摘要
中国大陆科学钻探工程(CCSD)主孔位于苏鲁超高压变质带南部的东海县毛北村。本文主要研究深度分别为734.21m～737.16m(Ⅰ)和929.67m～932.86m(Ⅱ)两段榴辉岩与片麻岩互层的主孔岩芯,利用激光氟化法和Fourier变换红外光谱技术分别进行矿物的氧同位素组成和结构羟基含量分析。氧同位素分析表明,两段岩芯的榴辉岩中石榴石的δ^(18)O分别为+1.0‰～+3.8‰和-4.1‰～-3.3‰,两段岩芯的片麻岩中石榴石的δ^(18)O均低达-4‰,表明片麻岩和榴辉岩的原岩均在经历超高压变质作用之前一同遭受过古大气降水的高温热液蚀变作用。在第Ⅰ段岩芯的榴辉岩和片麻岩接触界线附近发现氧同位素交换前锋,指示板块折返过程中靠近片麻岩的榴辉岩与主要来自片麻岩的退变质流体发生流体-岩石交换,且其退变质程度主要受流体的迁移距离控制。红外光谱分析研究表明,所有石榴石都含有以OH形式存在的结构水,其含量的变化范围很大,为50～776ppm;绿辉石中OH含量范围为321～636ppm;石英中含有以3596cm^(-1)为特征峰的结构水,但是含量很少。将剖面氧同位素和结构羟基含量的分析结果相结合,我们发现石榴石和绿辉石等矿物的氧同位素组成随着深度的变化与对应矿物的OH含量具有一定的相关性。这证明板块折返过程中的高压退变质作用及其伴生的局部流体活动可能是造成超高压变质矿物之间氧同位素不平衡的主要原因,并且氧同位素不平衡的程度主要受退变质作用发生时的局部水/岩比控制。因此,在深俯冲板块折返过程中,名义上无水矿物中结构水的出溶可以是退变质流体的重要来源。
Two continuous core segments between eclogite and gneiss from the main hole of CCSD project, at the depths of 734.21 to 737.16m (I) and 929.67 to 932.86m (II), respectively, are systematically analyzed for oxygen isotopes by the laser fluorination technique and for hydroxyl (OH) content in nominally anhydrous minerals by the Fourier transform infrared spectroscopy (FTIR). Oxygen isotope analysis shows that garnet from eclogites in the two core segments has delta(18)O values of +1.0parts per thousand to +3.8parts per thousand and -4.1parts per thousand to -3.3parts per thousand, respectively, and that garnet from gneisses in the two core segments has delta(18)O values as low as -4.0. These suggest that protoliths of both eclogite and gneiss experienced meteoric-hydrothermal alteration before continental subduction. An oxygen isotope front is observed in the contact zone between eclogite and gneiss in the core segment I. This is interpreted as a result of fluid-roek interaction between eclogite near the contact zone and fluid mainly from gneiss nearby, which is controlled by fluid transport distance. The FTIR analysis shows concentrations of hydroxyl in 50 to 776 ppm for garnet and in 321 to 1636 ppm for omphacite. For quartz, we have only found the characteristic band of hydroxyl at 3596 cm(-1) but its hydroxyl content is very low. A profile-comparison of oxygen isotope ratio and hydroxyl content shows a consistent change with depth between delta(18)O value and OH content in garnet, omphacite and other minerals. This suggests that retrograde alteration and associated local fluid flow may be the main cause for the observations that the ultrahigh- and high-pressure minerals deviate from oxygen isotope equilibrium fractionations and the degree of disequilibrium is controlled by local water/rock ratios. Therefore, the significant amounts of hydroxyl in garnet and omphacite, together with other nominally anhydrous minerals, may constitute a major source of retrograde fluid by releasing water during decompression exhumation.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2004年第5期1116-1132,共17页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划项目(2003CB716501)资助
