Volcanic-sedimentary Petrological Characteristics And Tectonic Attribute Of Nadigangri Formation In Qiangtang Basin

The tectonic framework and evolution of the Qiangtang Basin in the Late Triassic has long been a controversial issue.It is not only a key period for basin sedimentary transformation but also for tectonic attribute transformation.It is of great significance to correctly understand the evolution of the Paleo-Tethys Ocean and the Middle Tethys Ocean.Nadi Kangri is a set of volcanic-sedimentary rock combination formed in the sedimentary conversion stage of the basin.It has dual attributes of volcanic action and sedimentary strata.A thorough discussion of the tectonic attributes of Nadi Kangri plays a vital role in understanding the tectonic evolution of Qiangtang Basin.In this paper,the Nadi Kangri volcanic-sedimentary rocks are taken as the research object,and systematic petrography,major trace element geochemistry,zircon U-Pb chronology,zircon in-situ Lu-Hf isotope,whole rock Sr-Nd isotope and volcanic sedimentary response are studied to explore the genesis,tectonic background and dynamic mechanism of the Nadi Kangri.The precise SHRIMP and LA-ICP-MS zircon U-Pb chronology of the potential Nadi Kangri volcanic-sedimentary rocks in the Qiangtang Basin has been carried out.The ages of the 13 samples are 214.9±1.5 Ma,222.1±1.9 Ma,210.5±2.1 Ma,227.0±2.3 Ma,216.0±2.7 Ma,206.2±1.8 Ma,233.5±2.5 Ma,202.3±1.6 Ma,221.9±3.4 Ma,221.8±2.1 Ma,221.5±2.6 Ma,221.1±1.5 Ma and 216.4±1.1 Ma.The eruption-deposition age is mainly concentrated in 221²01 Ma and runs through the whole Norian and Rhaetian Age.Rock assemblages are mainly distributed along Wanwanliang-Quemocuo aulacogen,Xiaochaka-Biluocuo aulacogen and Tucuo-Tupocuo aulacogen,which are matched with the rift basement of Nadi Kangri.There are three types of sedimentary overburden:angular unconformity contact with Late Paleozoic fold strata and Xiaochaka,and integration contact with underlying strata.The volcanic-sedimentary rocks of Nadi Kangri are divided into two series of continental eruption and underwater deposition,with various lithofacies combinations such as overflow facies,eruption facies,eruption sedimentary facies,subvolcanic facies,delta tidal flat facies,fluvial facies and lake facies.The bottom conglomerate is divided into five types according to the composition of gravel,degree of rounding and cementation.Volcanic rocks are characterized by bimodal distribution and are mainly composed of two end members:intermediate basalts and acid rhyolite.Sedimentary rocks are mainly tuff mixed with terrigenous clastic rocks and have various transitional volcanic sedimentary clastic rocks.The volcanic rocks of Nadi Kangri are characterized by high Na₂O?2.08%⁸.07%?and low K₂O?0.26%³.05%?.Basalt slightly loses Nb or Ta while acidic rocks strongly lose Nb,Ta and Ti.Basalt has(⁸⁷Sr/⁸⁶Sr)_i=0.7047⁰.7150,?Nd?t?value is-10.42^-3.3,and Nd model age is 1.8Ga on average.The?Hf?t?values of acidic rocks are all negative,ranging from-3.2 to-19.1.The two-stage model age?TDM2?of Hf is 1.6 Ga to 20 Ga,with an average of 1.8Ga.It is shown that basalt has undergone different degrees of crustal contamination,while acid rock is about 1.8Ga old Proterozoic crust remelting,and both have the same crustal source.Presumably,basaltic magma underplating induced remelting of siliceous basement rocks in deep crust and rapid eruption formed basalt-rhyolite bimodal combination.Meantime,granite is basaltic magma staying in the crust and fully melting.At the same time,a small amount of hot mafic magma is injected into cold felsic magma to form dark inclusions and finally invades into rock mass.Basalt is mainly sodium alkaline coarse-grained basalt with a small amount of tholeiite,which has the characteristics of continental basalt and is formed in the continental intraplate extensional background related to rift.The study of volcanic sedimentary cycle shows that Nadi Kangri has at least 7large-scale volcanic eruptions and 4 main eruption peaks,which are divided into 3 main rifting stages.The Fanghu section as a whole reflects a continuous gradual process of gradual shrinkage and disappearance of Qiangtang foreland basin in Late Triassic and gradual opening of rifted basin,from the large set of sandstone in delta phase to the sand-mud combination in tidal flat phase in Nadi Kangri and then to the gravel filling in fluvial phase.After the Qiangtang Basin underwent the orogeny from the end of Middle Triassic to the beginning of Late Triassic,it has been merged with the Eurasia continent in the north.The Paleo-Tethys Ocean was closed.The Middle Tethys Ocean,represented by Bangong Lake and Nujiang Ocean,expanded rapidly in the south during the Norian Period.The Qiangtang Basin experienced large-scale rifting and gradually transformed from the active continental margin to the passive continental margin,forming the Nadi Kangri volcanic-sedimentary rocks with the nature of passive continental margin.