Identification and significance of fluid exsolution in high silica granite
High silica granite is characterized with low content of dark minerals,abundant SiO2,Rb,poor MgO,FeO,Sr,Ba,and enrichment of rare metal elements.Research on it is crucial to understand the petrogenesis of granite,the enrichment of rare metal elements and mineralization process.The petro-graphic and geochemical characteristics indicate that it has undergone a high degree of differentiation evolu-tion.Volatile components such as H2O,as incompatible components,are gradually enriched and eventually saturated in the residual melt,resulting in inevitable fluid dissolution in high silica granitic melts,but how to identify it is difficult.This paper summarizes the evidence and indicators for fluid exso-lution in high silica granite from the perspectives of petrography,whole-rock geochemistry,mineralogy,and metal stable isotopes(Li,Ba,Fe).The appearance of miarolitic structure,snowball texture and uni-directional solidification texture are important petrographic signs of fluid exsolution.In terms of whole-rock geochemistry,extremely low Nb/Ta values(＜5),Zr/Hf values and the tetrad effect of rare earth el-ements are effective identifiers of fluid-melt interaction.In mineralogy,metamictization and LREE enrich-ment of zircon and high Pb content in K-feldspar indicate the involvement of hydrothermal fluids.Com-pared to common granite,high silica granite is usually enriched with heavy Li,light Ba,and heavy Fe iso-topes.Fluid-melt interaction is probably the major factor in isotope fractionation of high silica granite.However,some geochemical evidence remains controversial,so we recommend to use together the various lines of evidence.After the two-stage enrichment process of magma evolution and fluid exsolution,rare metal elements can be extremely enriched in the exsolved fluid and then mineralized.
high silica granitefluid exsolutionmagmatic-hydrothermal transitionmetal stable iso-topesrare-metal mineralization
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