Study On The Formation Mechanism Of The Vein With Rutile In The High Pressure Metamorphic Belt Of Southwestern Tianshan,Xinjiang

The high pressure metamorphic rock in orogenic belt is the direct evidence of a series of tectonic geological effects of subduction-collision-exhumation between subduction zone and continental.Furthermore,the metamorphic vein in high pressure metamorphic rocks and the high pressure metamorphic minerals contained in the vein are the records of the fluid under high pressure conditions during this process.High pressure metamorphic belt in southwestern Tianshan in Xinjiang is one of the most important research areas related to subduction of oceanic crust metamorphic evolution.This paper discusses the formation mechanism of the omphacite vein with rutile in metamorphic rock with omphacite vein with rutile in metamorphic rock in southwestern Tianshan as the research object.Mineral combination of the vein includes omphacite,rutile,phengite and quartz,and a small amount of apatite,glaucophane,while the mineral composition of host rock includes garnet,epidote,clinozoisite,omphacite,glaucophane,phengite,rutile,ankerite and a small amount of titanite,quartz,pyrite and zircon.This paper focuses on the study of petrographic observation,geochemical element testing and electron probe testing of the omphacite vein with rutile and the host rock,in order to analyze the microstructure characteristics of the vein,the source of the fluid and the conditions for the formation of the vein.This research indicates that there are three main deformation mechanisms of the vein and host rock.The first is the diffusion mechanism of diffusive substances,including the pressure of fluid participation and the migration of solid state diffusion.The second is the dynamic recovery action,dislocation slip,dislocation climbing and dynamic recrystallization.The third is the micro-fracture deformation mechanism.The content of SiO₂,NaO₂ and P₂O₅ in the vein are significantly higher than that in the host rock,while the content of other major elements are lower.The content of trace elements in the vein is much lower than that of host rock.Also,the content of rare earth elements in the vein is 1-2orders of magnitude lower than that of host rock,which has similar pattern of REE assemblage to the Ocean Island basalts?OIB?.The garnet in the host rock has a growth zoning,while the content of MgO at the edge is slightly reduced,indicating that the edge of the garnet is transformed by the retrograde metamorphism or retrograde metamorphic fluid.The chemical contents of omphacite in the vein,host rock and omphacite inclusion in the garnet are roughly the same,and that the content of jadeite in omphacite in the vein is decreased from the core to the rim.In epidote,from the core to the rim,the content of Fe decrease,the contents of Al and Ca increase.The chemical composition of the rutile in the vein is basically consistent with that of the rutile which is included in garnet in the host rock.The fluid producing the vein proves to be mixed fluid that includes rich HFSE?Ti-Nb-Ta?fluid released from the host rock and rich Th,U and LREE fluid come from external.The temperature and pressure condition of the Pressure Peak period of host rock is:2.2-2.3GPa,600-604?;The temperature and pressure condition of the Temperature Peak period as follows:2.0-2.2 GPa,623-631?.The vein formed in the stage of pressure peak to early stage of retrograde metamorphism,which is composed of high pressure characteristic minerals,such as omphacite and rutile.During the process of the formation of the vein,it has undergone several times of cracks-healing process.The fluid transported through spread flow and channelized flow.The precipitation of the vein is mainly controlled by the change of the geological environment,the chemical action between the fluid and the host rock and the condition of the fluid medium.The vein is subjected to tension perpendicular to the wall,forming fibrous omphacites that is roughly perpendicular to the wall of the vein.