高温高压条件下实验变形石英闪长岩微观结构与熔体特征研究

本文以高温高压条件下石英闪长岩流变实验样品为研究对象,利用偏光显微镜进行微观结构观察,研究了样品在实验温度压力条件下的变形机制与斜长石结构对流变强度的影响;通过透射电镜能谱与电子探针,分析了熔体分布和成分特征,讨论了角闪石脱水熔融的影响因素与脱水熔融对岩石流变的影响。结果表明,随着温度升高,岩石从脆塑性过渡域逐渐向高温位错攀移和动态重结晶为主的塑性域转化。在高温条件下,角闪石出现了脱水与部分熔融,脱水熔融的熔体分布和成分体现出非均匀与非平衡熔融的特点,空间分布上,熔体主要出现在角闪石和黑云母矿物颗粒的边缘以及角闪石和长石颗粒之间的区域内;成分分布上,熔体的成分与参与熔融的矿物成分密切相关。角闪石边缘的熔体和黑云母边缘的熔体具有低硅铝、高铁镁特征,斜长石边缘的熔体具有高硅铝、低铁镁的特征,处于角闪石和斜长石颗粒中间的熔体,其成分间于斜长石与角闪石成分之间。实验中出现的非平衡非均匀部分熔融可以解释混合岩中的浅色体与暗色体的成因,富硅熔体可以形成富硅铝的花岗质岩石,而贫硅富铁镁的熔体可以形成基性岩。角闪石的脱水熔融程度依赖于样品的封闭条件,处于封闭环境的样品,角闪石不易脱水熔融,而处于开放环境时,角闪石脱水熔融显著。拆离断层带及其附近具备这样的开放环境,有利于角闪石发生脱水熔融。实验力学数据和微观结构显示,随机分布的斜长石对岩石强度影响并不明显,但斜长石的长轴方向与最大主应力方向呈大角度相交(近90°)会显著强化岩石的强度,这意味着岩石组构与主应力方向大角度相交或呈垂直方向时,不利于岩石变形和拆离断层的形成,反之,均匀岩石或岩石组构与最大主应力方向小角度相交,有利于岩石的变形,容易发育拆离断层。

Microstructures and melt characteristics of deformed quartz diorite under high temperature and high pressure.

This paper focus on microstructures and deformation mechanism as well as partial melt of dehydrated hornblende in experimental deformed quartz diorite samples. The effect of plagioclase fabric to the rheological strength was studied under optical microscope, and melt distribution and chemical composition were analyzed using microprobe. Microstructure observation reveals that the deformation mechanisms of deformed samples changed from brittle-plastic transition to dislocation climb and dynamic recrystallization with temperature increasing. Under high temperature, partial molten happened accompanied with dehydration of hornblende and the melt is nonequilibrium and heterogeneity both in distribution and chemical composition. Melt appeared around grain rims of dehydrated hornblende and biotite, or in grain boundaries of hornblende and plagioclase. The chemical composition of melts is strongly dependent to minerals which participated in melting. The contents of SiO₂ and Al₂O₃ in melts at the rims of hornblende and biotite are lower and contents of FeO and MgO are higher, but contents of SiO₂ and Al₂O₃ in melts at the rims of plagioclase are higher and contents of FeO and MgO are lower, the chemical composition of melts at grain boundaries of plagioclase and hornblende are between that of two minerals. The phenomenon of nonequilibrium and heterogenous melting could explain forming mechanism of leucosome and mesosome in migmatite, the melt with richer SiO₂ and Al₂O₃ could crystallize to leucogranite, and the melt with poor SiO₂ and Al₂O₃ could crystallize to mafic rocks. Dehydration of hornblende was effected by sealed environments of samples. It is found that hornblende did not dehydrated when samples were packed closely using thin nickel film, but most of hornblende grains dehydrated when samples were not packed, and water from dehydraion sealed to open environments, which implicated that open system in crust, such as detachment faults, could promote dehydration and partial melting of hornblende. The mechanical data and microstructures reveal that rheological strength is obviously higher when orientation of coarse grain plagioclase is nearly perpendicular with max principal stress, which suggest that rocks are hard to deformation when foliation is perpendicular to the orientation of the maximum principal stress, while parallel to foliation, it promotes rock deformation, and favorable for developing detachment fault.