PETROLOGY OF THE GRADATIONAL INTRUSIVE CONTACT BETWEEN THE PACKSADDLE SCHIST AND TOWN MOUNTAIN GRANITE, ENCHANTED ROCK STATE NATURAL AREA, LLANO UPLIFT, CENTRAL TEXAS

The Mesoproterozoic intrusive contact between the ~1.1 Ga Town Mountain Granite and the ~1.3 Ga Packsaddle Schist, exhumed within the Llano uplift, is associated with anorogenic magmatism that occurred during the final stages of the Grenville orogeny. This contact is well exposed along the southeastern margin of the Enchanted Rock batholith at Enchanted Rock State Natural Area in central Texas. Field observations, hand sample and thin section petrography, and handheld X-ray fluorescence (hXRF) analysis of hand samples are used to characterize the gradational nature of this contact, as well as the temperature and depth of emplacement of the Town Mountain Granite. Field observations along the contact indicate variation in igneous rock composition, style of foliation, the degree of contact concordance, and the gradational nature of the contact. The fine- to coarse-grained phaneritic and pegmatitic granitoids in the study area include quartz +K-feldspar + plagioclase ± biotite ± hornblende, and are classified as granite, quartz-rich granitoid, granodiorite, and alkali feldspar granite. The Packsaddle Schist near the contact has gneissic to fine-grained schistose foliation, and contains quartz + muscovite + biotite ± chlorite ± andalusite ± fibrolite. The presence of andalusite and possible fibrolitic sillimanite observed in hand sample is consistent with the prior description of a widespread, low-pressure metamorphic overprint during emplacement of the Town Mountain Granite. The contact between the Town Mountain Granite and the Packsaddle Schist is primarily an injected gradational contact, with complex relationships resulting from the injection of the granite along the foliation planes of the polydeformed Packsaddle Schist. Bulk rock hXRF analyses of hand samples of Town Mountain Granite and Packsaddle Schist will be presented.