华北新太古代和中生代大陆下地壳中的水:以汉诺坝地区为例

Water in Neoarchean and Mesozoic lower continental crust, North China: A case study from Hannuoba

应用显微傅里叶变换红外光谱技术,对华北克拉通北缘汉诺坝地区的太古宙地体麻粒岩和新生代玄武岩中捕虏体麻粒岩(中生代底侵成因)的主要组成矿物(单斜辉石、斜方辉石、斜长石和石榴子石)进行了详细分析。结果表明,无论是地体麻粒岩还是捕虏体麻粒岩,岩石中的主要组成矿物均含有结构水,主要以OH形式存在。同种产状麻粒岩的矿物水含量(H2O%)均存在较大的变化,而且都远远超出实验误差,表明下地壳内水的分布可能是不均一的。两种产状麻粒岩的矿物结构水含量有着显著的不同,地体麻粒岩中矿物的水含量是捕虏体麻粒岩的2倍以上。太古宙—古元古代形成的安徽女山新生代玄武岩中的捕虏体麻粒岩具有与汉诺坝太古宙地体麻粒岩相似的水含量范围,但明显高于汉诺坝中生代形成的捕虏体麻粒岩。由此看来,早期(太古宙—古元古代)下地壳的水含量要比中生代形成的下地壳高得多,这可能指示不同时代下地壳在形成机制和性质上的不同。

Nominally anhydrous minerals （clinopyroxene, orthopyroxene, plagioclase and garnet） within Archean terrain granuhtes and xenolith granulites captured by Cenozoic basalts from Hannuoba, North China are investigated by Fourier Transform Infrared spectroscopy （FTIR）. The results show that all these minerals can contain some water in the manner of OH. Calculated water contents are highly variable, indicating that the water distribution in the lower crust is very likely heterogeneous. However, the water contents of minerals in terrain granulites are obviously higher, by a factor of -2, than those in xenolith granulites, but similar to those in Nüshan xenolith granulites （Yang et al., 2005）. The water contrast between terrain and xenolith granulites suggests that the early （Neoarehean to Paleoproterozoic） lower crust is probably more hydrous than that formed in Mesozoic, implying that different nature and formation mechanisms are involved for the lower crust built in different times.