| The Central Asian Orogenic Belt(CAOB)is one of the most strongly accreted orogenic belts in the world and has undergone a series of evolutionary processes including subduction,collision,and collapse during the Phanerozoic.The Paleozoic evolution for the CAOB is particularly important,because the key issues such as when the Paleo-Asian Ocean(PAO)subducted,the polarity of the subduction,and when it closed,are still controversial.To solve these,this study looks to take the magmatic rocks and sedimentary strata from the Jining-Sonid Right Banner area as the object,and determine the tectonic background of object samples from different ages in this area through field investigations,isotope chronology,and geochemistry.At the last,by summarizing the previous results and data the Paleozoic tectonic evolutionary history and the accretionary process of the continental crust of the central segment of the northern margin of the North China Block(NCB)can be fully elucidated.The main observations and conclusions are as follows:1.Early Paleozoic SSZ ophiolite and AdakiteThe metamorphic gabbro and quartz diorite may represent the earliest magmatism in the area.The metamorphic gabbro formed at ca.490 Ma and has geochemical characteristics that are different from typical MORB and OIB-type basalts,which indicate that it was derived from partial melting of the subduction mantle wedge.And the quartz diorite has an age of 488.4±2.3 Ma,with typical plagioclase-granite characteristics,and was derived from partial melting of the High temperature shear zone in subducted oceanic crust.Both of them are part of the SSZ ophiolite,which is a symbol of the beginning of the subduction of the PAO during the Paleozoic.To compare,the trondhjemite is a typical adakite with an age of 448.1±2.1 Ma.Large-scale magmatism in the area persisted until the end of the Silurian,indicating a continuous southward subduction process of the PAO.2.Early Paleozoic sedimentary strataThe Early Paleozoic sedimentary strata is mainly composed of the respective Ordovician Ayadeng,Silurian Xuniwusu,and Xibiehe Formations.The Ayadeng Formation is exposed as klippe in the southern part,while its maximum depositional age is 490 Ma and the lithology shows that formed in a relatively stable environment amenable to deposition.The maximum depositional ages of the Xuniwusu and Xibiehe Formations are 440 Ma and 417 Ma,respectively,and they were formed before(in the case of Xuniwusu)and after(Xibiehe)the arc-continent collision.As concerns proximity to the NCB,the Ayadeng Formation has an ancient detrital zircon composition(700?1250 Ma)which,due to the significant difference as compared to sedimentary rocks in NCB,indicates that its deposition position was not local to it.Conversely,the pre-historic samples representing the NCB gradually increased in the Xuniwusu and Xibiehe Formations.This transiton of detrital zircon from the Ayadeng Formation to the other two formations reflects the processes of approaching,and collision between the Bainaimiao Arc and NCB at the end of the Early Paleozoic.3.Late Paleozoic sedimentary strataThe Benbatu Formation is a set of neritic facies clastic rocks,for which the zircon ages prove that its maximum depositional age is 296 Ma,and it shows a transgression sequence,while being formed in a local extension back-arc basin.The Sanmianjing Formation is volcanic sedimentation in littoral-neritic facies with a maximum depositional age of 272 Ma which formed in a compressional basin and an unstable environment.The sedimentary environment changed from marine to land facies,and from extension to extrusion,reflecting the process from subduction to the closure of the PAO in the Late Paleozoic.4.Permian volcanic rocksFor this study,most of the volcanic rocks in the area,which was previously mapped as Mesozoic,are re-defined as of the Permian Erlitu Formation.The samples range from basaltic andesites to rhyolites which present a sequence of continental arc rather than bimodal volcanic rocks.Their ages in the range of 278 Ma to 260 Ma,and specifically the basic volcanic rocks are significantly older than the acidic rocks,though it is possible that an eruption interrupted the formation of each set.Before 260 Ma,all the volcanic rocks have the characteristics of continental arc,while after this date,intraplate granite begins to appear and are characteristics of A-type granite.The?Hf(t)values of volcanic rocks on each side of the Xuniwusu Fault are different,displaying a decrease trend southward across the samples,which suggests the participation of the ancient crust of the NCB,the process of southward subduction of the PAO and accretion of the crust.5.Nature of the Bainaimiao ArcThe volcanic rocks and sedimentary strata here frequenlty show zircons with ages of 700 Ma to 1250 Ma,a range that is absent in NCB.Furthermore,the magmatic rocks that occurred in the Bainaimiao Arc have isotopic signatures that are distinct from those in NCB,implying that the Bainaimiao Arc has its unique basement.Comparing the zircon ages and fossils,it is possible that the it was an exotic terrane separated from other plates before integrating into the CAOB gradually after 500 Ma.6.Paleozoic two-stage tectonic evolution modelCombining the magmatic rocks and sedimentary stratigraphic data of the region,the work herein improves the two-stage evolutionary model of the northern margin of the NCB.The first stage is from the Late Cambrian to Late Silurian,which began with the southward subduction of the PAO and ended with the collision between the Bainaimiao Arc and the NCB.The second stage is from Devonian to Early Triassic,the former being a post-collisional extension stage,before the subduction restarted during the Carboniferous,and from the Late Permian to Early Triassic,the PAO closed from west to east in a scissors-type,which was concurrent with the disappearance of and the main ocean basin. |
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