CHEMICAL VARIATION IN TOWN MOUNTAIN GRANITE AND ASSIMILATION OF PACKSADDLE DOMAIN XENOLITHS ANALYZED BY HXRF, “THE SLAB” SWIMMING HOLE, LLANO UPLIFT, KINGSLAND, TEXAS

Dozens of xenoliths of the ~1.3 Ga Packsaddle Schist are hosted by the ~1.1 Ga Town Mountain Granite in an exceptional exposure within the Llano uplift at “The Slab” swimming hole along the Llano River in Kingsland, Texas. The post-tectonic Town Mountain Granite intruded the polydeformed and polymetamorphosed forearc basin deposits of the Packsaddle Domain during anorogenic magmatism in the wake of the Grenville Orogeny. The xenoliths exhibit a variety of shapes, sizes, textures, and contact styles. The contacts vary from sharp to gradational (permeated and/or injected). Injection of the Town Mountain Granite along foliation planes is likely the primary control on xenolith shape, as is suggested by a strong correlation between the trend of the long axes of each xenolith and the trend of the xenolith’s intersection lineation of foliation on the outcrop surface. The smoothly scoured pavement-style outcrops and their popularity among local residents for playing and wading means that sampling of the xenoliths is both impractical and irresponsible, so a handheld X-ray fluorescence analyzer (hXRF) was deployed to assess compositional variation across the fine-grained xenoliths and the granitic host. Preliminary hXRF analysis discriminates between the granite and the xenoliths, which are enriched in Al, K, and Fe relative to the granite. As expected, hXRF analyses across permeated gradational contacts exhibit varying chemical compositions that are intermediate between the chemical compositions of the granite and the xenoliths. However, analyses of the granite near sharp contacts exhibit greater chemical similarity to the xenoliths than to other granite analyses, suggesting greater interaction between the granite and the xenoliths than predicted by visual observations. This interaction may be the result of xenolith assimilation, chemical diffusion, and/or complicated three-dimensional contact relationships. Additional hXRF analysis is presented to assess the variation in analyses, compositional trends at contacts, and the usefulness of the hXRF in assessing spatially detailed compositional variation.