HXRF ANALYSIS OF METAMORPHIC XENOLITHS AND BULK COMPOSITIONAL COMPARISON WITH METAMORPHIC UNITS IN THE LLANO UPLIFT, TEXAS

Mafic xenoliths are exposed in knobby outcrops of the Mesoproterozoic Town Mountain Granite along the Llano River in Kingsland, Texas, at a location residents call “the Slab”. The anorogenic Town Mountain Granite intruded rocks metamorphosed during the Grenville orogeny, and is now exposed as part of the Llano uplift. The xenoliths are fine-grained, foliated metamorphic rocks, presumably from the nearby Packsaddle Schist, which includes the Click, Rough Ridge, Sandy, and Honey formations, the Valley Spring Gneiss, or the Coal Creek formations. Unfortunately, the xenoliths cannot be sampled because they are part of a popular local swimming hole, and it would be difficult to collect samples from these smooth, pavement-like outcrops. We used a handheld X-ray fluorescence (hXRF) device, which allows us to determine the bulk rock compositions of the xenoliths and then compare them to bulk rock analyses for metamorphic rocks in the area. Bulk rock composition data was collected using the hXRF to analyze points and traverses from the exposed faces of more than ten different xenoliths. The data was reduced and plotted on an ACF diagram. The xenolith compositions suggest origins from mafic and/or quartzofeldspathic protoliths. Previous workers have published bulk composition data for rocks from the Packsaddle Schist, Valley Spring Gneiss, and Coal Creek units. The Packsaddle Schist and Valley Spring Gneiss samples have quartzofeldspathic or mafic compositions, and the Coal Creek rocks have quartzofeldspathic and pelitic compositions. While one gneissic sample has a composition very similar to one of the quartzofeldspathic xenoliths, none of the other metamorphic rock samples have bulk compositions that suggest they are a possible source of the xenoliths. Further research will include additional analysis of metamorphic rocks in the area.