Material Composition And Tectonic Significance Of The Rumigou Ophiolite Melange In Muli Country, Sichuan Province

The Garze-Litang suture zone is a suture line for the splicing and collision of the Yangtze block and the Yidun Island arc.It is an important part of the southwestern�Sanjiang�composite orogenic belt and represents the ocean basin remains that opened in the Late Permian-Early Triassic,closed in the late Late Triassic.This thesis through the detailed field geological survey and indoor data compilation of the ophiolitic m�lange in the Rumigou area and its surrounding areas,from rock geochemistry,isotope geochronology and tectonic setting set out to,made the following recognition:The Garze-Litang ophiolite m�lange in the area is in fault contact with the eastern Maidilong continental margin and structural debris,multi-phase tectonic activities are developed in the belt and formed various wrinkle deformations,faults and ductile shear zones.Entirety manifest�unordered,partially ordered�.The material composition of the ophiolitic m�lange belt is rock block and matrix.The rock block is composed of dismembered ophiolite,ocean island basalt,limestone rock,etc.The matrix consists of oceanic sediments and terrigenous clastic sediments.According to rock combination,genesis and tectonic deformation,the ophiolitic m�lange can be divided into ocean basin system,ocean island-seamount combination and other units.The geochemical analysis of basalt and gabbro in the ophiolitic m�lange belt shows that the content of MgO,Na₂O and TiO₂ is high,the content of Al₂O₃ and CaO is medium,the content of K₂O is low,and the mostly rock Mg^#is lower than original basalt magma Mg^#,the rock is mainly alkaline,a small amount is calc-alkaline,distributed in sodium-potassium,and partial sodium,entirety belong to potassium-sodium alkaline tholeiitic series;In terms of trace elements,HFSE is enriched in rocks and LILE is depleted,P and Nb are weak negative anomalies,which are similar to the enriched mid-ocean ridge basalt in the Sanjiang area.Nb and Ta losses occur in a small number of rocks,showing the characteristics of island arc magmatic rock;In terms of rare earth elements,the distribution curve is right-dip,showing the characteristics of light rare earth enrichment and heavy rare earths loss,and no obvious Eu and Ce anomalies.Comprehensive analysis,the ophiolite in the area is formed in the mid-ocean ridge environment,The production the oceanic island basalt is affected by the hotspot of the mantle plume.The island arc basalt from the Yidun Island arc formed by the subduction of the oceanic crust and entered the m�lange belt by arc-land collision;The trace and rare earth element characteristics indicate that the basaltic magma forming the oceanic rock is from the enriched mantle,with a small amount of crustal material is mixed,and the mantle plume has a certain influence on the formation of the ocean ridge basalt.Four rock age data were measured through using the LA-ICP-MS zircon U-Pb method to measure year.The basic rocks of 255�2Ma and 249�6Ma are formed in the late Late Permian-early Late Triassic,representing the magmatic activity during the expansion of the ocean basin;The basic rocks of 214�7Ma and 210�2Ma are formed in the middle of the Late Triassic,representing the magmatism associated with the magma activity with respect to the cause of formation of the island arc during the subduction and subsidence period of the oceanic crust;Combined with the age of the rocks obtained by the predecessors in the ophiolitic m�lange belt and the identification of the siliceous radiolaria fossils,the age of the ophiolites in this area was initially determined as the late Late Permian-early Late Triassic.The period from the opening to the closing of the Garze-Litang ocean was limited to the Late Permian-late Late Triassic.According to the contact relationship between the ophiolitic m�lange block and the matrix in this area,the metamorphic deformation characteristics of the rock and the tectonic symbiosis combination developed in this area,and comparing it with the four emplacement mechanisms of the ophiolite,it is considered that the ophiolite emplacement mechanisms of the Rumigou area is angle flow type.Through the above research,the Late Permian subduction with the Jinsha River to the west,the western margin of the Yangtze block occured a cleavage to form the Ganzi-Litang ocean;the Early-Middle Triassic ocean basin continued to expand,the Middle Triassic Latin period-the Late Triassic Carni period reaches the largest range;the Late Triassic Carney in the middle and late-Norley period,the oceanic crust subducts westward,forming the Yidun Island arc magmatic rock belt;Late Triassic Resti-early Early Jurassic,the ocean basin was basically closed,and the Yidun Island arc magmatic rock belt collided with the east side of the Maidilong passive continental margin,and then entered the Yanshan-Hishan period intracontinental convergence,thrust nappe and translational shear stage,Garze-Litang ophiolite m�lange belt is the production of a series tectonic-magmatic activities.Compared and analyzed the Suoluogou gold deposit and the gold deposits?points?distributed in the ophiolitic m�lange belt at the study area.It is considered that the ophiolitic m�lange and later magmatism provide material sources for mineralization,and the ductile shear zone is Mineralization provides access and places.Therefore,the study area has great potential for forming ductile shear zone type gold deposits.