俄罗斯白海地区太古代榴辉岩的金红石Zr温度计应用及其地质意义

摘要

金红石Zr温度计在研究高级变质岩的热演化过程中可以发挥重要的作用.而电子探针显微分析技术得益于其较小的分析束斑(直径1 ～ 2μm)和适中的轰击能量,通过合理的实验条件设定和仪器参数设置,是对薄片中金红石Zr含量进行原位分析的理想实验手段.本文中,我们对俄罗斯白海地区的太古代榴辉岩-退变榴辉岩中的金红石Zr含量进行了电子探针原位分析,并进行了金红石Zr温度计计算.结果表明,其中石榴子石包裹体类型(产状1)金红石的Zr含量比较稳定,主要集中在400×10-6 ～500×10-6范围,个别金红石颗粒中的Zr含量可以达到1000×10-6;而基质后成合晶中(产状2)金红石的Zr含量的波动范围则相对更广一些(200×10-6 ～ 1000×10-6),这可能与后期退变过程中的Zr重置和/或扩散有关.总体上,不同产状金红石的Zr温度计计算结果都给出了两个主要的温度区间:T1=700～ 750℃和T2=800～ 850℃.结合前人对白海榴辉岩变质温压条件的研究以及金红石Zr体系封闭温度的控制,我们认为金红石Zr温度计的计算结果区间T1更有可能代表的是榴辉岩的退变冷却温度,而温度区间T2则反映的是榴辉岩在抬升过程中受到高温麻粒岩相变质作用阶段的温度峰期条件.%The application of Zr-in-rutile thermometer can be very useful in the study of the evolutionary path of high-grade metamorphic rocks.Electron Microprobe (EMP) analytical technique with appropriate settings is a quite optimal method to analyze the Zr content in rutile in thin sections owing to its small beam spot (1 ～2μm diameter) and moderate bombardment energy.In this work,we have completed in-situ EMP analyses of Zr content in rutiles from Precambrian (Archean) Belomorian eclogite and retrogressed (apo-) eclogite samples and applied practical Zr-in-rutile thermometer to estimate the temperature.The results show that the inclusion-in-garnet type rutile (type 1) mainly contains Zr in the peak range of 400 × 10-6 ～ 500 × 10-6 with some exceptions over 1000 × 10-6;while the matrix type rutile (type 2) contains varying Zr in a wide range (200 × 10-6 ～ 1000 × 10-6),which is probably related to the Zr resetting and/or diffusion during retrogression.In despite of the Zr content discrepancy in two different types rutiles,the Zr-in-rutile thermometric calculation yields statically one predominate temperature range of 700 ～ 750℃ (T1) and another subordinate range of 800 ～850℃ (T2).Considering the earlier proposed metamorphic PT conditions of the Belomorian eclogite and possible Zr closure temperature control in rutile,we think the obtained T1 =700 ～ 750℃ is more likely representative of the cooling stage during the retrogression of Belomorian eclogite;meanwhile,the obtained T2 =800 ～ 850℃ may well correspond to the high-temperature granulite-facies overprint stage during the eclogite's exhumation.