摘要
金红石是榴辉岩中的主要含钛副矿物。中国大陆科学钻探工程主孔100-2000m岩心样品中,金红石榴辉岩、多硅白云母榴辉岩和蓝晶石榴辉岩中都程度不等地含有金红石。金红石既可以与其它矿物一起包裹在主要变质矿物中,也可以呈粒间矿物,但在榴辉岩经受角闪岩相退变质作用过程中,金红石亦会退变为榍石。本文利用电子探针除了分析了金红石的主要元素外,还仔细测量了Nb、Cr、Zr含量。结果显示,Nb平均含量为147ppm,最高含量为670ppm,Cr的平均含量为614ppm,最高含量为3630ppm,低Nb特征(<1000ppm)显示榴辉岩原岩为镁铁质岩石;此外,三类榴辉岩也具有不同的金红石Nb、Cr地球化学特征,即金红石榴辉岩中的金红石表现为低Cr(<500ppm)、Nb变化大(0-670ppm)的特征,多硅白云母榴辉岩中的金红石以中等Cr含量(500-1200ppm)、Nb变化较大(0-480ppm)为特征,而蓝晶石榴辉岩中的金红石显著富Cr(2000-3630ppm),而Nb则非常贫乏(<140ppm)。在总共289个金红石Zr含量数据中,大部分Zr含量分布在150-240ppm之间,均值约为200ppm;利用Zacketal.(2004)提出的金红石温度计,计算得到金红石的形成温度介于690℃和7870℃之间。研究结果表明,金红石的微量元素分析是研究榴辉岩原岩特征及其钛成矿作用的实用方法之一。
Rutile is a common Ti-bearing accessory mineral in eclogites. In the 100-2000m borehole samples of the Chinese Continental Scientific Drilling Project, rutile appears more or less in rutile eclogite, phengite eclogite and kyanite eclogite. It is mainly included in major metamorphic minerals, or as intergranular grains between them. When eclogite suffered amphibolite-facies retrograde metamorphism, rutile was often retrograded to produce sphene aureole. Apart from the major components, trace elements such as Nb, Cr and Zr are measured by electron microprobe with specific operating conditions. Rutiles contain on average 147ppm Nb with the maximum 670ppm, while the average Cr content is 614ppm with the maximum value of 3630ppm. Overall, low Nb concentrations in rutile (<1000ppm) indicate a mafic source-rock for the eclogites. Additionally, three types of eclogites are also distinguishable on the basis of their Nb, Cr concentrations. Rutiles from the rutile eclogites display low Cr (<500ppm) and variable Nb contents (0-670ppm), and those from the phengite eclogites are characterized by moderate Cr (500-1200ppm) and also variable Nb (0-480ppm). Rutiles from kyanite eclogites are diagnostic, being highly enriched in Cr (2000-3630ppm) but very poor in Nb (<140ppm). Although Zr is very low in rutile, most of Zr contents range between 150 and 240ppm with an average of 200ppm. The highest Zr contents of each samples is used to estimate formation temperature of rutiles, based on a new Zr-in-rutile geothermometer proposed by Zack et al. (2004) , which vary from 690℃ to 787℃. The study shows that trace-element geochemistry can serve as a potential tool for provenance of eclogites and for estimation of temperature of associated Ti mineralization.