Basalt Characteristics And Tectonic Significance In The Southern Of Garze-Litang Suture Zone

The Garze-Litang suture zone is an important part of the Sanjiang Tethys orogenic belt in the continental crust of China and the Songpan-Ganzi orogenic belt.It is a suture of the collision between the ancient island arc and the Yangtze block in the west of Sichuan.It represents the closure of the Late Permian early three fold and closed late three fold ocean basins.Basalt is an important part of the ophiolites mélange in the Garze-Litang suture zone.Its geological and geochemical characteristics are one of the most important objects in the exploration of the geological tectonic evolution of the suture zone and the Songpan-Ganzi orogenic belt.In this paper,the basalt in the ophiolites mélange,the area of Muli,Daocheng and Litang counties,in the southern part of the Garze-Litang union belt,Sichuan Province,has been studied in detail in the field of field geological survey and indoor comprehensive data.From the aspects of petrology,geochemistry,isotopic chronology and geotectonic background,the composition and composition of the ophiolites mélange are analyzed in detail,and the geological and geochemical characteristics,tectonic environment and the age of formation of basalt in the ophiolites mélange are deeply studied,and the combination of Garze-Litang combination is discussed.The geological tectonic evolution of the southern section.The boundaries of the Garze-Litang ophiolites mélange zone are all fractured,and there are internal faults and ductile shear zones which are consistent with the surrounding structural lines.The belt has very complex material composition,mainly composed of a set of"totally disordered and locally ordered geological bodies.According to its material composition,forming environment and time,it can be divided into several units,such as oceanic crust fragments,foreign rocks?slope-basin margin deposit?,ocean island-seamount assemblage,oceanic arc and ocean basin system.The ultrabasic rocks and basic volcanic rocks,deep-sea clastic rocks and mixed limestone blocks form the Garze-Litang ophiolites mélange belt.The basalt is widely distributed in the Garze-Litang ophiolites mélange belt,which can be divided into oceanic island basalt and middle oceanic ridge basalt based on petrological and geochemical characteristics.Ocean island basalt and thick bedded massive-grain limestone symbiosis,belong to ocean island-Litang melange belt seamounts combination snake.The characteristics of low K₂O,Na₂O,high MgO and Fe O belong to the natrial basalt basalt;the rare earth element content is high and the degree of fractionation of heavy and heavy rare earth is high,which indicates that the mantle rock is produced by the low partial melting of the mantle rock;in the trace element,the large ion stone elements in the rocks change greatly,probably due to later alteration and weathering.The formation of action does not represent the characteristics of primary rocks.The middle ridge basalt is a part of the Garze-Litang ophiolite,which has the characteristics of low TiO₂,K₂O,Na₂O,high MgO and FeO.The REE distribution pattern is similar to the Hawaii and other oceanic island basalts or the middle ocean ridge basalt,which reflects the complexity of the middle ocean ridge basalt.The trace element spider web diagram has the characteristics of Ba,Th and Ta enrichment,and the loss of K and Nb.Comprehensive analysis showed that the oceanic-ocean ridge basalt in the oceanic environment,which belongs to the mid ocean ridge tholeiitic series.The formation of the initial magma of the ocean bottom oceanic ridge basalts may be related to the partial melting of the spinel peridotite,and the formation of the initial magma of the oceanic island-seamount basalt may be related to the partial melting of garnet peridotite.The single zircon U-Pb age of mid ocean ridge basalt is 219.73-222.17Ma and the age of island arc basalt is 219.35±0.45Ma,indicating that the age of the zircons in the island arc basalts was late at the late stage of the world.The radiolarian fossils are obtainedinthesiliceousrocks,whicharePseudostylosphaerasp.and Cryptostephanidium sp.,whose age is the middle three Latino period,which may be a period of intense expansion of the Garze-Litang ocean.The above studies show that the langang ophiolites mélange belt in the area was subducted to the West with the closure of the Lancangjiang ocean and the Jinsha Jiang Yang shell at the end of the Late Permian,and began to form the Garze-Litang basin.In the early-middle three fold,the Garze-Litang ocean expanded rapidly and the middle three fold expanded to the maximum.The late three fold Garze-Litang oceanic crust subducted from east to west,and closed at the end of the late three fold,forming the Yun island arc volcanic zone.In the late Norian-Rhaetian of the late Triassic epoch,the island arc of the Yi Dun volcano collided with the eastern edge of Yajiang on the eastern side,forming a collision orogenic belt,and Garze-Litang ophiolites mélange was the product of this plate collision.