华南火成黄英岩的特征和成因机理

THE CHARACTERISTICS AND GENETIC MECHANISM OF IGNEOUS TOPAZITES IN SOUTH CHINA

新近在华南发现的黄英岩与富氟花岗岩存在密切的时空联系。黄英岩具典型火成斑状结构。斑晶为黄玉和石英,基质由柱伏-针状黄玉微晶和他形石英组成。相对于花岗岩,黄英岩富SiO,(65.06％-81.12％)和Al_2O_3(13.01％-18.09％);贫MgO和Na_2O_5,强过铝质(A/NKC=3.204-37.313);在地球化学特征上,明显亏损锶、钡,富集锡、钨、铌、氟。锡和部分钨的丰度高于维氏酸性岩平均值1-2个数量级。相关的富氟花岗岩具高I_s(0.7103-0.7460)和δ^(18)O‰(9.5-14.24),低的ε_(Nd)(T)(-4.8--9.4)值,成因上与S型花岗岩相似。石英斑晶中常含羟基化硅酸盐熔体包裹体,由数颗石英加水质流体组成。捕获温度约520℃.

Igneous topazites found recently in South China are closely associated with F-rich granites in time and space. They have a typical igneous porphyritic texture. The phenocrysts are quartz and topaz, while the groundm ass consists mainly of aci-cular or prismatic topaz microlites and anhedral quartz. Compared with granites, topazites are rich in SiO2 (65.06%-81.12% ) and Al2O3 (13.01% -18.09% ) but poor in MgO and Na2O and strongly peraluminous with A/NKC - 3.204-37.313. Geo. chemically, the rocks are evidently depleted in Sr and Ba, but enriched in Sn, W, Nb and F(see Fig. 2). The concentrations of Sn and partial W in the topazites are 1-2 orders of magnitude higher than that in average acid rocks. The F-riched granit es genetically related to the topazites in the area usually have much higher ISr (0.7103-0.7460) and δ518O‰,(9.5-14.24) of quartz, but relatively lower εNd(T)( - 4.8——9.4). It suggests that the topazite is similar to S-type granites in genesis.Hydroxylated silicate melt inclusions, consisting of some grains of quartz and aqueous fluids, have been found in the quartz phenocrysts. The entrapment temperature of these inclusions is about at 520℃.