Dry and strong quartz during deformation of the lower crust in the presence of melt
Dry and strong quartz during deformation of the lower crust in the presence of melt
复制标题
在熔体存在的情况下下地壳变形过程中干燥且坚固的石英
DOI：
--
复制
发表时间：
2011
期刊：
影响因子：
--
通讯作者：
E. Ravna
中科院分区：
文献类型：
--
作者：
L. Menegon;P. Nasipuri;H. Stünitz;H. Behrens;E. Ravna
[1]挪威北方塞兰火成岩省麻粒岩相混合片麻岩是在深部地壳剪切过程中，黑云母脱水熔融形成的熔体作用下发生变形的。在570 ~ 520 Ma的陆内裂谷环境中，下地壳底部的大型辉长岩侵入过程中发生了部分熔融和变形。混合片麻岩由高纵横比的富含浅色体的区域和一个浅色体贫乏的石英质残积区域组成。根据同运动矿物组合的热力学模拟，变形发生在T = 760°C-820°C，P = 0.75-0.95 GPa和残余熔体体积分数≤ 5%的条件下。有直接的证据，从显微结构观察，傅立叶变换红外测量，热力学建模，和钛在石英测温的干石英在白细胞体穷人域变形在高差应力（50-100 MPa）的位错蠕变。高应力表现在再结晶晶粒的局部层中石英的小晶粒尺寸（11-17 μm），其中石英中的钛温度测量产生770°C-815°C。干燥而坚固的石英在混合片麻岩中形成了一个承重框架，其中存在0.5%的熔体，但由于它位于孤立的口袋中，因此不控制机械行为。石英的高应力变形叠加了较早的低应力变形，特别是在分离的熔体袋附近。颗粒尺度熔体分布、含水量及分布、石英显微构造的叠加关系表明，石英中的黑云母在位错蠕变变形过程中发生脱水熔融。水分配到隔离的熔体结晶在隔离的口袋，在附近的石英显示出较高的晶内水含量和大的晶粒尺寸。相反，岩石中缺乏白细胞体的区域，由于熔体被移除，变得干燥，从而机械强度更高。大规模的熔体去除将产生足够干燥以具有机械强度的下地壳。应用从湿石英中导出的流动定律来估计下地壳麻粒岩相部分的行为是不合适的。
[1] Granulite facies migmatitic gneisses from the Seiland Igneous Province (northern Norway) were deformed during deep crustal shearing in the presence of melt, which formed by dehydration melting of biotite. Partial melting and deformation occurred during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspect-ratio leucosome-rich domains and a leucosome-poor, restitic domain of quartzitic composition. According to thermodynamic modeling using synkinematic mineral assemblages, deformation occurred at T = 760°C–820°C, P = 0.75–0.95 GPa and in the presence of ≤5 vol % of residual melt. There is direct evidence from microstructural observations, Fourier transform infrared measurements, thermodynamic modeling, and titanium-in-quartz thermometry that dry quartz in the leucosome-poor domain deformed at high differential stress (50–100 MPa) by dislocation creep. High stresses are demonstrated by the small grain size (11–17 μm) of quartz in localized layers of recrystallized grains, where titanium-in-quartz thermometry yields 770°C–815°C. Dry and strong quartz forms a load-bearing framework in the migmatitic gneisses, where ∼5% melt is present, but does not control the mechanical behavior because it is located in isolated pockets. The high stress deformation of quartz overprints an earlier, lower stress deformation, which is preserved particularly in the vicinity of segregated melt pockets. The grain-scale melt distribution, water content and distribution, and the overprinting relationships of quartz microstructures indicate that biotite dehydration melting occurred during deformation by dislocation creep in quartz. The water partitioned into the segregated melt crystallizing in isolated pockets, in the vicinity of which quartz shows a higher intracrystalline water content and a large grain size. On the contrary, the leucosome-poor domain of the rock, from which melt was removed, became dry and thereby mechanically stronger. Melt removal at larger scale will result in a lower crust which is dry enough to be mechanically strong. The application of flow laws derived for wet quartz is not appropriate to estimate the behavior of such granulite facies parts of the lower crust.