Metastability of granulites and processes of eclogitisation in the UHP region of western Norway
Metastability of granulites and processes of eclogitisation in the UHP region of western Norway
复制标题
挪威西部超高压地区麻粒岩的亚稳定性和榴辉岩化过程
DOI：
--
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
H. Austrheim
中科院分区：
文献类型：
--
作者：
A. Wain;D. Waters;H. Austrheim
西部片麻岩地区麻粒岩相低应变包体Flatraket杂岩为榴辉岩相条件下含斜长石组合的亚稳定性提供了极好的例证。柯石英榴辉岩在平地台杂岩上方<；200 m，下方-lt；1 km处，由角闪岩相片麻岩隔开，与普遍的晚造山变形和叠加有关。Flatraket杂岩中的花岗岩在9-11 kbar，700-800°C时达到平衡，早于榴辉岩相变质作用，并在榴辉岩相条件下稳定地保存在干燥的未变形带中。大约5%的杂岩在流体渗入和变形带中转化为榴辉岩，集中在杂岩边缘的岩性接触带上。榴辉化是通过主要的水化反应使斜长石重新平衡和不平衡分解而进行的。水化和无水斜长石的分解反应都与流体的输入有动力学联系。在折返过程中，杂岩发生了更普遍的水化作用，流体渗透与外部片麻岩的脱水有关。杂岩内部榴辉岩相剪切带的平衡温度为20~23 kbar，温度为650~800℃，与柯石英的缺乏和外部高压榴辉岩的平衡条件相一致。如果杂岩所承受的压力与附近柯石英榴辉岩(>； 28 kbar)的压力相等，则表明斜长石和石英具有前所未有的亚稳性。另一方面，在平坦杂岩和超高压榴辉岩之间存在构造破裂，支持西片麻岩地区超高压带构造边界的概念。榴辉岩和角闪岩相变质叠加的分布表明，地壳在深埋藏和折返过程中的反应性强烈地受控于流体的可获得性，并且是原岩的函数。
The Flatraket Complex, a granulite facies low strain enclave within the Western Gneiss Region, provides an excellent example of metastability of plagioclase‐bearing assemblages under eclogite facies conditions. Coesite eclogites are found <200 m structurally above and <1 km below the Flatraket Complex, and are separated from it by amphibolite facies gneisses related to pervasive late‐orogenic deformation and overprinting. Granulites within the Flatraket Complex equilibrated at 9–11 kbar, 700–800°C. These predate eclogite facies metamorphism and were preserved metastably in dry undeformed zones under eclogite facies conditions. Approximately 5% of the complex was transformed to eclogite in zones of fluid infiltration and deformation, which were focused along lithological contacts in the margin of the complex. Eclogitisation proceeded by domainal re‐equilibration and disequilibrium breakdown of plagioclase by predominantly hydration reactions. Both hydration and anhydrous plagioclase breakdown reactions were kinetically linked to input of fluid. More pervasive hydration of the complex occurred during exhumation, with fluid infiltration linked to dehydration of external gneisses. Eclogite facies shear zones within the complex equilibrated at 20–23 kbar, 650–800°C, consistent with the lack of coesite and with the equilibration conditions of external HP eclogites. If the complex experienced pressures equivalent to those of nearby coesite eclogites (> 28 kbar), unprecedented metastability of plagioclase and quartz is implied. Alternatively, a tectonic break exists between the Flatraket Complex and UHP eclogites, supporting the concept of a tectonic boundary to the UHP zone of the Western Gneiss Region. The distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability, and is a function of the protolith.