金红石和榍石Zr温度计在新疆西南天山榴辉岩中的应用
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国家重点基础发展研究计划(2015CB856105);国家自然科学基金项目(41272069,41330210)


The application of Zr-in-rutile and Zr-in-titanite geothermometers to eclogites from southwestern Tianshan Mountains, Xinjiang
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    摘要:

    应用LA-ICP-MS方法对新疆西南天山高压-超高压变质带中的榴辉岩及其高压脉体中的金红石和榍石进行了Zr含量的检测和Zr温度计的计算。榴辉岩中位于石榴石幔部且与绿辉石共生的金红石包体Zr含量都集中于10~20 μg/g;而基质金红石的Zr含量为30~50 μg/g,高于包体金红石。榍石均为金红石退变质的产物,且各样品间的榍石Zr含量较均一,都集中在3~5 μg/g之间。脉体金红石Zr含量则与榴辉岩中基质金红石的Zr含量相当甚至偏高一些,为30~60 μg/g。金红石和榍石的Zr温度计研究也表明,榴辉岩石榴石中的金红石包体生长于压力峰期阶段,温压条件为480~540℃、2.7~3.0 GPa;基质金红石随温度增加达到退变质再平衡,记录了温度峰期的条件,约530~590℃、2.4~2.7 GPa;榴辉岩中高压脉体中的金红石则生长于退变质榴辉岩相阶段,金红石Zr温度计给出结果为540~580℃、1.5~2.1 GPa,记录了近等温降压的过程;榴辉岩中的榍石在1.0 GPa左右达到平衡,榍石Zr温度计给出的温度为540~560℃,记录了进一步的近等温降压的过程。根据以上4个阶段的分析结果,得出一个较完整的顺时针p-T轨迹,且与相平衡模拟所限定的p-T轨迹相一致。金红石的Zr含量可以作为压力的指示,表明压力校正在金红石Zr温度计中起到了重要作用。在对金红石和榍石Zr温度计进行应用时,要结合细致的岩相学观察,综合考虑压力、活度、扩散速率、退变质作用和流体影响等方面的因素,才能得到比较精确的温压估算结果和p-T轨迹。

    Abstract:

    LA-ICP-MS analyses of rutile and titanite from eclogites and veins from high pressure-ultrahigh pressure (HP-UHP) metamorphic belt of southwestern Tianshan Mountains in Xinjiang were carried out for testing the Zr content and calculating the Zr-in-rutile and Zr-in-titanite thermometers. Rutile inclusions in garnet mantle, coexisting with omphacite, have a narrower range of Zr values of 10~20 μg/g. The Zr content of matrix rutiles is higher than that of rutile inclusions in garnet, with Zr values being 30~50 μg/g. Titanite is the retrograde product of rutile. Zr values of titanite are homogeneous between individual grains, even among different samples, concentrated between 3 μg/g and 5 μg/g. The Zr concentrations in rutile from the veins are equivalent to those from the eclogites, or even higher. The Zr concentrations of rutile in vein are 30~60 μg/g. Rutile inclusions in garnet mantle grow in the peak pressure metamorphic stage and Zr-in-rutile thermometers gave the estimated temperature of 480~540℃ at 2.7~3.0 GPa. Matrix rutiles can re-equilibrate easily along with the increase of temperature during retrogression, and record the peak temperature condition of 530~590℃, 2.4~2.7 GPa. Rutiles in high pressure veins grow in eclogite facies metamorphism stage, and the growth temperature of vein rutile is 540~580℃, 1.5~2.1 GPa. Titanite in eclogite is the secondary product of rutile, and reaches equilibrium at about 1.0 GPa. Zr-in-titanite thermometry gave the estimated temperature of 540~560℃, recording the further isothermal decompression process. Combined with the analytical results above, the authors constructed a relatively complete clockwise p-T path, which is consistent with the result of phase equilibrium modeling. Zr in rutile from eclogites and veins can serve as a pressure indicator that suggests that the pressure has a significant effect on the Zr-in-rutile thermometry. In the specific case, combined with detailed petrographic observation and consideration of the pressure, the activity of TiO2 and SiO2, diffusion rate, retrograde metamorphism and fluid effect, Zr-in-rutile and Zr-in-titanite thermometers can give an accurate p-T estimate and p-T path.

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张丽娟,张立飞,2016,金红石和榍石Zr温度计在新疆西南天山榴辉岩中的应用[J].岩石矿物学杂志,35(5):840~854. ZHANG Li-juan, ZHANG Li-fei,2016,The application of Zr-in-rutile and Zr-in-titanite geothermometers to eclogites from southwestern Tianshan Mountains, Xinjiang[J]. Acta Petrologica et Mineralogica,35(5):840~854.

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  • 收稿日期:2016-05-24
  • 最后修改日期:2016-06-12
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  • 在线发布日期: 2016-09-23
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