新疆西南天山哈布腾苏河高压-超高压变质带中榴辉岩和蓝片岩的岩石学及变质演化
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中国地质调查局工作项目 (1212011121276);国家杰出青年科学基金 (40725007);创新研究群体科学基金项目 (40921001);国土资源部公益性行业科研专项项目 (201011034)


Petrologic characteristics and metamorphic evolution of eclogites and blueschists from the high- to ultrahigh-pressure metamorphic belt in Habutengsu Valley, southwestern Tianshan, Xinjiang
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    摘要:

    对西南天山哈布腾苏河一带出露的典型榴辉岩和蓝片岩进行了详细的岩相学、矿物化学和温压条件综合研究。榴辉岩可分为蓝闪石榴辉岩、钠云母榴辉岩、绿帘石榴辉岩和蓝闪石榴角闪岩(退变榴辉岩)4类,蓝片岩可分为含蓝闪石石榴白云母钠长片岩、石榴白云母蓝闪片岩和石榴白云母蓝闪石英片岩3类。新鲜榴辉岩主要矿物组合为石榴石+绿辉石+钠云母+绿帘石,退变榴辉岩则为石榴石+蓝闪石+角闪石;蓝片岩主要矿物组合为石榴石+蓝闪石+多硅白云母+钠云母+钠长石+石英。榴辉岩和蓝片岩中石榴石变斑晶均保存进变质生长环带,从核部到边部XMnXFe降低,XMgXCa升高,指示了升温进变质的演化过程。根据榴辉岩矿物共生组合、石榴石内部包体组合分布特征及传统地质温压计估算结果,确定榴辉岩经历了4阶段的变质演化:早期硬柱石蓝片岩相进变质阶段、峰期榴辉岩相变质阶段(t=543~579℃,p=1.5~1.6 GPa)、峰后绿帘蓝片岩相退变质阶段(t=~450℃,p<1.0 GPa)和晚期蓝闪绿片岩相退变质阶段(t<400℃,p<0.5 GPa)。利用p-T视剖面图计算的榴辉岩、蓝片岩峰期变质温压条件与传统地质温压计估算结果十分相近,其中榴辉岩的峰期变质条件t=520~550℃,p=1.7~1.9 GPa;蓝片岩峰期变质条件t=520~620℃,p=1.7~2.3 GPa。本文估算的榴辉岩峰期变质压力条件与前人根据柯石英的发现而认为研究区部分榴辉岩及其围岩曾经历超高压变质作用的认识明显相悖,原因可能如下:① 后期退变质作用引起研究区榴辉岩全岩成分、矿物化学成分的调整,在采用Grt-Cpx-Phe温压计和以全岩成分为基础的相平衡模拟方法估算峰期温压条件时受到影响,从而使估算峰期压力条件普遍偏低;② 西南天山的榴辉岩可能并非全都经历了超高压变质作用,高压、超高压榴辉岩可能分别代表了不同变基性岩块在不同俯冲深度变质的产物。

    Abstract:

    Eclogites and blueschists are exposed mainly as slices or variable tectonic lenses within garnet phengite schists in Habutengsu area, southwestern Tianshan Mountains, Xinjiang. In this paper, petrography, mineralogical chemistry and p-T conditions of the eclogites and blueschists were studied in detail. The eclogites could be divided into four groups, i.e., glaucophane eclogites, paragonite eclogites, epidote eclogites, and glaucophane-garnet amphibolites (retrograde eclogites), whereas blueschists could be divided into three groups, namely glaucophane-bearing garnet-muscovite-albite schists, garnet-muscovite-glaucophane schists and garnet-muscovite-glaucophane-quartz schists. Major mineral assemblages are Grt+Omp+Pg+Ep for the fresh eclogites, Grt+Gln+Amp for the retrograde eclogites, and Grt+Gln+Phe+Pg+Ab+Qtz for the blueschists. Porphyroblastic garnets show perfect prograde zoning, with XMn and XFe deceasing and XMg and XCa increasing from the core to the rim, which indicates a process of increasing temperature and prograde metamorphism. According to the paragenetic assemblages of inclusions within garnet as well as in matrix, four stages of metamorphism were recognized, viz., pre-peak lawsonite-blueschist facies metamorphic stage, peak eclogite facies stage (t=543~579℃, p=1.5~1.6 GPa), retrograde epidote-blueschist facies stage (t=~450℃, p<1.0 GPa), and post-peak retrograde blueschist-greenschist facies stage (t<400℃, p<0.5 GPa). The p-T pseudosection calculation shows that the peak metamorphic conditions were 520~550℃ and 1.7~1.9 GPa for the eclogites, and 520~620℃ and 1.7~2.3 GPa for the blueschists, similar to the results of traditional geothermobarometer. Previous researchers reported coesites preserved in some eclogites and country rocks, which suggested that partial eclogites and partial country rocks within the southwestern Tianshan metamorphic belt underwent UHP metamorphism, but this conclusion is inconsistent with the results constrained by the eclogites in this study. The possible reasons might be as follows: ① the eclogites were affected by later thermal events, which led to the partial variation of both minerals and whole rock compositions. On the basis of the changed whole-rock compositions, the traditional thermobarometer Grt-Cpx-Phe and phase equilibrium simulations certainly show relatively low pressure results, and ② not all eclogites from the southwestern Tianshan underwent UHP metamorphism, and HP-UHP eclogites may represent the metabasites subducted to different depths.

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施建荣,刘福来,刘平华,孟恩,刘超辉,杨红,王舫,蔡佳,2014,新疆西南天山哈布腾苏河高压-超高压变质带中榴辉岩和蓝片岩的岩石学及变质演化[J].岩石矿物学杂志,33(1):29~50.

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  • 收稿日期:2013-10-20
  • 最后修改日期:2013-12-13
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  • 在线发布日期: 2014-01-20
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