The Sequences And Structure Significance Of The Qushenla Volcanic-Sedimentary Strata,Gaize,Tibet

Study area is located in the hinterland of the Qinghai-Tibet Plateau,belonging to Gaize of Ali as a the administrative part.In the view of tectonic,the study area is located in the Binding sites south Qiangtang and northern Lhasa terrane.On the basis of regional geological field survey,this paper research the volcanic-sedimentary strata of Qushenla formation(former Nadingcuo formation,Paleogene)which is located in the eastern Gaize,combined with the route field geological survey and measured geological section,rock thin sections,geochemical analysis of samples and rock-age samples.We determine the belonging of volcanic-sedimentary strata,timescale and facies of the rock,then inferred the sedimentary facies and structure settings.In addition,we also analyze the forming of Qushenla volcanic-sedimentary strata and its tectonic significance,according to geochemistry data and previous study of the igneous of the adjacent area,such as Qiangtang terrane and northern Lhasa terrane.We achieve follow understanding:1.By using zircon LA-ICP-MS U-Pb dating test technology,we obtain the age 106.0±0.8 Ma and 106.7±0.7 Ma of the young zircon,indicating that the age of effusive eruption of this igneous should be Late Early Cretaceous.2.Constrained by kinds of reasons,former scientists haven't do deep research for this igneous,as a consequence,it is named Nadingcuo formation of Paleogene.This paper rename is as Qushenla formation according to the dating of U-Pb and the adjacent Qushenla trench.The name also applying for the volcanic-sedimentary strata.3.Based on field reconnaissance of the route,we chose sites that have clear outcrops and helpful to exhibit the strata characteristic as models of geological profile.This paper divide the Qushenla formation into four sections: 1st section is characterized of major basalt and a small number of andesite,with volcanic breccia in lower part,and partly volcanic agglomerate;2nd section have a great number of sedimentary rock,such as sandstone and conglomerate,with a 1 or 2 mezzanines of volcanic;3rd section consist majorly of neutral volcanic breccia,with a small number of basalt and marlstone lenticel;4th section consist majorly of clastic rocks,with a small number of tuff and marlstone,and has a characteristic of upward thickening vertically.4.After building of the sequence of the formation,this paper intend to compare with the similar Meiriqiecuo formation,which finally been confirmed that don't exist according to former study materials.We chose the adjacent Qushenla formation in Bange to compare with.Then we achieved that the volcanic exist almost in every inter-volcanism(volcanic sedimentary period of stagnation),and consist of two great eruption cycles.5.In the aspect of sedimentary,by analysis of the petrologic feature,sedimentary structure and sedimentary sequence,we finally work out the face of volcanic – sedimentary.The 1st section is divided into 22 eruption rhythms and 3 eruption cycles.The first eruption cycle consist of 2 eruption rhythms of eruption face and effusion face.The second eruption cycle consist of volcanic sedimentary face and effusion face.The third eruption cycle predominantly due to basalt with andesite of effusion face,also with zebra limestone with fint,which indicate basin carbonatite sedimentary environment in the inter-volcanism.The 2nd section has a predominantly fluvial face,and several subfacies of riverway and riverbed.The effusion face still exists.The 3rd section is divided into 7 eruption rhythms and 3 eruption cycles,majorly eruption face and effusion face.The 4th section has a whole fluvial face as sedimentary face.6.On the geology background,combined with the analysis of volcanic –sedimentary face,geochemistry data plot and previous study for the structure setting of this area.,we propose that the forming of the volcanic of Qushenla formation attribute to the magma eruption through the continental rift,which is due to the strong extension of the active continental margin after the convergence of Bangonghu-Nu river belt and the Qiangtang terrane.