大容山花岗岩带中石榴子石、堇青石的成因研究

GNNETIC STUDIES OF GARNET AND CORDIERITE IN THE DARONGSHAN GRAITE ZONE

该花岗岩带中石榴子石富MgO而贫MnO,并且具正成分环带构造,证明是原岩残留晶;堇青石与残留包体中堇青石成分相似,均属镁堇青石,Na_2O<0.33％,表明不是岩浆晶出矿物,而是岩浆源区残留晶。矿物谱学工作表明,堇青石中赋存原生CO_2和H_2O,某组成是矿物形成温度的灵敏指示剂;另一方面,由于挥发分的存在,“湿”堇青石畸变系数△不能反映硅铝有序度,而红外分裂指数能够及“干”堇青石△可能能够反映硅铝有序度。石榴子石-堇青石组合是理想的地质温压逸度计,它给出了大容山岩体及旧州岩体源区的T、P、f_(H_2O)和P_(H_2O)。

Garnet in the Darongshan granite zone is rich in MgO but poor in MnOwith a girdle structure, indicating that garnet is the residual crystal from theoriginal rocks. Cordierite is similar to that present in residual inclusions, bothbelonging to magnesium cordierite with Na_2O<0.33%, demonstrating that theyare not mineral crystallized from the magma, but are residual crystals in themagma provenance. Spectroscopy of minerals indicates that cordierite contains primary CO_2 andH_2O, and its chemical composition is a sensitive indicator of mineral-for-mingtemperature. On the other hand, due to the presence of volatile components,the deformation coefficient △ for 'wet' cordierite cannot reflect the Si-Al ordering. However, the infrared splitting index and the deformation coefficient for 'dry' cordierite may probably reflect the Si-Al ordering. The garnet-cordi-erite pair is an ideal geothermobarometer, by which the values of T, P, f_(H2O).and P_(H2O) for the source areas where the Darongshan and Jiuzhou batholithsare located.