CHEMICAL COMPOSITION OF TOURMAILINES FROM THE B-RICH GRANITE AND MIAROLITIC CAVITIES IN YINPING,GUANGXI AND ITS IMPLICATIONS FOR EVOLUTION OF THE MAGMATIC-HYDROTHERMAL SYSTEM

In this paper,mineralogical and geochemical researches on tourmalines from the matrix and miarolitic cavities of the fine granite, which is the latest facies in the Huashan A-type granites,were conducted systematically.The results showed that the tourmalines from the matrix and miarolitic cavities are much similar in chemical composition,characterized by high Fe (2.78-3.44 apfu),low Al(5.62-6.20 apfu),low Mg(0.05-0.12 apfu) and non-existence of Li,F and Cl.The chemical compositions of tourmalines are very close to those of the end-member of schorl in the Fe-Mg diagram.Al3+=Fe3+ and R++R2+=R3++ □are the two major substitutions in tourmalines.In Al-Fe-Mg triangle diagram,tourmalines of magmatic origin and hydrothermal origin are under the schorl-dravite line,indicating that high Fe3+ contents enter into the structure of tourmalines and magma differentiated under a relatively high oxygen fugacity condition.We deduced that hydrous fluid exsolution from magma didn't occur during the evolution of the Huashan A-type granites for plentiful development of the miarolitic cavities, ranging from several to dozens of centimeters in size and deposition of abundant tourmalines in the miarolitic cavities.Because of the lack of evolution for the magmatic-hydrothermal system,the Yinping granitic pluton and/or nearby areas may not be suitable ideal targets for exploration of cassiterite-quartz ore deposits.