Limitations of Traditional Thermobarometer in Applications to Low-Temperature Eclogites: A Case Study of UHP Metamorphic Belt in Southwest Tianshan
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摘要: 榴辉岩相变质岩石的温压研究对理解高压-超高压变质带的形成和演化具有重要意义，但西南天山低温榴辉岩运用石榴石-绿辉石（-多硅白云母）温压计计算的压力普遍低于相平衡模拟的结果.为此，在
Zhang et al.（2017） 对含霓辉石榴辉岩研究结果的基础上，对该区域内榴辉岩及其脉体中的绿辉石进行了岩相学和矿物化学的研究，结果表明绿辉石普遍发育环带结构：从核部到边部，Fe3+含量降低，Al含量增加，Fe3+/Al比值的降低对应于霓石含量的降低和硬玉含量的升高.相平衡模拟中硬玉分子等值线的计算结果表明具有最高硬玉含量的边部绿辉石在降压阶段生长.因此，具有最高含量的硬玉组分的绿辉石并不一定代表峰期压力，在应用石榴石-单斜辉石（-多硅白云母）传统温压计时需谨慎，尤其是应用于低温的、具有复杂环带模式的矿物组合时要尤为慎重.Abstract: The temperature and pressure of eclogite facies metamorphic rocks are very important to understand the formation and evolution of HP-UHP metamorphic belt. However, the pressures calculated by the traditional thermobarometer that involves omphacite are generally lower than those of the phase equilibrium modeling, for eclogites from the UHP metamorphic belt of the Southwest Tianshan. Petrographic and mineral chemical analyses on omphacite in eclogites and HP veins are conducted, inspired by the study ofZhang et al.(2017) . The results show that the omphacite is typically zoned. From core to rim, Fe3+ content continuously decreases, and Al content increases in response; the decrease in Fe3+/Al ratio corresponds to a general decline in the aegirine content and increase in the jadeite content. Phase equilibrium modeling shows that the omphacite rim with the highest jadeite content grew during decompression after the pressure peak. Thus, the maximum in jadeite component does not necessarily represents peak pressure, and the conventional thermobarometers should be used with caution, especially for the low-temperature assemblages with complicated zonation patterns.-
Key words:
- traditional thermobarometer /
- pseudosection /
- omphacite /
- low-temperature eclogites /
- Southwest Tianshan /
- petrology
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传统矿物对温压计，例如石榴石-单斜辉石温度计(Ellis and Green, 1979；Powell, 1985；Krogh, 1988；Ravna, 2000)、石榴石-单斜辉石-白云母压力计(Ravna and Terry, 2004)等，以及近二十年来通过变质相平衡模拟估算温压条件(所谓的“视剖面图温压计”，Powell and Holland, 2008)被广泛应用于造山带高压-超高压榴辉岩的研究中，对理解高压-超高压变质带的形成和演化具有重要意义(Zheng et al., 2011).西南天山超高压变质带是目前已知出露规模最大的洋壳岩石深俯冲形成的超高压变质地体(张立飞等，2007；Zhang et al., 2013)，该超高压变质带内广泛发育高压-超高压榴辉岩.有关该变质带榴辉岩的变质温压条件前人报道了大量的研究成果(图 1).
图 1 西南天山超高压变质带榴辉岩变质P-T轨迹(实曲线)和P-T条件(方形阴影)总结Fig. 1. Summary of P-T paths (solid curves) and P-T conditions (shaded squares) derived from eclogites from the Southwest Tianshan UHP belt西南天山超高压变质带中榴辉岩的峰期条件估算范围为470~610 ℃、16~33 kbar(图 1).Gao et al.(1999)根据绿辉石中的硬玉含量，以及出现钠云母而不是蓝晶石和硬柱石，估计峰期变质条件为14~21 kbar、540±50 ℃；Klemd et al.(2002)和Gao et al.(2007)用Grt-Cpx温压计分别得到18~21 kbar、490~570 ℃和19 kbar、500~600 ℃的变质条件；John et al.(2008)、van der Straaten et al.(2008)和Beinlich et al.(2010)利用Grt-Cpx-Ph温压计分别得到530~560 ℃、19 kbar, 650 ℃、21 kbar以及510±30 ℃、21.0±1.5 kbar的温压条件.Li et al.(2012)利用Perple_X相平衡模拟得到的结果是21.8~23.1 kbar、540~565 ℃和22.5~23.5 kbar、580~590 ℃.Du et al.(2014a, 2014b)利用Thermocalc估算了含有硬柱石的榴辉岩的峰期压力条件为23~27 kbar、490~540 ℃，随后伴随热弛豫达到峰期温度条件为550~590 ℃、19~21 kbar.Li et al.(2015)利用Perple_X对互层状的榴辉岩、大理岩、片岩的岩心样品进行了复杂的地质温压计算，得到了一致的P-T轨迹，峰期条件为485~510 ℃、22~24 kbar.Zhang et al.(2002a)根据绿辉石中的Ca-Eskola含量和Grt-Cpx-Ph温压计得到榴辉岩的峰期变质条件为25.7~26.7 kbar、496~598 ℃；Zhang et al.(2002b)利用Thermocalc对含菱镁矿的榴辉岩进行相平衡模拟并结合Grt-Cpx温压计，得到榴辉岩的峰期变质条件为27~28 kbar、525~607 ℃；Zhang et al.(2003)利用Grt-Cpx-Ph温压计得到榴辉岩的峰期变质条件为24.6~28.9 kbar、525~607 ℃.Lü et al.(2009, 2012)分别利用Domino和Grt-Cpx温压计得到含柯石英榴辉岩的峰期变质条件分别为24~27 kbar、470~510 ℃和＞27 kbar、490 ℃；Tan et al.(2017)利用Perple_X和Thermocalc得到含柯石英榴辉岩的峰期变质条件为27.5~31.5 kbar、490~530 ℃.Tian and Wei(2013)和Zhang et al.(2016)利用Thermocalc得到榴辉岩的峰期变质条件分别为29~30 kbar、526~540 ℃和29~32 kbar、500~543 ℃.这些P-T轨迹普遍为压力峰期之后出现的热弛豫现象，随后为近等温降压过程，详见Li et al.(2015)和Tan et al.(2017)的研究.
西南天山石榴石中柯石英包体及其假象的存在表明该变质带确实经历了超高压变质作用(Zhang et al., 2002a, 2002b; Lü et al., 2008, 2009, 2012; Lü and Zhang, 2012; Yang et al., 2013; Zhang et al., 2016；田作林等，2016; Tan et al., 2017).然而，应用传统温压计计算的榴辉岩的峰期压力多位于14~23 kbar、500~600 ℃，与超高压的事实背道而驰；而相平衡模拟的结果可以接近或达到超高压.可见，对于西南天山榴辉岩应用传统温压计计算得到的P-T条件的解释仍存在争议(Klemd et al., 2002; Gao et al., 1999, 2007; John et al., 2008; Beinlich et al., 2010).那么，造成西南天山榴辉岩运用传统温压计计算的压力普遍偏低的原因何在？
最近，Zhang et al.(2017)在西南天山超高压变质带发现了一种特殊的富Fe3+的含霓辉石榴辉岩，其单斜辉石具有3个不同的生长分区.电子背散射图像可以清楚地分辨出自形富霓石的明亮核部，灰色自形的过渡幔部和黑色他形的绿辉石边部.单斜辉石从核部到边部，霓石含量降低，硬玉含量升高.值得指出的是单斜辉石中霓石成分随升温轨迹降低，单斜辉石幔部则对应峰期条件，而具有最高硬玉含量的边部绿辉石却是在降压阶段生长的.据此Zhang et al.(2017)提出了具有最高硬玉成分含量的单斜辉石并不一定代表峰期压力，在应用石榴石-单斜辉石(-多硅白云母)传统温压计时需谨慎，尤其是应用于低温的、具有复杂环带模式的矿物组合时要尤为慎重.但对榴辉岩来说含霓辉石的矿物组合并不常见，且这一认识又与人们传统的认识是不相符的，那么这一认识是否正确，又是否普遍适用于西南天山的榴辉岩，甚至于全球的低温榴辉岩呢？
因此，本文是在Zhang et al.(2017)含霓辉石榴辉岩研究的基础上，对西南天山高压-超高压变质带内榴辉岩中的绿辉石进行了岩相学和矿物化学的研究，并通过视剖面图中硬玉分子等值线计算研究，阐明了传统的石榴石-单斜辉石(-多硅白云母)传统地质温压计在低温榴辉岩应用中的一些挑战，以期对传统温压计应用于低温榴辉岩提供一定的启示.
1. 地质背景与样品描述
西南天山高压-超高压变质带是中亚南天山造山带的重要组成部分，由塔里木板块向伊犁-中天山板块俯冲形成，标志着中亚造山带西部的最终拼合.该变质带呈近北东东-南西西向延展分布，全长约200 km.其北部与中天山地块被中天山南缘断裂隔开，南部与早古生代浅变质碳酸盐地层相接.该变质带出露一套以榴辉岩、蓝片岩、石榴石云母片岩、大理岩为代表的高压-超高压岩石组合，以云母片岩为主体岩石，榴辉岩和蓝片岩呈团块状、透镜状或层状零星产于片岩中(Zhang et al., 2001；张立飞等，2005).榴辉岩主要在该带偏北侧出露，而在南侧和东端的科克苏河一带较少出露.蛇纹岩等超基性岩则较少产出，仅在木扎尔特河一带出露.此外，西南天山高压-超高压变质带高压脉体分布广泛，表明流体活动较为强烈.既有内部流体成因和外部流体成因以及两者共同作用产生的进变质脉体(Gao and Klemd, 2001; Gao et al., 2007; John et al., 2008; Beinlich et al., 2010; Li et al., 2013, 2015)，也有抬升折返阶段形成的退变质脉体(van der Straaten et al., 2008; Lü et al., 2012; Zhang et al., 2016).不同种类脉体的出现表明西南天山高压-超高压变质带流体复杂的成因和演化过程.
本文研究样品与Zhang et al.(2016)以及张丽娟和张立飞(2016)所报道的为同一批样品，即5个榴辉岩及其高压脉体样品对(HB121-8和HB121-8v，HB121-10和HB121-10v，HB121-13和HB121-13v，HB121-21和HB121-21v，HB123-5和HB123-5v).所有榴辉岩均呈斑状变晶结构，矿物组合为：石榴石+绿辉石+绿帘石+白云母+石英+榍石+金红石+磷灰石±黄铁矿±蓝闪石/冻蓝闪石±角闪石±钠长石±绿泥石±碳酸盐.高压脉体切穿榴辉岩，主要由石英+绿辉石±绿帘石±白云母±磷灰石±榍石±金红石组成.详细的样品描述及采样点位见Zhang et al.(2016)、张丽娟和张立飞(2016)的研究.
2. 绿辉石岩相学及主量元素
榴辉岩中的绿辉石主要以半自形-他形细粒(0.01~0.20 mm)的形式出现在基质中，是构成榴辉岩最主要的基质矿物(图 2a, 2c, 2e, 2g, 2i)；另外，绿辉石也以包体形式分布在石榴石、绿帘石、钠云母和(或)多硅白云母斑晶内.脉体中绿辉石多呈粗粒棱柱状，普遍较寄主榴辉岩中的绿辉石粒度大，多为0.5~2.0 mm (图 2b, 2d, 2f, 2h, 2j)；绿辉石散乱地分布在高压脉体中，有的甚至垂直于脉体边缘生长(图 2h).
图 2 榴辉岩和高压脉体中的绿辉石环带背散射照片a.榴辉岩中绿辉石核部呈半自形到他形浅灰色，边部呈他形深灰色(HB121-8)；b.高压脉体中绿辉石颗粒粗大，核部呈自形浅灰色，边部则呈自形-半自形深灰色(HB121-8v)；c.部分绿辉石颗粒保留了成分环带，其他颗粒则已难以区分(HB121-10)；d.石英脉中粗粒绿辉石，边部发育角闪石，环带已不明显(HB121-10v)；e.榴辉岩中绿辉石的核部呈半自形灰色，边部呈他形黑色(HB121-13)；f.石英脉中粗粒绿辉石，核部较高的硬玉含量是由于绿辉石中发育裂隙并包裹石英所致(HB121-13v)；g.退变质作用非常强烈，已很难区分出核-边结构，但仍可分出深色和浅色区域，浅色的绿辉石硬玉含量低，深色的绿辉石硬玉含量高，且越靠近脉体处的绿辉石颜色越深(HB121-21)；h.石英脉中粗粒绿辉石垂直脉体边缘生长，边部发育角闪石，环带已不明显(HB121-21v)；i.少数绿辉石颗粒仍保留成分环带，大多颗粒则已难以区分(HB123-5)；j.绿辉石较为粗大，且发生碎裂，边部退变为角闪石，仍可辨别出核-边结构(HB123-5v).白色数字代表绿辉石硬玉(Jd)组分，且从核部到边部硬玉含量总体升高.矿物简写据Whitney and Evans(2010)：Grt.石榴石；Omp.绿辉石；Rt.金红石；Ttn.榍石；Ph.多硅白云母；Pg.钠云母；Amp.角闪石；Czo.斜黝帘石；Dol.白云石；Cal.方解石；Qz.石英；Zrn.锆石Fig. 2. BSE photographs of omphacite zonations in eclogites and HP veins绿辉石的主量元素分析在北京大学造山带与地壳演化教育部重点实验室JXA-8230型电子探针仪器上完成，加速电压为15 kV，束流为10 nA，束斑直径为2 μm，采用PRZ方法修正，标准样品采用美国SPI公司53种矿物.一般来说，电子背散射图像中的暗色区域对应于相对低的平均原子量，亮色区域对应于相对高的平均原子量.绿辉石代表性主量元素成分见表 1.所有样品均投于绿辉石区域(图 3).绿辉石化学式中Fe3+含量根据电价平衡计算，并通过Na+K-Cr-Al估算，两种方法对多数绿辉石，特别是对于铁含量较高的样品，得到的成分结果未见明显差别(表 1).
表 1 榴辉岩和高压脉体中绿辉石代表性主量元素成分Table Supplementary Table Representative major element compositions of omphcites in eclogites and HP veinsMineral HB121-8 HB121-8v HB121-10 HB121-10v o-c浅 o-r深 o-in-pg o-in-ep o-in-g-r o-c浅 o-r深 o-浅 o-深 o-in-g-m o-c o-r SiO2 55.31 55.20 55.59 54.41 56.27 56.72 58.41 56.07 55.66 55.55 56.40 56.18 TiO2 0.07 0.03 0.05 0 0.03 0.05 0.07 0.04 0.05 0 0 0.05 Al2O3 9.80 11.44 11.82 11.49 11.08 11.16 12.55 11.36 12.31 9.48 10.53 10.91 Cr2O3 0.05 0.12 0 0.09 0.01 0.01 0.09 0 0.03 0.11 0 0.02 FeO 7.21 4.12 3.29 5.05 7.13 6.88 3.57 3.90 2.97 8.75 6.63 5.81 MnO 0 0.01 0 0.04 0.02 0.04 0 0.03 0 0.02 0.03 0.01 MgO 6.82 7.66 8.00 8.71 6.25 6.38 7.29 7.56 7.82 6.69 7.04 6.64 CaO 12.08 12.95 12.98 11.87 10.78 11.15 11.58 12.88 12.65 12.55 11.94 11.49 Na2O 7.46 7.18 7.03 6.39 8.28 8.01 7.69 7.82 7.52 7.79 7.85 7.85 K2O 0.01 0 0 0.04 0.02 0.01 0 0.01 0.03 0.03 0 0.01 Totals 98.80 98.71 98.76 98.10 99.86 100.41 101.25 99.68 99.04 100.97 100.43 98.96 Si 2.001 1.981 1.989 1.971 2.004 2.013 2.027 1.984 1.978 1.971 1.998 2.017 Ti 0.002 0.001 0.001 0 0.001 0.001 0.002 0.001 0.001 0 0 0.001 Al 0.418 0.484 0.499 0.491 0.465 0.467 0.513 0.474 0.516 0.397 0.440 0.462 Cr 0.001 0.003 0 0.003 0.000 0 0.003 0 0.001 0.003 0 0.001 *Fe3+ 0.100 0.049 0.008 0.016 0.097 0.056 0 0.093 0.044 0.195 0.103 0.047 **Fe3+ 0.105 0.012 0 0 0.108 0.085 0.001 0.063 0.002 0.137 0.099 0.084 Fe2+ 0.119 0.074 0.090 0.137 0.115 0.148 0.104 0.023 0.044 0.065 0.094 0.127 Mn 0 0 0 0.001 0.001 0.001 0 0.001 0 0.001 0.001 0 Mg 0.368 0.410 0.427 0.470 0.332 0.337 0.377 0.399 0.414 0.354 0.372 0.355 Ca 0.468 0.498 0.498 0.460 0.411 0.424 0.431 0.488 0.482 0.477 0.453 0.442 Na 0.524 0.499 0.488 0.449 0.572 0.551 0.517 0.537 0.518 0.536 0.539 0.547 K 0 0 0 0.002 0.001 0.001 0 0 0.001 0.001 0 0 Sum 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Jd 41.900 46.500 48.800 46.200 46.900 48.000 54.000 45.800 49.400 36.800 43.800 47.900 Ae 10.000 4.900 0.800 1.600 9.700 5.600 0.000 9.300 4.400 19.500 10.300 4.700 WEE 48.100 48.600 50.400 52.200 43.400 46.400 46.000 44.900 46.200 43.700 45.900 47.400 Mineral HB121-13 HB121-13v HB121-21 HB121-21v HB123-5 HB123-5v o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 SiO2 56.22 56.78 55.30 57.18 55.75 57.54 56.35 56.18 56.18 56.49 56.72 57.65 TiO2 0.12 0.03 0.09 0.06 0.03 0 0.03 0.02 0 0.05 0.05 0.01 Al2O3 10.33 12.61 10.01 12.77 9.43 13.71 10.22 11.54 11.53 11.25 10.04 12.02 Cr2O3 0.02 0 0 0.02 0 0.01 0 0 0.09 0.06 0 0.01 Fe2O3 4.82 0 6.01 0 4.63 0.40 5.47 2.75 2.95 2.23 2.16 2.66 FeO 9.77 1.99 9.94 2.72 9.25 4.62 9.86 7.56 2.80 3.11 6.62 5.70 MnO 0.04 0 0.06 0.01 0 0 0.07 0.04 0.01 0.01 0.06 0.06 MgO 5.30 7.19 5.01 6.51 6.16 5.97 5.15 5.37 9.00 8.33 7.29 6.56 CaO 10.35 11.93 10.04 11.74 12.10 10.55 10.60 10.22 13.96 13.55 12.54 11.30 Na2O 8.43 7.58 8.58 7.74 7.57 8.70 8.54 8.49 7.24 7.38 7.34 8.49 K2O 0.02 0 0 0 0.01 0 0 0 0.01 0.02 0 0 Totals 100.60 98.12 99.03 98.76 100.29 101.11 100.83 99.43 101.11 100.25 100.67 101.81 Si 2.007 2.023 2.004 2.029 2.001 2.008 2.007 2.014 1.963 1.987 2.013 2.005 Ti 0.003 0.001 0.002 0.002 0.001 0 0.001 0.001 0 0.001 0.001 0 Al 0.435 0.530 0.428 0.534 0.399 0.564 0.429 0.488 0.475 0.467 0.420 0.493 Cr 0.001 0 0 0.001 0 0 0 0 0.002 0.002 0 0 *Fe3+ 0.129 0 0.163 0 0.125 0.009 0.145 0.073 0.077 0.059 0.057 0.068 **Fe3+ 0.148 0 0.175 0 0.128 0.025 0.161 0.102 0.014 0.035 0.085 0.080 Fe2+ 0.162 0.059 0.138 0.081 0.152 0.126 0.148 0.154 0.004 0.032 0.140 0.097 Mn 0.001 0 0.002 0 0 0 0.002 0.001 0 0 0.002 0.002 Mg 0.282 0.382 0.271 0.344 0.329 0.310 0.273 0.287 0.469 0.437 0.386 0.340 Ca 0.396 0.455 0.390 0.446 0.465 0.394 0.404 0.393 0.523 0.511 0.477 0.421 Na 0.583 0.524 0.603 0.532 0.527 0.589 0.590 0.590 0.491 0.503 0.505 0.573 K 0.001 0 0 0 0 0 0 0 0 0.001 0 0 Sum 4 4 4 4 4 4 4 4 4 4 4 4 Jd 44.2 52.4 43.2 53.2 40 57.2 43.6 50.2 43.8 45.4 43.3 49.8 Ae 12.9 0 16.3 0 12.5 0.9 14.5 7.3 7.7 5.9 5.7 6.8 WEE 42.9 47.6 40.5 46.8 47.5 41.9 41.9 42.5 48.5 48.7 51.0 43.4 注：o-c.绿辉石核部；o-r.绿辉石边部；o-in-pg.钠云母中的绿辉石包体；o-in-ep.绿帘石中的绿辉石包体；o-in-g-r.石榴石边部的绿辉石包体；o-in-g-m.石榴石幔部的绿辉石包体；深和浅代表背散射图像中的颜色；其中绿辉石化学式中Fe3+含量计算方法：*Fe3+由AX程序根据电价平衡计算(T.J.B.Holland, http://www.esc.cam.ac.uk/astaff/holland/as.html)；**Fe3+通过Na+K-Cr-Al估算. 
图 3 绿辉石成分分类图解自核部向边部绿辉石霓石组分逐渐降低，硬玉组分逐渐升高，底图据Morimoto (1988)Fig. 3. Compositional ternary classification diagram of omphacite与含霓辉石榴辉岩(Zhang et al., 2017)相似，榴辉岩样品中的绿辉石大部分发育成分环带.如样品HB121-8(图 2a)和HB121-13(图 2e)中几乎每颗绿辉石都发育很好的成分环带，背散射图像显示绿辉石较亮核部呈半自形-他形，暗色边部则呈他形，从核部到边部Fe3+含量降低，Al含量增加，Fe3+/Al比值的降低对应于霓石组分的降低和硬玉组分的升高，与含霓辉石榴辉岩中的单斜辉石环带特征极为相似.HB121-10(图 2c)和HB123-5(图 2i)中的绿辉石则只有部分颗粒保留了成分环带，其他则已难以区分.HB121-21则由于退变质作用非常强烈，已很难区分出核-边结构(图 2g)，但仍可分出深色和浅色区域，浅色的绿辉石硬玉含量低，深色的绿辉石硬玉含量高，但仔细观察仍可发现越靠近脉体处的绿辉石颜色越深(图 2g)，可见浅色区域仍然应该为核部，深色区域为边部.包裹在石榴石幔部的绿辉石的硬玉含量与基质绿辉石核部的硬玉含量相当，但较基质绿辉石边部的硬玉含量低；包裹在绿帘石和钠云母中的绿辉石的硬玉含量与基质绿辉石边部的硬玉含量相当(表 1).
高压脉体中绿辉石成分与榴辉岩中的绿辉石成分相似，其硬玉含量甚至普遍较榴辉岩中偏高(表 1).高压脉体中的绿辉石同样发育成分环带，样品HB121-8v(图 2b)和HB121-13v(图 2f)的背散射图像显示绿辉石的核部呈自形-半自形浅灰色，边部则呈半自形-他形深灰色，且从核部到边部Fe3+含量降低，Al含量增加，霓石组分(Ae)降低，硬玉含量(Jd)逐渐升高.样品HB121-13v脉体中绿辉石核部记录了较高的硬玉含量值(图 2f), 这是由于绿辉石中发育裂隙并包裹石英所致.样品HB121-10v(图 2d)、HB121-21v(图 2h)和HB123-5v(图 2j)中的绿辉石由于退变质作用较为严重，边部有角闪石发育，环带已不明显，但从核部到边部总体仍表现出硬玉含量(Jd)有升高的趋势.
3. 绿辉石硬玉等值线
Zhang et al.(2016)中的相平衡研究和岩相学观察均表明，榴辉岩经历了超高压变质作用，压力峰期条件为29~32 kbar、480~540 ℃；随后降压升温达到温度峰期条件560~590 ℃、24~27 kbar.高压脉体则被认为是在榴辉岩抬升折返过程中由硬柱石脱水作用形成的，为内部流体成因，大致的温压条件为19~21 kbar、570~590 ℃.同时，金红石和榍石的Zr温度计研究也表明，榴辉岩石榴石幔部的金红石包体生长于压力峰期阶段，温压条件为480~540 ℃、27~30 kbar；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，~530~590 ℃、24~27 kbar；榴辉岩高压脉体中的金红石则生长于退变质榴辉岩相阶段，在15~21 kbar压力条件下，金红石Zr温度计给出结果为540~580 ℃，记录了近等温降压的过程；榴辉岩中的榍石则为金红石的退变质产物，在约10 kbar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过程(张丽娟和张立飞, 2016).
本文在Zhang et al.(2016)相平衡模拟的基础上叠加了绿辉石的j(o)(=硬玉+霓石组分)等值线(图 4).在含硬柱石的矿物组合(Grt+Gln+Omp+Lws)中，绿辉石j(o)等值线几乎与石榴石的镁铝榴石等值线平行，主要受温度的控制，并随温度的升高而增加；沿降压升温轨迹，j(o)不断增加.而在含绿帘石的矿物组合(Grt+Gln+Omp+Ep+Pg)中，绿辉石j(o)等值线则与石榴石钙铝榴石和镁铝榴石等值线具有相似的斜率，受温度和压力两者共同控制，随温度和(或)压力的降低而降低；沿降压轨迹，j(o)不断降低.
图 4 含柯石英假象榴辉岩样品HB121-10 P-T视剖面图叠加绿辉石j(o)等值线图修改自Zhang et al.(2016)；a.以全岩成分计算的P-T视剖面图；b.以有效全岩成分计算的P-T视剖面图；图中j(o)=硬玉+霓石，以橙色粗实线表示，数值为j47-j58.图中黑色方框里所标数值为样品HB121-10中基质绿辉石实测值；紫色方框所标数值为脉体绿辉石的实测值.矿物简写：Gln.蓝闪石；Lws.硬柱石；Tlc.滑石；Coe.柯石英；Ep.绿帘石；Ky.蓝晶石；Chl.绿泥石；Pl.斜长石；Bt.黑云母；其他矿物简写同上Fig. 4. P-T pseudosection calculated for coesite pseudomorph bearing eclogite HB121-10 contoured with j(o) isopleths4. 讨论
通常，人们用具有最高Mg/(Mg+Fe2+)的石榴石边部成分和具有最高硬玉含量的单斜辉石来估计变质峰期条件(Ravna, 2000; Ravna and Terry, 2004).然而，视剖面分析表明，具有最高硬玉含量的边部绿辉石事实上是在减压阶段生长的(图 4).由于西南天山特有的热弛豫现象，在压力峰期之后有一段降压升温过程，使得绿辉石的j(o)等值线(51.4~56.3)随温度的增加而增加(图 4).因此，这样的榴辉岩中具有最高硬玉分子的绿辉石其实是在减压阶段生长的，并不能代表压力峰期.脉体中的绿辉石也发育同样的成分环带，且脉体绿辉石成分与榴辉岩中的绿辉石成分相似，甚至脉体绿辉石的硬玉含量普遍较榴辉岩中绿辉石的硬玉含量偏高，这也与脉体发育于退变质减压阶段的事实相符合(图 4).而且脉体的绿辉石从核到边硬玉含量依旧升高，并未出现硬玉含量降低的成分环带，也进一步说明脉体的形成压力在20 kbar左右(Zhang et al., 2016).
传统上最高硬玉含量对应于峰期压力的假设是源于斜长石的分解反应(Ab=Jd+Q; Holland, 1980; Ghent et al., 1988).在没有斜长石的矿物组合中，平衡反应Di+Mus=Grs+Pyr+Cel被用作压力计(Ravna and Terry, 2004).欲得到最高的压力值，需要使用最高的Si含量(即白云母中最高的Si含量)与最低的Di含量(即单斜辉石中最低的Di含量).最低的Di含量通常对应于单斜辉石中最高的Jd含量(Ravna and Terry, 2004).而单斜辉石的硬玉含量是一个二维函数：Na/Ca交换[j(cpx)]和Fe3+/Al交换[f(cpx)]，Fe3+/(Fe3++Al)比值不仅受P-T条件的控制，也受全岩成分和矿物组合控制(Zhang et al., 2017).因此，最高的硬玉含量的单斜辉石不能直接对应于压力峰期矿物组合.正如含霓辉石榴辉岩单斜辉石边部绿辉石最高的硬玉含量并不是峰期矿物组合，反而是单斜辉石幔部才是峰期矿物组合(Zhang et al., 2017)；再如本文普通榴辉岩中绿辉石的边部具有最高的硬玉含量，但事实上却是在减压阶段生长的(图 4).因此，传统的石榴石-绿辉石(-多硅白云母)温压计的使用是有一定条件的.
首先，石榴石-绿辉石(-多硅白云母)温压计的准确性严重依赖于单斜辉石中Fe3+含量，尤其是在低温条件(Carswell and Zhang, 1999; Ravna and Terry, 2004; Powell and Holland, 2008；魏春景等, 2009).然而，研究人员很难用电子探针数据精确计算Fe3+/FeT的比值(Sobolev et al., 1999; Schmid et al., 2003; Proyer et al., 2004; Li et al., 2005).对于富霓石的单斜辉石，Fe3+/FeT比值的不准确将导致在P-T计算时出现很大的误差.相反，用视剖面图分析计算得到的P和T的值仅用石榴石成分即可.相关系和石榴石、多硅白云母等值线几乎不受全岩Fe3+/FeT变化的控制，石榴石的成分行为受矿物组合控制，多硅白云母Si含量主要受控于压力(Zhang et al., 2017).
第二，Fe-Mg交换温度计是在600~1 500 ℃下通过高温实验校正得到的，所以低温外推法不可避免会产生一定的误差(Powell and Holland, 2008).而西南天山的榴辉岩均为低温榴辉岩，温度小于600 ℃(Li et al., 2015; Tan et al., 2017)，所以其对西南天山榴辉岩可能并不适用.而且对于硬柱石榴辉岩来说，用传统温压计计算得到的P-T条件通常要明显低于用视剖面图计算得到的结果(Wei and Clarke, 2011).
第三，传统地质温压计的应用需要判断平衡矿物组合.然而在低温岩石中，矿物发育并保留有成分环带，因此平衡矿物组合可能会被矿物生长环带、晶内扩散和(或)退变质改造所掩盖，难以区分(Faryad and Chakraborty, 2005; Powell and Holland, 2008; Wei et al., 2009).在低温岩石中，石榴石、单斜辉石、角闪石等矿物往往发育并保留成分环带，仅仅通过岩相学分析很难判断不同矿物不同环带之间的平衡关系.很显然，对于西南天山的榴辉岩具有最高硬玉含量的绿辉石并不是峰期矿物，而是在减压过程中生长的，没有和峰期石榴石达到平衡.而在视剖面图中，确定变质温压条件可以只用一个矿物的不同等值线交点，故避免了这个问题.
第四，在硬柱石稳定的低温高压-超高压榴辉岩组合中，硬柱石与石榴石之间的平衡控制了钙铝榴石组分含量，表现为随着压力增加或温度降低，硬柱石含量增加，石榴石中钙铝榴石组分降低.因此，在含有硬柱石的榴辉岩中，会由于石榴石贫钙而导致石榴石-绿辉石-多硅白云母(GCP)压力计结果偏低，尤其在低钾的基性岩中，多硅白云母的含量很低，GCP转换反应对石榴石成分的影响也会微乎其微(魏春景等，2009).
利用传统温压计得到的温压条件远低于相平衡模拟得到的温压条件，这种偏差对含硬柱石榴辉岩来说非常普遍(魏春景等，2009; Wei and Clarke, 2011).相对而言，视剖面图增加了温压计算的维度，通过更多信息(如矿物环带，体积分数等)的比较可以给出更一致的P-T限制.但值得注意的是，视剖面图分析也有来自于热力学数据库、矿物活度模型和对模型及全岩体系所作简化而导致的误差，而传统温压计是通过野外或实验数据的拟合而得出，可以在原理上部分避免这些误差.
5. 结论
西南天山榴辉岩及其高压脉体中的绿辉石普遍发育成分环带，从较亮的核部到暗色他形的边部，Fe3+含量不断降低，Al含量不断增加，Fe3+/Al比值不断降低，对应于霓石含量的降低和硬玉含量的升高.结合相平衡模拟中硬玉分子等值线的计算，结果显示具有最高含量的硬玉成分并不一定代表峰期压力.本文以西南天山低温榴辉岩为例，解释了为什么用传统的石榴石-单斜辉石(-多硅白云母)温压计所计算的温压条件普遍较相平衡模拟的结果偏低，并讨论了视剖面分析如何克服了传统温压计的一些限制.在实际应用中，其与传统温压计的相对优劣需要根据具体情况综合考虑.总之，在解释传统温压计时应该谨慎，尤其是对低温的、具有复杂环带模式的矿物组合时要尤为慎重.
致谢: 仅以此文纪念董申保院士诞辰100周年!感谢匿名审稿专家提供的建设性意见! -
图 1 西南天山超高压变质带榴辉岩变质P-T轨迹(实曲线)和P-T条件(方形阴影)总结
Fig. 1. Summary of P-T paths (solid curves) and P-T conditions (shaded squares) derived from eclogites from the Southwest Tianshan UHP belt
图 2 榴辉岩和高压脉体中的绿辉石环带背散射照片
a.榴辉岩中绿辉石核部呈半自形到他形浅灰色，边部呈他形深灰色(HB121-8)；b.高压脉体中绿辉石颗粒粗大，核部呈自形浅灰色，边部则呈自形-半自形深灰色(HB121-8v)；c.部分绿辉石颗粒保留了成分环带，其他颗粒则已难以区分(HB121-10)；d.石英脉中粗粒绿辉石，边部发育角闪石，环带已不明显(HB121-10v)；e.榴辉岩中绿辉石的核部呈半自形灰色，边部呈他形黑色(HB121-13)；f.石英脉中粗粒绿辉石，核部较高的硬玉含量是由于绿辉石中发育裂隙并包裹石英所致(HB121-13v)；g.退变质作用非常强烈，已很难区分出核-边结构，但仍可分出深色和浅色区域，浅色的绿辉石硬玉含量低，深色的绿辉石硬玉含量高，且越靠近脉体处的绿辉石颜色越深(HB121-21)；h.石英脉中粗粒绿辉石垂直脉体边缘生长，边部发育角闪石，环带已不明显(HB121-21v)；i.少数绿辉石颗粒仍保留成分环带，大多颗粒则已难以区分(HB123-5)；j.绿辉石较为粗大，且发生碎裂，边部退变为角闪石，仍可辨别出核-边结构(HB123-5v).白色数字代表绿辉石硬玉(Jd)组分，且从核部到边部硬玉含量总体升高.矿物简写据Whitney and Evans(2010)：Grt.石榴石；Omp.绿辉石；Rt.金红石；Ttn.榍石；Ph.多硅白云母；Pg.钠云母；Amp.角闪石；Czo.斜黝帘石；Dol.白云石；Cal.方解石；Qz.石英；Zrn.锆石
Fig. 2. BSE photographs of omphacite zonations in eclogites and HP veins
图 3 绿辉石成分分类图解
自核部向边部绿辉石霓石组分逐渐降低，硬玉组分逐渐升高，底图据Morimoto (1988)
Fig. 3. Compositional ternary classification diagram of omphacite
图 4 含柯石英假象榴辉岩样品HB121-10 P-T视剖面图叠加绿辉石j(o)等值线
图修改自Zhang et al.(2016)；a.以全岩成分计算的P-T视剖面图；b.以有效全岩成分计算的P-T视剖面图；图中j(o)=硬玉+霓石，以橙色粗实线表示，数值为j47-j58.图中黑色方框里所标数值为样品HB121-10中基质绿辉石实测值；紫色方框所标数值为脉体绿辉石的实测值.矿物简写：Gln.蓝闪石；Lws.硬柱石；Tlc.滑石；Coe.柯石英；Ep.绿帘石；Ky.蓝晶石；Chl.绿泥石；Pl.斜长石；Bt.黑云母；其他矿物简写同上
Fig. 4. P-T pseudosection calculated for coesite pseudomorph bearing eclogite HB121-10 contoured with j(o) isopleths
表 1 榴辉岩和高压脉体中绿辉石代表性主量元素成分
Table 1. Representative major element compositions of omphcites in eclogites and HP veins
Mineral HB121-8 HB121-8v HB121-10 HB121-10v o-c浅 o-r深 o-in-pg o-in-ep o-in-g-r o-c浅 o-r深 o-浅 o-深 o-in-g-m o-c o-r SiO2 55.31 55.20 55.59 54.41 56.27 56.72 58.41 56.07 55.66 55.55 56.40 56.18 TiO2 0.07 0.03 0.05 0 0.03 0.05 0.07 0.04 0.05 0 0 0.05 Al2O3 9.80 11.44 11.82 11.49 11.08 11.16 12.55 11.36 12.31 9.48 10.53 10.91 Cr2O3 0.05 0.12 0 0.09 0.01 0.01 0.09 0 0.03 0.11 0 0.02 FeO 7.21 4.12 3.29 5.05 7.13 6.88 3.57 3.90 2.97 8.75 6.63 5.81 MnO 0 0.01 0 0.04 0.02 0.04 0 0.03 0 0.02 0.03 0.01 MgO 6.82 7.66 8.00 8.71 6.25 6.38 7.29 7.56 7.82 6.69 7.04 6.64 CaO 12.08 12.95 12.98 11.87 10.78 11.15 11.58 12.88 12.65 12.55 11.94 11.49 Na2O 7.46 7.18 7.03 6.39 8.28 8.01 7.69 7.82 7.52 7.79 7.85 7.85 K2O 0.01 0 0 0.04 0.02 0.01 0 0.01 0.03 0.03 0 0.01 Totals 98.80 98.71 98.76 98.10 99.86 100.41 101.25 99.68 99.04 100.97 100.43 98.96 Si 2.001 1.981 1.989 1.971 2.004 2.013 2.027 1.984 1.978 1.971 1.998 2.017 Ti 0.002 0.001 0.001 0 0.001 0.001 0.002 0.001 0.001 0 0 0.001 Al 0.418 0.484 0.499 0.491 0.465 0.467 0.513 0.474 0.516 0.397 0.440 0.462 Cr 0.001 0.003 0 0.003 0.000 0 0.003 0 0.001 0.003 0 0.001 *Fe3+ 0.100 0.049 0.008 0.016 0.097 0.056 0 0.093 0.044 0.195 0.103 0.047 **Fe3+ 0.105 0.012 0 0 0.108 0.085 0.001 0.063 0.002 0.137 0.099 0.084 Fe2+ 0.119 0.074 0.090 0.137 0.115 0.148 0.104 0.023 0.044 0.065 0.094 0.127 Mn 0 0 0 0.001 0.001 0.001 0 0.001 0 0.001 0.001 0 Mg 0.368 0.410 0.427 0.470 0.332 0.337 0.377 0.399 0.414 0.354 0.372 0.355 Ca 0.468 0.498 0.498 0.460 0.411 0.424 0.431 0.488 0.482 0.477 0.453 0.442 Na 0.524 0.499 0.488 0.449 0.572 0.551 0.517 0.537 0.518 0.536 0.539 0.547 K 0 0 0 0.002 0.001 0.001 0 0 0.001 0.001 0 0 Sum 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 Jd 41.900 46.500 48.800 46.200 46.900 48.000 54.000 45.800 49.400 36.800 43.800 47.900 Ae 10.000 4.900 0.800 1.600 9.700 5.600 0.000 9.300 4.400 19.500 10.300 4.700 WEE 48.100 48.600 50.400 52.200 43.400 46.400 46.000 44.900 46.200 43.700 45.900 47.400 Mineral HB121-13 HB121-13v HB121-21 HB121-21v HB123-5 HB123-5v o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 o-c浅 o-r深 SiO2 56.22 56.78 55.30 57.18 55.75 57.54 56.35 56.18 56.18 56.49 56.72 57.65 TiO2 0.12 0.03 0.09 0.06 0.03 0 0.03 0.02 0 0.05 0.05 0.01 Al2O3 10.33 12.61 10.01 12.77 9.43 13.71 10.22 11.54 11.53 11.25 10.04 12.02 Cr2O3 0.02 0 0 0.02 0 0.01 0 0 0.09 0.06 0 0.01 Fe2O3 4.82 0 6.01 0 4.63 0.40 5.47 2.75 2.95 2.23 2.16 2.66 FeO 9.77 1.99 9.94 2.72 9.25 4.62 9.86 7.56 2.80 3.11 6.62 5.70 MnO 0.04 0 0.06 0.01 0 0 0.07 0.04 0.01 0.01 0.06 0.06 MgO 5.30 7.19 5.01 6.51 6.16 5.97 5.15 5.37 9.00 8.33 7.29 6.56 CaO 10.35 11.93 10.04 11.74 12.10 10.55 10.60 10.22 13.96 13.55 12.54 11.30 Na2O 8.43 7.58 8.58 7.74 7.57 8.70 8.54 8.49 7.24 7.38 7.34 8.49 K2O 0.02 0 0 0 0.01 0 0 0 0.01 0.02 0 0 Totals 100.60 98.12 99.03 98.76 100.29 101.11 100.83 99.43 101.11 100.25 100.67 101.81 Si 2.007 2.023 2.004 2.029 2.001 2.008 2.007 2.014 1.963 1.987 2.013 2.005 Ti 0.003 0.001 0.002 0.002 0.001 0 0.001 0.001 0 0.001 0.001 0 Al 0.435 0.530 0.428 0.534 0.399 0.564 0.429 0.488 0.475 0.467 0.420 0.493 Cr 0.001 0 0 0.001 0 0 0 0 0.002 0.002 0 0 *Fe3+ 0.129 0 0.163 0 0.125 0.009 0.145 0.073 0.077 0.059 0.057 0.068 **Fe3+ 0.148 0 0.175 0 0.128 0.025 0.161 0.102 0.014 0.035 0.085 0.080 Fe2+ 0.162 0.059 0.138 0.081 0.152 0.126 0.148 0.154 0.004 0.032 0.140 0.097 Mn 0.001 0 0.002 0 0 0 0.002 0.001 0 0 0.002 0.002 Mg 0.282 0.382 0.271 0.344 0.329 0.310 0.273 0.287 0.469 0.437 0.386 0.340 Ca 0.396 0.455 0.390 0.446 0.465 0.394 0.404 0.393 0.523 0.511 0.477 0.421 Na 0.583 0.524 0.603 0.532 0.527 0.589 0.590 0.590 0.491 0.503 0.505 0.573 K 0.001 0 0 0 0 0 0 0 0 0.001 0 0 Sum 4 4 4 4 4 4 4 4 4 4 4 4 Jd 44.2 52.4 43.2 53.2 40 57.2 43.6 50.2 43.8 45.4 43.3 49.8 Ae 12.9 0 16.3 0 12.5 0.9 14.5 7.3 7.7 5.9 5.7 6.8 WEE 42.9 47.6 40.5 46.8 47.5 41.9 41.9 42.5 48.5 48.7 51.0 43.4 注：o-c.绿辉石核部；o-r.绿辉石边部；o-in-pg.钠云母中的绿辉石包体；o-in-ep.绿帘石中的绿辉石包体；o-in-g-r.石榴石边部的绿辉石包体；o-in-g-m.石榴石幔部的绿辉石包体；深和浅代表背散射图像中的颜色；其中绿辉石化学式中Fe3+含量计算方法：*Fe3+由AX程序根据电价平衡计算(T.J.B.Holland, http://www.esc.cam.ac.uk/astaff/holland/as.html)；**Fe3+通过Na+K-Cr-Al估算. 
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