Geological Characteristics And Tectonic Evolution Of Ophiolitic Melange In Litang,Sichuan Province

The Ganzi-Litang combination belt is a significant segment of the Sanjiang Tethys orogenic belt and the Songpan-Ganzi orogenic belt,as well as the place where the Yidun island arc and the Yangtze plate subducted and collided.In this paper,the ophiolitic melange in central part of the northwestern Sanjiang area is selected as the research object with the guidance theory of ocean plate stratigraphy to divide the oceanic plate stratigraphic system in the Ganzi-Litang junction zone.The Litang ophiolite melange group in the middle of the survey area is disintegrated into the Triassic Samalongwa Formation?Tsm?,the Triassic Yicelongwa Formation?Tyc?,and the Permian Cunge Formation?Pcg?,the Permian-Triassic Caganglongwa Formation?PTc?,the Triassic Dasenlongwa Formation?Tds?,and the Ordovician Douwuba Formation?Odw?.Combined with the field geological investigation,the basite and ultrabasite of the Caganglongwa Formation and the Dasenlongwa Formation are determined by petrology,petrochemistry and chronology to analyze the formation environment of the rock and discuss the tectonic evolution of the ophiolitic melange in the area.The basite and ultrabasite rock,such as olivine pyroxenite,basalt and gabbro in the Caganglongwa Formation?PTc?,are rich in Fe,Mg and sharply depleted in Na,K according to the main-micro analysis.Meanwhile the total amount of rare earth(18.64×10^-6180.45×10^-6)vary greatly.The normalized rare earth distribution curve of chondrite shows two distribution patterns:?1?light rare earth is slightly enriched,and the curve is slightly rightward.?La/Yb?_N=2.13³.42,the total amount of rare earth is higher;?2?The light rare earth is slightly enriched,while some is slightly deficient,and the curve is relatively flat.?La/Yb?_N=0.82².68,the total amount of rare earth is low.Backed by the structural discriminant analysis,the rocks are caused by Mid-Ocean Ridge Basalt?MORB?environment,but may be contaminated by different degrees of island arc basalt?OIB?environmental substances.When it comes to the main-micro analysis of basic rocks,such as basalt,diabase and plagioclase in the Dasenlongwa Formation?Tds?,they are rich in Fe,Mg and Ti.The larger of range of MgO content,the higher the total amount of rare earth(121.05×10^-6312.35×10^-6).The normalized rare earth distribution of chondrite is slow right-dip curved.The phenomenon of light and heavy rare earth fractionation is clear,?La/Yb?_N=3.27¹1.70,without obviously Eu abnormality.Large ion lipophilic elements,such as Rb,Ba and K are relatively enriched,while high-field strength elements,such as Nb,Ta and Hf are poor.According to the structural discriminant analysis,the rock is formed within the Within Plate Basalt?WPB?environment.The basalt zircon U-Pb age is 238.8±5Ma,indicating that the rift existed in the Middle Triassic.During the Late Paleozoic Devonian period,the western margin of the Yangtze block was cracked and the ocean basin continued to develop.The area may be the earliest in the Carboniferous to form the Litang ocean basin.The deep water basin was developed during the Middle Triassic period,and the formation of volcanic rocks in the middle oceanic ridge of the Caganglongwa formation was deposited.The study newly discovered that during the Triassic period,the eastern margin of the Litang ocean basin re-expanded and developed the Dasenlongwa Formation?Tds?continental margin rift volcano-deposition combination,which may directly lead to the split of Douwuba Formation?Odw?from the western edge of the Yangtze.