Ma intermediate to felsic volcanic rocks?rhyolites,minor dacites?and²55 Ma alkaline granites?monzogranites and K-feldspar granite?linearly distributed in the research area.They have similar petrographic and geochemical features:all of them are composed mainly of alkali feldspar with minor mafic minerals and geochemically metaluminous with high K₂O+Na₂O and low CaO,MgO,Al₂O₃ contents,as well as high?K₂O+Na₂O?/CaO,FeOt/MgO and 10000×Ga/Al ratios.They are also depleted in Sr,Eu,Ba and enriched in?REE,Rb,Th,U,Pb,Zr and Hf contents.These features indicate that they are A-type magma,which is distinctly different from the calc-alkaline arc-related rocks during Carboniferous and early Permian.These A-type rock suites with high SiO₂ and K₂O+Na₂O contents crop out mainly in the northern margin of the NCC with nearly E-W direction linearly distribution.Together with their low Y/Nb ratios and some arc-like signals?negative Nb,Ta anomalies?indicate that they were formed partial melting of continental crust materials with subsequently fractional crystallization in a post collisional extensional setting.From north to south,they are the middle Permian rhyolites and minor dacites and late Permian alkaline granites respectivley.At nearly the same time?²60 Ma?,there are also some arc-like magmatic rocks such as diorite and basalt,basaltic andesite in this study.Their arc-like signals probably inherited from earlier Carboniferous to early Permian continental arc magmatism.Moreover,there is a significant input of mantle contribution to their magma sources,as suggested by the change of Hf isotopes from only negative to highly evolved and some positive Hf isotopes.Thus,our results favor a model of slab breakoff of the PAO in the middle Permian.The widely outcropped Late Permian granites?²55 Ma?also show an A-type affinity.Furthermore,they have various continental crust materials involved in the magma sources compared to the rhyolites and thus could be attributed to the continuous extension after slab breakoff of the PAO.Thus,the western segment of the northern NCC was in a post-collisional extension in the the middle to late Permian?ca.270²55 Ma?probably triggered by slab breakoff.?3?Our work indicate that the southward subduction of the PAO existed and the North China accretion-collage system was the products of it.The Wenduermiao SSZ-type ophiolitic melange in the Ondor Sum accretionary complex and the bainaimiao arc-related magmatism were accretion and reworking products of Bainaimiao possible Precambrian basement rocks,respectively.This indicates the southward subduction initiated at least in the middle Ordovician,even in the Cambrian.Our documented ca.270²55 Ma large-scaled post-collisional extensional magmatic rocks indicate a continent/arc-to-continent collisional event occurred prior to ca.270 Ma.Given the continuation of the southward subduction of the PAO in the Carboniferous to early Permian suggested by contemporaneous arc-related magmatic rocks,we suggest that this collisional event represents the termination of the southward subduction of the PAO which occurred probably in the latest Early Permian.This seems supported by the occurrence of a⁵0 m thick conglomerate layer intercalated within the Permian volcanic rocks in the Damao region.Due to the northward subduction of the PAO finished at the late Carboniferous?ca.300 Ma?,we infer the termination of the southward subduction of the PAO at the western segment of the northern NCC indicate the final closure of the PAO there.Given the timing of closure of the PAO in areas to the eastern and western of the research area,our results favor a scissor-like closure of the PAO from west to east.?4?Among these rock suites formed during the southward subduction of the PAO in the North China accretion-collage system,the Wenduermiao ophiolitic melange in the Ondor Sum accretionary complex are typical accretion-related rocks,and most of them in the Bainaimiao arc terrane and Northern margin of the NCC are mainly the reworking products of ancient continental crust(including the late Archean to Paleoproterozoic basement rocks and Mesoproterozoic covering sediments.Thus,there are both the accretion and reworking during the southward subduction.Combined with previous work,both of them are common and thus,they and their collages play a important role in the CAOB evolution.Thus,the CAOB is formed by the accretion with formation of juvenile crust?e.g.Intraoceanic arc,seamount,accretionary wedge,etc.?and reworking of microcontinents and subsequent collages of them.