大别山榴辉岩中石榴石的结构水:红外光谱分析

WATER IN GARNETS FROM DABIESHAN ECLOGITES:FTIR ANALYSIS

对大别山(包括南大别和北大别)几个典型产地榴辉岩中的石榴石进行了傅里叶变换红外光谱(FTIR)分析,结果显示所有石榴石颗粒都含有一定量的结构水,以OH-的形式存在,结构水含量w(H2O)最高可达1859×10-6。水含量在同一榴辉岩样品不同石榴石颗粒间表现出较大的差异,反映了原岩中水含量的不均一分布。与以前报道的南大别碧溪岭榴辉岩中石榴石情况相似,水含量的不均一性表明:①榴辉岩原岩经历的水-岩交换作用的方式是“隧道式”而非“弥散式”;②超高压变质过程中流体的活动性十分有限;③整个超高压板块俯冲-折返的过程十分快速。南大别石榴石水含量总体上高于北大别,这种差异可能与北大别所经历的高温麻粒岩相变质作用有关。

Garnets in UHP eclogites from several localities in north and south Dabieshan were investigated by Fourier transform infrared spectroscopy （FTIR）. The results demonstrated that all garnet grains contain structural water occurring as hydroxyl （OH） with the content up to 1859 × 10^-6（ H2O wt. ）. Varying water contents were observed among different grains in the same sample, suggesting heterogeneity of water in the eclogites＇ protoliths. Heterogeneous water contents in Dabieshan garnets suggested that the water-rock interaction which the eclogites＇ protoliths experienced was channeled other than pervasive, the mobility of fluids during UHP metan^rphism was very limited, and both the subduction and exhumation processes of UHP rocks occurred in a short time interval. Water contents in garnets from eclogites in south Dabieshan are higher than those in north Dabieshan; this difference is probably related to possible high-temperature granulitefaces retroade metamorphism that occurred in north Dabieshan.