The Formation Mechanism Of The Omphacite Veins In Blueschists From Southwest Tianshan

The subduction channel is a highly reactive interface,and the transfer of matter and the exchange of elements are very active.Fluid is not only the carrier of material transfer,but also the participant of element interaction,which has a major impact on a series of effects during plate subduction.Vein-bearing rocks are regarded as direct evidence of fluid activity in the subduction zone.High-pressure veins are widely developed in eclogites and blueschists in the low-temperature(ultra)-high-pressure metamorphic belt of the southwestern Tianshan in Xinjiang.There are many types of high-pressure veins in the belt,and the causes are complex and diverse,and the source of the veining fluid is different.Therefore,it is a natural laboratory for studying fluid activities.This paper selects the omphacite veins in the blueschist in the study area to study,which can provide a basis for the discussion of the temperature and pressure conditions,metamorphic stages,fluid behavior and properties of the formation of the omphacite veins,and is of great significance to the discussion of the mechanism of vein formation.This paper focuses on the study of the omphacite vein in the blueschists in the Habutengsu River Basin of the HP-UHP belt of southwest Tianshan.The petrological research results show that the omphacite vein has an inequigranular granoblastic texture,and the main minerals are omphacite,quartz,zoisite,garnet and muscovite;the host rock blueschists have a fine granoblastic texture and a massive structure,mainly composed of glaucophane,garnet,muscovite and zoisite;the eclogitic selvage has a fine granoblastic texture between the omphacite vein and the host rock blueschists,and the main minerals are omphacite,zoisite,muscovite,garnet and titanite,which are in abrupt contact with the omphacite vein and transitional contact with the host rock blueschist.In addition,there are three different kinds of garnets in the vein-bearing rock.They are the garnet G_A formed in the metamorphic process,the garnet G_B formed by the internal fluid near the pressure peak period,and the garnet G_C formed by the external fluid during the process of increasing temperature and decreasing pressure respectively.The origins of the omphacite vein and the eclogitic selvage are all related to the external fluid.Therefore,the omphacite vein is formed in the metamorphic stage of increasing temperature and decreasing pressure by combining the phase balance simulation of the host rock blueschist and the compositional characteristics of garnets,and the temperature and pressure conditions of forming the omphacite vein are 22.5?24.8 kar,534?557°C.At the same time,the temperature range of forming the omphacite vein was calculated from 547?565°C by using a traditional G-Cpx thermometer.It is also confirmed that the omphacite vein was formed in the progressive metamorphism stage of temperature rising and pressure lowering.Geochemical research results show that the concentration of Ca,LILE(Large Ion Lithophile Elements),S,P,Li,LREE(Light Rare Earth Elements)and CO2 of the veining fluid is very high,while the concentration of HREE(Heavy Rare Earth Elements)and HFSE(High Field Strength Elements)is very low.The mass balance calculation results show that the concentration of Ca in the selvage exceeds 20%,the loss of LILE in the selvage is 10%?-29%,and the gain of LILE in local host rock is 55%?95%.Except for the fractional distillation of Nb-Ta,the other HFSE(Ti,Zr,Hf)are basically not activated.The gain of LREE in the selvage is 19%?33%,and the gain of LREE in local host rock is 2%?25%;while the gain of HREE is 2%?13%,and the gain of HREE in local host rock is 5%?24%.Through comprehensive analysis,it is found that the formation of the omphacite vein is related to the introduction of Ca-enriched external fluid.Due to the decomposition of water-bearing minerals in the rock,a large amount of fluid is released,but a large amount of Ca precipitates with the crystallization of zoisite to form a Ca-poor fluid,which squeezing the surrounding rock to form small fissures,and activate a large amount of LILE,nearly15%of LREE,and a small amount of HFSE and HREE.These fractures form a channel network,causing external Ca,P,S,Li and nearly 12%LREE fluids to enter the fracture channels to mix with internal fluids.The mixed fluids form veins and interact with surrounding rocks to form a vein-bearing system.