Concentric patterns in the granites of the Llano-Burnet region, Texas

Abstract

The larger bodies of pre-Cambrian coarse-grained granites in the Llano-Burnet uplift of central Texas have been studied with special attention to structure and texture. They are steep-walled, roughly circular massifs averaging 12 miles in diameter. Flow layers or schlieren with parallel schist xenoliths are well developed near the borders. Lineation within the flow layers was not detected. The flow layers follow the outlines of the massifs and usually dip outward at angles of 70 degrees or more. Vertical joints normal to the strike of the flow layers are characteristic of the outer portions of the massifs. Pegmatite and aplite dikes near the borders dip toward the centers of the massifs, suggesting marginal fissure fillings. The granites exhibit a concentric arrangement of three textural varieties. These are, in order of their solidification, an outer coarse-grained granite, an intermediate zone of porphyritic coarse-grained granite, and a core of medium-grained granite. The adjacent textural varieties are mutually gradational indicating simultaneous intrusion. Chemical and petrographic analyses show that the three textural varieties are similar. At a later stage small bodies of fine-grained granite were intruded into the centers of the massifs, around the margins, and in radial dikes.