下地壳含水性的演化 ——来自不同时代麻粒岩中长石水含量的证据

利用傅里叶变换红外光谱(FTIR)和电子探针(EMP)分析了早古生代的松树沟麻粒岩和桐柏麻粒岩地体以及古元古代的莒南麻粒岩包体(其寄主岩石为新生代玄武岩)中长石的水含量和化学成分。结果显示,麻粒岩中的长石均含有以OH和H2O形式存在的结构水;3个地点的长石水含量分别为465×10-6~733×10-6、210×10-6~993×10-6和717×10-6~1 239×10-6。对比前人报道的中生代(道县和汉诺坝包体)和古元古代(女山包体、汉诺坝地体)的麻粒岩研究结果,发现早古生代样品和古元古代样品中长石的水含量都比中生代样品明显的高,而早古生代和古元古代样品之间却没有差别,指示了中国东部下地壳在中生代之前更加富水。

Temporal variation of water content in the lower continental crust: Evidence from feldspar in mafic granulites

Water contents and chemical compositions of normal anhydrous minerals from Early-Paleozoic Songshugou and Tongbai terrain granulites and Paleo-proterozoic Junan granulite xenoliths were obtained by Fourier transform infrared spectrometer(FTIR) and electron microprobe(EMP) respectively.All granulites characterized by mafic nature were formed under the lowest crustal conditions.Plagioclases in granulites contain trace amounts of water in the form of structurally bound hydroxyl and less molecular water.Water contents(H2O wt.) of plagioclase range from 465×10-6 to 733×10-6 at Songshugou,from 210×10-6 to 993×10-6 at Tongbai,and from 717×10-6 to 1 239×10-6 at Junan,respectively.Combined with previous results for Paleo-proterozoic granulites(Junan and Nushan xenoliths and Hannuoba terrain) and Mesozoic granulites(Daoxia and Hannuoba xenoliths),it seems that water contents in the lower continental crust of eastern China were variable with time.Water contents of Paleozoic and Proterozoic granulites are much higher than those of the Mesozoic granulites and there is no difference between Paleozoic and Proterozoic granulites,implying the existence of a more hydrous lower continental crust of eastern China before Mesozoic.