Spherulitic omphacite in pseudotachylytes: Microstructures related to fast crystal growth from seismic melt at eclogite-facies conditions

The microstructures and microtexture of omphacite spherulites from an eclogite-facies pseudotachylyte from central Zambia was studied by electron backscattered diffraction and transmission electron microscopy. The spherulitic growth is characteristic for fast growth rates at high undercoolings resulting from quenching a melt of seismic origin to the eclogite-facies conditions. Its preservation constrains that no significant deformation was present after the seismic event and during uplift. The analysis of grain misorientations in spherulites indicates – in addition to the overall radial alignment of grains – that crystallization took place in a highly viscous medium, which does not allow grain reorientation during growth. The microstructure of omphacite is diverse and ambiguous: Grain contacts appear both curved and with 120° triple junctions with few recrystallized grains. Dislocations are frequent, but heterogeneously distributed and occur in dislocations networks, subgrain boundaries and as free dislocations with locally high densities. Planar defects (chain multiplicity faults parallel (0 1 0) ending at partial dislocations) are abundant, while twinning parallel to (1 0 0) is rare. Anti-phase domains with variable domain sizes within a single grain and the disappearance of domain walls around the planar defects constrain that most of the microstructure must be formed during or shortly after crystallization.Omphacite microstructures are commonly discussed as result of deformational stresses due to a tectonic forcing. Here, we hypothesize that thermal stress during growth also can provide a feasible explanation for this melt-grown omphacite followed with partial recovery under static conditions. The observation does not exclude repeated stress loading of the shear veins in general, but suggest that only the latest seismic event is preserved in the studied samples as fine-grained texture of the spherulites. Moreover, anti-phase domain sizes constrain that the rock must be uplifted rather shortly after the seismic event.     

变质岩中的球状结构及其对变质作用的指示

球状结构是指由纤维状晶体组成的放射状集合体,其为一种高度不平衡的结构。它们常见于火山岩、假玄武玻璃、沉积岩以及热液沉积物中,在不同变质级别的变质岩中也有广泛报道。变质岩中的球状结构记录了丰富的变质作用信息,然而其在变质岩中的成因机制和对变质作用的指示意义尚缺少详细的研究。球状结构的生长需要温度、压力或成分的改变使得结晶体系远离平衡,它的结晶动力学过程受物质的扩散控制,要求晶体生长速率远大于物质扩散速率。详细解析球状结构和分析导致不平衡的因素有助于限定变质作用的条件和过程。本文拟从变质岩中报道的代表性球状结构出发,基于球状结构结晶所需的热力学和动力学条件,总结变质岩中球状结构的四种可能成因机制:流体结晶、变质熔体结晶、冲击变质作用和高度不平衡的变质反应。组成球状结构的纤维状晶体具有大的表面能,在持续保持温压和流体条件不变的情况下会快速重结晶使得球状结构消失,球状结构得以保留意味着其所处的环境要迅速转变为它不再结晶的条件。因此,变质岩中的球状结构指示寄主岩石经历了持续时间很短的非平衡变质作用过程。借鉴其它学科研究球状结构的定量方法和在变质作用条件下开展球状结构的实验研究是变质岩中球状结构研究的潜在方向。