Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif
Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif
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DOI：
10.1016/s0024-4937(03)00097-5
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发表时间：
2003-10-01
期刊：
影响因子：
3.5
通讯作者：
Hermann, J
中科院分区：
文献类型：
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作者：
Hermann, J
结合KCMASH系统实验结果和白片岩岩石学资料，对西阿尔卑斯多拉-迈拉地块俯冲陆壳的峰值变质条件进行了修正。白片岩的结构观测表明，石榴石+辉石+蓝晶石+辉石+/-滑石的峰变质组合来源于蓝晶石+滑石+辉石+液体的反应。在实验确定的成岩网格中，这种反应发生在45千巴以上，温度接近730摄氏度。在较低的压力下，滑石或与正辉石和辉石反应，或与辉石一起反应成黑云母、辉石和蓝晶石。释放出来的液体可能含有等量的水和溶解的花岗岩成分。在早期的研究中未知的白片岩在变质峰条件下的液体成分可能与实验中产生的液体成分非常相似。因此，实验确定的岩石成因网格是估计白片岩峰值变质条件的良好模型。在天然白片岩中测量到的硅= 3.60的白云石，实验确定的白云石硅等条进一步限制了峰值压力接近43 kbar。这些资料均为白片岩达到金刚石相条件提供了证据。根据实验中石榴石和白云石的组成，标定了4蓝晶石+ 3青瓷石= 4钴石+ 3白云石+白云石的无流体平衡，并作为超高压（UHP）变质长岩的地温晴雨表。对于构成白片岩围岩的含石墨偏长岩和蓝晶石-辉云岩榴辉岩，根据该气压计计算出的峰值变质压力约为44.3 kbar，温度与由石榴石-辉云岩测温法估计的750℃相似。因此，整个多拉-迈拉地块的超高压单元极有可能经历了金刚石稳定场的峰值变质条件。虽然石墨在变长岩中很常见，但迄今为止尚未发现钻石。没有变质微金刚石的原因可能是变质作用温度低，变质岩中缺乏自由流体相，在金刚石相条件下停留时间短，导致石墨转化为金刚石的动力学问题。(C) 2003 Elsevier B.V.版权所有
The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite+/-talc originates from the reaction kyanite+talcgarnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at similar to730 degreesC. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H2O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to similar to43 kbar for the measured Si = 3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions.The fluid-absent equilibrium 4 kyanite + 3 celadonite = 4 coesite + 3 muscovite + pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite-phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of similar to750 degreesC estimated from garnet-phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond. (C) 2003 Elsevier B.V. All rights reserved.