GEOLOGY AND HXRF GEOCHEMISTRY OF METAMORPHIC XENOLITHS IN TOWN MOUNTAIN GRANITE AT “THE SLAB” SWIMMING HOLE, LLANO UPLIFT, KINGSLAND, TEXAS

Abstract

Abstract Metamorphic xenoliths in the Town Mountain Granite are exposed at “The Slab,” a low-water crossing in Kingsland, Texas, that is popular both for recreation and as a stop on regional geology field trips to the Llano uplift. Difficulty in sampling the pavement-style outcrops limits observation and analysis to that which can be completed at the outcrop. We use structural orientation measurements and a handheld X-ray fluorescence spectrometer (hXRF) to study xenolith orientation in context of regional fabrics, assess evidence of xenolith assimilation into the Town Mountain Granite, and investigate the origin of the xenoliths based on bulk composition. A nearly 1:1 correlation between the orientations of the xenoliths’ long axes and their internal foliation suggests that granite intrusion along foliation strongly controlled the xenoliths’ shapes. The orientation of the xenoliths’ foliation is consistent with dominant regional foliations. The hXRF analyses are able to discriminate between the compositions of the granite and the xenoliths. Xenolith compositions are consistent with a mafic protolith and suggest they have the same or similar sources; different analytical techniques for previously published bulk compositional analyses prevent direct comparison with our data or confident identification of a source unit. Limited assimilation of the xenoliths into the granite is suggested by magma injections into xenoliths, mafic residue from partial melting, chemical gradations at contacts between the granite and xenoliths, and possible enrichment of xenoliths in FeO + MgO relative to bulk compositions of the metamorphic country rock. These results encourage future use of hXRF for outcrop-based geological studies.