Thermobarometry of phengite-bearing eclogites in the Dabie Mountains of central China
Thermobarometry of phengite-bearing eclogites in the Dabie Mountains of central China
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DOI：
10.1111/j.1525-1314.1997.00014.x
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发表时间：
1997-03-01
影响因子：
3.4
通讯作者：
Zhai, M
中科院分区：
文献类型：
--
作者：
Carswell, DA;OBrien, PJ;Zhai, M
通过Fe ~（2+）-Mg ~（2+）在石榴子石-绿辉石和石榴子石-多硅白云母对之间的分配，以及反应平衡为绿辉石+镁铝榴石+钙铝榴石=白云母+透辉石，对大别山含多硅白云母榴辉岩峰期变质矿物组合形成的压力-温度条件进行了评价。其中包括一个强压力依赖性的逆Tschermak的分子取代的多硅白云母的程度的评价。对于后一种均衡，沃茨和马丁（1993）指出的校准和推荐的活性-组成模型已经被采用，重要的是，得到的结果与某些样品中柯石英和其他样品中石英在峰值条件下的稳定性的岩相学证据一致。榴辉岩样品峰值硅酸盐矿物组合在围压下达到平衡，围压足以使柯石英稳定（在某些情况下甚至是金刚石），而不是否定了以前的建议，柯石英可能已经稳定在只有非常本地化，可能只是晶内，域。由于矿物中Fe 3 +/Fe 2+比值的不确定性，从矿物相之间的Fe 2 +-Mg 2+配分评价峰变质温度存在固有的困难（特别是绿辉石），并在一些样品中广泛的逆行叠印期间重新平衡，我们的研究结果表明，在南-大别山中部的多硅白云母榴辉岩具有明显不同的平衡条件下，在两个结构不同的构造变质岩。因此，我们的数据不支持早先的论点，大别山中南部包括一个结构连贯的大陆地壳的地幔与区域P-T梯度信号以前最深的俯冲在北部。相反，我们认识到大别山超高压（柯石英榴辉岩轴承）在结构上被南大别山高压（石英榴辉岩轴承）在一个主要的南倾剪切带，沿沿着晚造山伸展塌陷似乎已经消除了至少20公里的地壳部分覆盖。
Pressure-temperature conditions for formation of the peak metamorphic mineral assemblages in phengite-bearing eclogites from Dabieshan have been assessed through a consideration of Fe2+-Mg2+ partitioning between garnet-omphacite and garnet-phengite pairs and of the reaction equilibrium celadonite + pyrope + grossular = muscovite + diopside, which incorporates an evaluation of the extent of the strongly pressure-dependent inverse Tschermak's molecule substitution in the phengites. For the latter equilibrium, the calibration and recommended activity-composition models indicated by Waters & Martin (1993) have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others.Confirmation that in some phengite-eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite (and in some cases even diamond) rather negates previous suggestions that coesite may have been stabilized in only very localized, possibly just intracrystalline, domains. Inherent difficulties in the evaluation of peak metamorphic temperatures from Fe2+-Mg2+ partitioning between mineral phases, due to uncertainties over Fe3+/Fe2+ ratios in the minerals (especially omphacites), and to re-equilibration during extensive retrograde overprinting in some samples, are also assessed and discussed.Our results indicate the existence in south-central Dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes. Thus our data do not support earlier contentions that south-central Dabieshan comprises a structurally coherent continental-crust terrane with a regional P-T gradient signalling previous deepest-level subduction in the north. Instead, we recognize the Central Dabie ultra-high-pressure (coesite eclogite-bearing) terrane to be structurally overlain by a Southern Dabie high-pressure (quartz eclogite-bearing) terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section.