Textural and chemical evolution of pyroxene during hydration and deformation: A consequence of retrograde metamorphism
Textural and chemical evolution of pyroxene during hydration and deformation: A consequence of retrograde metamorphism
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DOI：
10.1016/j.lithos.2017.11.002
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发表时间：
2018
期刊：
影响因子：
3.5
通讯作者：
Stephen Centrella;A. Putnis;P. Lanari;H. Austrheim
中科院分区：
文献类型：
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作者：
Stephen Centrella;A. Putnis;P. Lanari;H. Austrheim
在挪威西部的卑尔根弧的麻粒岩斜长岩中，厘米级的富铝单斜辉石颗粒受到断层和微剪切带（扭结）的变形，流体沿这些剪切带沿着被引入。单斜辉石（11 wt% Al 2 O 3）以两种不同的方式与变形和水化反应：沿沿着扭折反应生成石榴石（Alm 41 Prp 32 Grs 21）加上含铝量较少的辉石（3 wt% Al 2 O 3），沿沿着解理面反应取代富铝单斜辉石。这些反应仅发生在手标本的水合部分，该水合部分通过限定水合极限的尖锐界面与干燥的未反应的麻粒岩分离。我们使用电子探针微区分析（EPMA）和X-射线映射与电子背散射衍射（EBSD）映射一起调查这两个共生体之间的空间和可能的时间关系。Gresens分析（Gresens，1967）已被用于确定与两个反应相关的质量平衡和局部体积变化。石榴石+低铝单斜辉石的反应引起固相体积的损失，而钙钛矿形成获得体积。应变的变化导致了母体单斜辉石中波状消光的局部变化。EBSD结果表明，石榴子石+低铝单斜辉石的密度增加反应发生在应变最高的地方，而密度降低反应形成远离剪切带EBSD没有显着的应变。相平衡模型表明，剪切带内组合的热力学压力比相同温度范围（~ 650 °C）的整个岩石的压力条件高出> 6 kbar。这一结果表明，岩石内部的应力重新分布可能在决定退变质作用期间发生的反应中起作用。
Centimetre-sized grains of Al-rich clinopyroxene within the granulitic anorthosites of the Bergen Arcs, W-Norway undergo deformation by faults and micro-shear zones (kinks) along which fluid has been introduced. The clinopyroxene (11 wt% Al2O3) reacts to the deformation and hydration in two different ways: reaction to garnet (Alm41Prp32Grs21) plus a less aluminous pyroxene (3 wt% Al2O3) along kinks and the replacement of the Al-rich clinopyroxene by chlorite along cleavage planes. These reactions only take place in the hydrated part of a hand specimen that is separated from dry, unreacted granulite by a sharp interface that defines the limit of hydration. We use electron probe microanalysis (EPMA) and X-Ray mapping together with electron backscatter diffraction (EBSD) mapping to investigate the spatial and possible temporal relationships between these two parageneses. Gresens' analysis (Gresens, 1967) has been used to determine the mass balance and the local volume changes associated with the two reactions. The reaction to garnet + low-Al clinopyroxene induces a loss in volume of the solid phases whereas the chlorite formation gains volume. Strain variations result in local variation in undulose extinction in the parent clinopyroxene. EBSD results suggest that the density-increasing reaction to garnet + low-Al clinopyroxene takes place where the strain is highest whereas the density-decreasing reaction to chlorite forms away from shear zones where EBSD shows no significant strain. Modelling of phase equilibria suggest that the thermodynamic pressure of the assemblage within the shear zones is > 6 kbar higher than the pressure conditions for the whole rock for the same range of temperature (~ 650 °C). This result suggests that the stress redistribution within a rock may play a role in determining the reactions that take place during retrograde metamorphism.