Quantitative Textural and Microstructural Study of Orthogneiss Deformed at Increasing

Quantitative Textural and Microstructural Study of Orthogneiss Deformed at Increasing

Quantitative textural and microstructural study of orthogneiss deformed at increasing metamorphic grade: contributioon of solid state annealing to metamorphic textures

Pavla Stipska, Ondrej Lexa Karel Schulamann

Institute of Petrology and Structural Geology, Charles University, Albertov 6, 14200 Prague, Czech Republic

Microstructures and textures of feldspars and quartz from naturally deformed orthogneiss were investigated from a nappe pile with inverted Barrovian metamorphic zones. Detailed microstructural and textural work combined with existing PT data allows us to correlate microstructural and textural evolution in orthogneiss sheets with process of continental underthrusting associated with continuous yet slow temperature increase.

The quantitative textural analysis was applied on orthogneiss samples from three metamorphic zones with temperature varying from 500 to 650°C. The SEM backscatter images have been recorded with Camscan instrument and digitised for image analysis. The above procedure was carried out in ArcView GIS environment in order to maintain the correct topology of grain boundaries. The grains (polygons) and grain boundaries (polylines) defined in Arc View environment, have been imported into MATLAB environment where the statistical manipulation and calculation was performed using PolyLX toolbox (Lexa, 2000). This analysis included study of grain size distribution, planimetry, grain shape and grain boundaries preferred orientation and grain contact frequency analysis.

The results of orientation analysis were expressed using rose diagrams where the circular variance and standard deviation were calculated and Rf/ diagrams which represent axial ratios of mineral grains plotted against orientation of their long axis. Measured crystal size distributions (CSD) were used to discuss the kinetics and dynamics of crystallization. The phase distributions of K-feldspar, plagioclase and quartz have been studied using grain contact frequency method in order to examine the deviation from random distribution of contact relation between individual minerals. The effect of rock anisotropy on grain contact distribution was quantified by determination of preferred orientations of like/like and unlike boundaries. These grain boundaries (polylines) were divided into individual linear segments of unequal length, the orientation density of which is evaluated using orientation tensor analysis. The eigenvalues and eigenvectors are further calculated to quantify an intensity and preferred orientations of individual types of grain contacts. The grain shape and grain boundary analysis clearly show decreasing shape preferred orientation of all phases with increasing metamorphic grade. The probability plots of grain size distributions show that in all studied samples the grain size distributions can be best described by lognormal distribution. The log normal plots show increasing grain size and growing contribution of solid state annealing with increasing metamorphic grade. The grain contact frequency plots and eigen-value analysis of grain boundaries indicate increase of randomness and decrease of anisotropy with increasing degree of metamorphism. This microstructural and textural evolution of gneisses is related to mechanisms of continental underthrusting. This process is associated with decreasing flow stress from 110 Mpa to 30 Mpa as estimated using quartz piezometry, constant strain rate 10-13-10-14s-1 and slowly increasing temperature. Our work show continuous increase of solid state annealing contribution with increasing temperature which may modify microstructures and textures associated with main tectonic process.