藏南过铝花岗岩中电气石的矿物化学特征及成因意义

讨论了藏南过铝花岗岩中电气石的地质产状、矿物学和矿物化学特征。结果表明:(1)在以氧原子数为24.5计算的化学式中,电气石的(Fe+Mg)/Mg比值在2.32~5.37之间,指示花岗岩和伟晶岩中的电气石均为黑电气石系列,而且属镁电气石—铁电气石系列中的较富铁电气石的成员;(2)电气石的FeO/(FeO+MgO)值高达0.70~0.89,与贫Li花岗岩接近,Al-Al50Fe50-Al50Mg50图解和Fe-Mg-Ca图解投点均位于贫Li花岗岩区,属于贫Li花岗岩有关的电气石;(3)TiO2-MnO/CaO-MgO/FeO三元图解可判定属于第Ⅰ类,即MgO和FeO含量同步消长,且较贫Mg富Fe,而MnO和TiO2含量为异步消长,这与电气石的FeO/(FeO+MgO)值所反映的性质相同;(4)地质产状、矿物学及矿物化学揭示的成因信息表明藏南过铝花岗岩中的电气石为酸性侵入体岩浆期后热液成因。

Mineral Chemistry and Its Petrological Implications in Tourmaline of the Peraluminous Granites, South Tibet

This paper describes in detail the features of mineralogy and mineral chemistry about tourmaline from peraluminous granite in south Tibet(PGST).The analysis results show that(Fe+Mg)/Mg ratio is between 2.32-5.37 in tourmaline.It indicates the tourmaline in granite and pegmatite is iron tourmaline series,belongs to pierrepontite in magnesium tourmaline-iron tourmaline series.FeO/(FeO+MgO)ratio of 0.70-0.89 is similar to the feature of Li-poor granite.The Al-Al50Fe50-Al50Mg50 diagram and the Fe-Mg-Ca diagram spots are also locating in lean Li-poor granite area.TiO2-MnO/CaO-MgO/FeO diagram indicates the tourmaline is as type Ⅰ,in which MgO content is synchronous ebb and flow with FeO,and the tourmaline has low Mg and high Fe content.But MnO is opposed to TiO2 in content,which represent same implication as FeO/(FeO+ MgO)ratio in tourmaline.It indicates the tourmaline in PGST is hydrothermal genesis during post-magmatic stage of acid intrusion with the features of geology mineralogy and mineral chemistry.