EXPLORING PRIMARY COOLING PHASES AND CHARACTERISTICS OF PSEUDOTACHYLITES FROM THE IKERTÔQ SHEAR ZONE THROUGH EXAMINATION OF MICROLITIC TEXTURES

Pseudotachylites are thin, glassy veins that intersect rocks along fault planes resulting from the frictional melting during seismic or tectonic events. Under high magnification, pseudotachylites display microlitic textures of differing morphologies. The origin of these microlites could be from the primary crystallization of the pseudotachylitic melt, or devitrification of the fault glass after cooling. In this study, we utilized SEM analysis at both Virginia Tech and Montana State to look at vein morphology, and later EPMA and WDS at Concord University to calculate quantitative analyses for plots. These methods provided data regarding chemistry and morphology used to reconstruct the conditions of their formation and subsequent cooling phases. Evidently, the morphology showed that the margins of the veins were predominantly cryptocrystalline to microcrystalline, becoming more dendritic and spherulitic towards the centers. The geochemistry showed that the microlites were primarily composed of plagioclase feldspar, plotting largely in the andesine range indicative of high temperatures (An₃₇ - An₄₇). Plagioclase clasts within the melt and the tonalitic gneiss host rock, however, showed a more sodic chemistry indicative of lower temperatures ((An₃₀−An₃₁) and (An₃₂-An₃₄) respectively). The microlites also displayed a chemical gradient, with the inner portions of the spherulites being Sodium rich and depleting towards the margins, where the chemistry became Calcium dominated. Low values of Potassium within the microlites support devitrification as the mechanism of texturization; however, changes in texture across the vein shows a correlation between microlite growth and vein morphology, indicating that these textures formed during the primary cooling of the melt. Therefore, the contradicting data warrants further research in order to conclude whether these processes are occurring separately or simultaneously.