A FIELD-BASED STRUCTURAL AND HXRF STUDY OF THE ORIGIN AND ASSIMILATION OF PACKSADDLE DOMAIN XENOLITHS IN THE TOWN MOUNTAIN GRANITE, “THE SLAB” SWIMMING HOLE, LLANO UPLIFT, KINGSLAND, TEXAS

The ~1.1 Ga Town Mountain Granite exposed at “The Slab” swimming hole on the Llano River in Kingsland, Texas, hosts dozens of xenoliths of the ~1.3 Ga polymetamorphosed, forearc basin deposits of the Packsaddle Domain. This site is an excellent location for introducing students to relative age relationships and a popular destination for geologic field trips; however, students often come away with many additional questions about intrusive processes and xenolith formation. The outcrops at The Slab are popular among local residents due to the ease of playing and wading among the smoothly scoured granite knobs, so responsible study of this site does not allow for sample collection. We employed field-based methods for studying the structural orientation, compositional variation, and assimilation textures of the xenoliths.

A field-based undergraduate research project analyzed 53 xenoliths, each measuring at least 14 cm in length. Data collected for each xenolith include length, width, the plunge and bearing of the long axis of the xenolith’s exposed surface, the plunge and bearing of the xenolith’s foliation as it intersects with the outcrop’s surface, and the strike and dip of the xenolith’s foliation, if accessible. The aspect ratio for each xenolith was determined, and the data were examined for trends. A strong correlation between the long axis of each xenolith and the trend of its foliation suggests that xenolith shape is strongly controlled by cleaving or magma injection along foliation planes. The scatter of foliation orientations is interpreted to represent pre-intrusion folding of the country rock.

While many of the Packsaddle xenoliths have sharp contacts with the Town Mountain Granite, others exhibit gradational contacts, including both concordant and discordant injections, and permeated contacts that are suggestive of xenolith assimilation by the granite. A newly acquired handheld XRF analyzer (hXRF) is employed to assess compositional variation across the fine-grained xenoliths and into the granitic matrix. This data from the first field test of our new hXRF will be used to evaluate the degree of xenolith assimilation and the usefulness of this tool on a typical regional outcrop where geologic sampling is not acceptable.