Early Paleozoic Tectonic Movement On The Tibetan Plateau And Its Adjacent Areas

Existing preliminary studies indicate that Tibetan Plateau and its adjacent areas were located on the northern margin of the East Gondwana continent during the Early Paleozoic,preserved relevant information on the Gondwana supercontinent convergence event.It is one of ideal regions for restoring and reconstructing the early Paleozoic tectonic evolution of the northern margin of the East Gondwana mainland.The predecessors have studied the early Paleozoic stratigraphy,magmatic rocks,tectonics,and metamorphism on the Tibetan Plateau and its neighboring regions,and they have preliminarily concluded that the Tibetan Plateau and its adjacent areas underwent a strong tectonic movement during the early Palaeozoic.However,there are many debates about the early Palaeozoic tectonic movement,as a result of bad climate,traffic condition and complex evolution history on these areas.For example:What is the distribution scale of this tectonic movement?What is the timing of tectonic movement?What kind of temporal and spatial distribution characteristics of this tectonic movement?How does it relate to the Pan-African Orogeny?It is generally believed that the regional sedimentary unconformity and its upper and lower strata are the loyal recorders of tectonic movements and are the best objects for restoring the tectonic movement process and nature.Therefore,studying the Cambrian-Ordovician unconformity is the key to unveiling the mysterious early Paleozoic tectonic movement on the Tibetan Plateau and adjacent areas.Examples of the Cambrian-Ordovician unconformity are commonly observed in Tibetan Plateau and its adjacent areas,but there is still a different understanding of the nature of Cambrian-Ordovician unconformity.To sum up,the main three viewpoints are as follows:The first view believed that the Cambrian-Ordovician unconformity is the product of Pan-African Orogeny or late Pan-African Orogeny,based on the existence of a large number of early Paleozoic medium-high grade metamorphism in the Gondwana interior and the phenomenon of coarse Ordovician conglomerates covering the Cambrian-Precambrian strata with angular unconformity;The second point of view is based on the theory of the earth volume balance.After the end of the Pan-African Orogeny,a new subduction zone will form on the periphery of the Gondwana continent that has just been formed by the collage of small blocks.Therefore,the Cambrian-Ordovician unconformity is a part of Andean-type accretionary orogenic belt.The third view support that the Cambrian-Ordovician unconformity is formed in an extensional setting,because some Ordovician stratum overlie normal faults and regional magmatism is related to the lithosphere extension.On the basis of previous work,this paper selected the several typical examples of Cambrian-Ordovician unconformity that have been reported and newly discovered,as key research objects.By identifying their distributions and material compositions,and determining their ages of formation and diachronism,so as to discuss the nature of unconformity.Combining with the contemporaneous sedimentary formations,metamorphic deformations and magmatic rocks,eventually we restore and reconstruct the spatial and temporal evolution process of the early Paleozoic tectonic movement on the Tibetan Plateau and its adjacent areas.The Cambrian-Ordovician unconformity is found at Chahe area,where there is a<5°discordance(paraconformity)between upper Cambrian Baoshan Formation and overlying middle Ordovician Shidian Formation and red glutenite,and is the most typical example in the eastern of the Southern Qiangtang-Baoshan(SQB)block.Three samples of siltstone from the lower part of the Ordovician strata above unconformity at Chahe,Shidian and Yiwanshui area,were collected for LA-ICP-MS zircon U-Pb dating,The zircon of these samples yielded a youngest age was 551 Ma,530 Ma,and 529 Ma respectively.These youngest detrital zircon ages are significantly older than the depositional age of the strata given by Ordovician fossils,and this suggests the site of deposition may have been located on a passive continental margin far from the active magmatic belt.The Cambrian-Ordovician unconformity at Rongma area(central SQB block)show that the Lower Ordovician Xiagula Formation and conglomerate cover in parallel unconformity above the Cambrian Rongma Formation.Based on the comparison of the stratigraphic section,this paper redefined the upper limit of the age of unconformity from Middle Ordovician to Early Ordovician.It can be seen that the examples of Cambrian-Ordovician unconformity in the SQB block are generally dominated by parallel unconformity,and the slightly angle unconformity only appear in local areas,which is quite different from the typical unconformity of the orogenic system.The Shenzha Cambrian-Ordovician unconformity located on the central Gangdise block,is currently the most typical,the best preserved unconformity,and it is also the first Cambrian-Ordovician unconformity that has been reported outside the Himalaya block.The Lower Ordovician Zhakang Formation and conglomerate overlie the Cambrian Zhaqian Group,and its age is between Late Cambrian and Early Ordovician.Cambrian-Ordovician unconformity at Kangmar area,located in the Himalaya block,shows that the Middle-Upper Ordovician Zeguo Group rests unconformably on Precambrian Laguigangri Group,with local lenses of residual conglomerate.A clastic sample was taken from the base of overlying Zeguo Group for the LA-ICP-MS zircon U-Pb dating,and yielded two youngest zircon ages of 462 Ma,which are consistent with Middle-Late Ordovician fossils and indicate a Middle Ordovician depositional age.Moreover,two granitic gneiss samples were collected in the Kangmar area for the LA-ICP-MS zircon U-Pb dating.The age results of the original rock were 521Ma and 495Ma,respectively,indicating that the lower limit of timing of the Kangmar is in the Early Cambrian.Based on previous and our recent research,we conclude that the examples of the Cambrian-Ordovician unconformity were widely exposed on the Tibetan Plateau and its adjacent areas,including the Himalaya,Gangdise and SQB blocks.The northern boundary is the Longmuco-Shuanghu-Lancangjiang suture zone(LSLSZ),and its distribution range is about 2000×600km~2.By statistically analyzing the regional paleontological fossil,zircon ages data of magmatic rocks(volcanic rock interlayers and intrusive rocks)and clastic rocks,we found that hiatus time of the Cambrian-Ordovician unconformity is gradually decreasing along the direction from India to the LSLSZ.In this same direction,the nature of the unconformity changes from an angular unconformity to a paraconformity,even partially may be integration.Combined with the evidence of the Ordovician basal conglomerate,contemporaneous metamorphic deformation and bimodal rock assemblage,we conclude that the examples of the Cambrian-Ordovician unconformity may be the overlap unconformity,which formed in an extensional background.We tentatively speculate that the early Paleozoic tectonic movement on the Tibetan Plateau and its neighboring regions may represent the extensional movement after Pan-African Orogeny,instead of Pan-African Orogeny or Andean-type accretionary orogeny.The processes of early Paleozoic tectonic movement on the Tibetan Plateau and its adjacent areas can be roughly divided into three stages:The first stage was in the late Neoproterozoic to early Late Cambrian period.The Tibetan Plateau and its adjacent areas,located on the marginal basin in the north of East Gondwana,were far away from Pan-African orogenic belts of Gondwana interior.During this period,it was affected by the Pan-African Orogeny and uplifted,causing a large-scale regression.In the second stage,during the late Late Cambrian-early Early Ordovician,the Pan-African orogenic belt had begun to collapse,and its tectonic regime changed dramatically.The examples of the Cambrian-Ordovician unconformity record the switch from orogenesis to post-orogenic extension.However,the disappearance of the orogenic belt is a slow process,and the surface will not immediately change from regression to transgression.In the third stage,after the late Early Ordovician,owing to the entire end of the Pan-African Orogeny,the settlement process of the crust caused a new round of transgression.Finally,the Tibetan Plateau and its adjacent areas in the Late Ordovician entered a stable passive continental margin,and deposited large and thick marine strata.