| Magma oxygen fugacity computation based on zircon trace element compoistion has been studied more than 16 years.Though this method has been widely used,the computed results were always distributing in very wide ranges without any reasonable explanation.In this study,we collected magmatic rock samples from Zhe-Gan-Wan,Xiaoqinling and Beijing West Mountain regions,and carried out systematic analyse on bulk-rock geochemistry,mineral geological chemistry,and geochronology,in an attempt to investigate the wide-range variation of magma oxygen fugacity computed based on zircon trace element composition.The results suggested that the wide-range variation of magma oxygen fugacity may be related to several factors,including the incorporation of non-native zircon,which may increase the degree of broadness or increase the number of false positive analysises within a specified range of oxygen fugacity;the micro-mineral inclusions in zircon,which may lead to oxygen fugacity underestimation;and the ion-competitive effect of the trace element displacement reaction during zircon crystallization,which may lead to overestimation of oxygen fugacity.Compared with the above factors,the effect of temperature on oxygen fugacity calculation is small.The study also found that the use of traditional chronology,statistics and outliers outliers can not effectively reduce the impact of wide-range varation problem,and the use of average value of the analysis to represent the final oxygen fugacity calculation needs to be considered with great caution.In order to obtain a more ideal oxygen fugacity calculation result,it is necessary to comprehensively consider the U-Pb age of zircon?age concentration?,zircon reflection photo?no micro-mineral inclusion?,spectrum signal curve?light rare earth has no obvious peak?,CL image of zircon?dark part in image?,relationship between?REE+Y?3+and P in zircon??REE+Y?3+/P>1?and Th/U ratio?Th/U?>1),and using the zircon lattice strain model instead of La-Pr interpolation to calculate Ce anomaly and oxygen fugacity.The above efforts may effectively avoid the abnormal analysis of zircons associated with non-native zircon,micro-mineral inclusions,and ion competition effects,thereby obtaining a more accurate calculation of the oxygen fugacity.Based on the above research,a program named ZOFIT used for zircon Ti temperature and oxygen fugacity calculation was written by VBA programming language.The oxygen fugacity of the Cretaceous S-type Ehu granite in the Cretaceous in Jiangxi Province was calculated to be FMQ-0.29.Because Ehu granite was remelted from the crust and has not experienced strong crust-mantle interaction,its low oxygen fugacity characteristics indicate that the oxygen fugacity of magma from the deep mantle source in the Cretaceous period would not increased by crust-mixed process or by crustal material contamination.Therefore,the Cretaceous magma is mostly a reducing magma,which is not conducive to the formation of copper-molybdenum gold deposits. |
PDF Full Text Request