A STUDY OF MICROSCOPIC AND MACROSCOPIC FRACTURES IN CRYSTALLINE ROCKS FROM THE LLANO UPLIFT, TEXAS: IMPLICATIONS FOR PROVENANCE STUDIES OF QUARTZ

Scanned cathodoluminescence (SEM-CL) of detrital quartz grains frequently reveals cracks that are filled with non-luminescent quartz. Reported from plutonic rocks by various investigators, they have been attributed to healing of post-magmatic micro-fractures (thermal and tectonic stresses), and possibly to volume change at the a/b transition. In crystalline rocks from the Llano Uplift these cracks are not restricted to plutonic quartz, but occur in metamorphic quartz as well. Thus, the question arises whether they can be used as a reliable indicator of plutonic provenance for detrital quartz grains. Outcrop joint patterns and microscopic SEM-CL fractures of oriented samples show good agreement between dominant fracture directions, indicating that macroscopic and microscopic fractures originated at the same time. Across igneous-metamorphic contacts, outcrops show equivalent fracture directions in both lithologies, suggesting that they originated from the same stress field. Because fracture intensity in metamorphic rocks declines with distance to the igneous-metamorphic contact, we have concluded that these fractures most likely reflect the cooling of plutons and propagated into the surrounding country rock. Hydrothermal circulation related to pluton cooling led to healing of microfractures with non-luminescent (low temperature) quartz. Quartz grains with non-luminescent (black) SEM-CL fractures probably occur wherever hydrothermal fluids percolate through fractured rocks (igneous and subvolcanic intrusions, contact metamorphic zones). Emplacement of these fracture fills does not “erase” pre-existing SEM-CL textures. Textures of metamorphic and volcanic quartz are still readily visible through the overprint. Thus, presence of these fractures in absence of metamorphic and volcanic SEM-CL textures most likely indicates quartz grains of plutonic derivation.