Research Of Xialu Rodingites From Xigaze Ophiolite,Southern Tibet

As a special kind of metasomatite,rodingites are common end-members of ophiolites.The processes of rodingitizition frequently occur during the process of ophiolite emplacement,thus,the rodingites are considered to be markers which enable a better chronological evaluation of events with respect to the entire history of ophiolite emplacement.Besides,the element migration of rodingites could reflect different environment(e.g.the fluid composition and P-T conditions)during the metasomatism processes.Therefore,the rodingites are very important and valuable to researchAs the fluid composition continiusly changes during the rondingitization process,the element migration also shows a different behavior for different elements in the different stages.A low pressure,temperature early generation of prehnite rodingites occur along tensile or extensional fractures within the ultramafic massif of the Xialu ophiolitic massif,Xigaze,southern Tibet.Rodingite samples were taken from the center to the margin of a big rodingitized intrusion,showing a variation in bulk compositions,mineralogy and textural propertiesThe mineral assemblages vary from relict clinopyxene+amphibole+clinoziosite+playgioclse+prehnite+ albite+chlorite+titanite+ ilmenite,through relict clinopyxene+new formed late diopside+amphibole+ clinoziosite+prehnite+albite+chlorite+titanite to amphibole+clinoziosite+prehnite+albite+chlorite+titaniteThe analysis of whole-rock chemistry indicates an increase of CaO,decrease of SiO2 and almost a complete removal of Na2O+K2O,as the metasoamtic process proceeded.Mass-balance diagrams show relative enrichments in the large ion lithophile element(LILE)Rb,Cs,Ba and Pb and also Ni.In addition,rare earth elements(REEs),high field strength elements(HFSEs,e.g.Zr,Nb,Ta,Hf,Y)as well as some highly compatible elements as Cr and Sc are decreasing during the early stages of rodingitization.Thermodynamic models suggest the Xialu rodingites occur in a H2O-saturated,CO2-rich,limited calcium increase and low-desilication environment and experienced P-T conditions of<285-300?/3.2 kbar,representing prehnite-pumpellyite facies metamorphism.In such an environment,REEs and HFSEs are soluble in the fluid and were partly removed.In addition,the prehnite rodingite experienced conditions close to the Fe2+/Fe3+redox boundary,but in a relatively reduced environment on the basis of the stable clinozoisite-bearing assemblage of the fo2-T diagram.