Evolution Of The Changning–Menglian Tethyan Ocean

The evolution of the Paleo–Tethys has been a hot topic in geological studies.The Changning–Menglian Suture Zone in southwest China is one of the most comprehensive suture zones in the world which preserve the volcanic–sedimentary records of the Paleo–Tethys.It is the main suture zone of the Paleo–Tethys as well as the principal boundary between the Gondwana and Asian Hun superterrane.Whether the evolution of the Changning–Menglian Tethyan Ocean is thoroughly studied directly affects the understanding of scientific problems such as the relationship between the Proto and Paleo Tethys,and the mechanisms of rifting and drifting of the microplates surrounded the Gondwana.However,these significant tectonic events such as the opening,subduction and closure were not well defined due to the restrictions of research methods and materials.The Changning–Menglian Suture Zone has been divided into three sub–zones: the west,central and east sub-zones.The west sub-zone composed sandstone,shale,and bedded chert deposited on the eastern continental margin of the Baoshan Block.The central sub-zone comprised pelagic radiolarian chert,shale,sea-mountain sequence and ophiolite.The east sub-zone is represented by the sandstone and argillaceous deposited on the western continental margin of the Lincang Block.In this dissertation,clastic rocks within the suture zone are studied to reveal the sedimentary response of the opening,initial subduction and closure of the Changning–Menglian Tethyan Ocean from the perspective of provenance by integrating the methods of petrology,sedimentology,detrital zircon chronology and ocean plate stratigraphy.The main research results are as follows:(1)The Devonian Wenquan Formation in the west–sub zone is a slope facies sedimentation on the eastern passive continental margin of the Baoshan Block.Detrital zircon from the Wenquan Formation show two distinctive age peaks at ～440Ma and～960Ma.Zircons with ～440Ma have a wide range of Lu–Hf isotopic compositions with in-situ εHf(t)values from-11.9 to +7.2.Zircons comprising the age peak at ～440Ma are from the Proto Tethyan magmatic rocks in the east of the suture zone,while zircons forming the age peak at ～960Ma come from the Gondwana.The above results show that the Wenquan Formation is jointly influenced by clastic on both side of continental margins,and the Changning–Menglian Tethyan Ocean is a narrow basin at that time.(2)The Carboniferous–Permian Nanduan–Laba formations in the east sub-zone and Mae Tha Formation–Ngao Group in eastern part of the Chiang Mai–Chiang Rai Suture Zone are continuous depositional sequence which recorded the conversion of the sedimentary basin from the passive continental margin to the active continental margin.Detrital zircons from the Nanduan Formation and the lower Laba Formation are characterized by two age peaks of ～550Ma and ～960Ma,and their provenance are the Lancang Group.Detrital zircons from the middle to lower part of the Laba Formation show a dominant age peak at ～280Ma,and other age groups around 550 Ma and 960 Ma,respectively.Zircons with ～280Ma have a wide range of Lu–Hf isotopic compositions with in-situ εHf(t)values from-12.8t o +7.9.Its provenance are the Jinggu Island Arc and the Lancang Group.Detrital zircons of the Mae Tha Formation are characterized by two major peaks of ～550Ma and ～960Ma,and the provenance is probably the Lancang Group or the Pre-Carboniferous strata on the Sukhothai Block that can be equivalent to the Lancang Group.The conversion from passive continental margin to active continental margin in the Nanduan–Laba formations depositional sequance takes place at least in the middle to lower part of the Laba Fomation,around ～285Ma.In the Chiang Mai–Chiang Rai Suture Zone,this transition takes place in the upper part of the Mae Tha Formation,at ～315Ma.The change of the provenance is the response of the eastward subduction of the Changning–Menglian–Chiang Mai–Chiang Rai Tethyan Ocean,and the initial subduction occurs in the Late Carboniferous(～315Ma).(3)Late Permian–Early Triassic sedimentary strata comprising the Papai,Muyinhe and Akabai formations are important materials to reveal the initial collision of the Baoshan and Lincang blocks.Quartz sandstone and lithic sandstone occur alternately in these strata.Detrital zircons from the quartz sandstone are characterized by two major age peaks of ～550Ma and ～960Ma,whose provenance is the Baoshan Block.Detrital zircons from the lithic sandstone show a distinctive peak of ～250Ma,and zircon Hf isotopes of ～250Ma age grains show wide range of εHf(t)values from-24.8 to +10.8.Its provenance is the magmatic rocks formed by eastward subduction of the Changning–Menglian Tethyan Ocean.The tectonic background of the Papai Formation is the Foreland Basin.The development of foreland basin on the Baoshan Block,the appearance of the clastic materials of the active continental margin in the passive continental margin of the Baoshan Block,and the change of provenance in the continuous sedimentary sequence of the remnant ocean basin indicate that the initial collision between the Baoshan and Lincang blocks has occurred,and its time is limited to ～252Ma.Base on available data from predecessors and our study,we argue that the Pre-Devonian is the raising stage.The westward subduction of Yunxian–Menghai Tethyan Ocean led to the development of extensional tectonics setting between the Lincang and Baoshan blocks,and create the conditions for the formation of the Changning–Menglian Tethyan Ocean;The Devonian is the rifting stage.Controlled by the Post Orogenesis of the Proto–Tethyan orogenic belt,the Changning–Menglian sedimentary basin opens as a narrow basin influenced by continental material from both continental margins with limited sedimentary facies variation.The Early Carboniferous is the mature and stable stage,the sedimentary facies variation reached its maximum value in the sedimentary basin at that time.Completed seamount–oceanic island volcano–sedimentary sequence,ophiolite and pelagic facies sedimentary are developed in the sedimentary basin.Both continental margins are passive continental margin,and the initial oceanic crust is formed around the late Late Devonian to Early Carboniferous.The Late Carboniferous to Late Permian is the subduction stage,where the Changning–Menglian Tethyan Ocean subducted eastward.The sedimentary basin of the eastern continental margin has transformed from passive continental margin to active continental margin.The initial subduction occurs in the Late Carboniferous,around315 Ma.The Early Permian to Middle Triassic is the closure stage,and the initial collision between the Baoshan and Lincang blocks is happened in the late Changxingian of the Late Permina(～252Ma),and fully amalgamate in the Middle Triassic.