REFINING CONSTRAINTS ON THE TIMING OF METAMORPHISM IN THE LLANO UPLIFT: EVALUATING GEOCHRONOMETERS VIA HIGH RESOLUTION PETROGRAPHY OF THE PACKSADDLE SCHIST, ENCHANTED ROCK STATE NATURAL AREA, TEXAS

Existing constraints on the timing of metamorphism during the Mesoproterozoic Grenville orogeny in the Llano uplift of central Texas may be refined by newer in situ geochronologic and petrochronologic methods. The Llano uplift exhumes rocks with a complex history of metamorphism, deformation, and magmatism. Three recognized metamorphic events include a high-pressure event (M₁: 610-775 °C, 14-24 kbar), a regional event at upper amphibolite facies (M₂: ~700 °C, ~7 kbar), and a widespread, static event (M₃: 525-625 °C, ~3 kbar) during the emplacement of the Town Mountain Granite (~1.1 Ga).

Most U-Pb zircon dates from the Valley Spring (~1366-1232 Ma), Coal Creek (~1326-1275 Ma), and Packsaddle (~1257-1215 Ma) domains are interpreted as crystallization ages for igneous protoliths, or for syntectonic (~1119-1116 Ma) and post-tectonic (~1098-1071 Ma) plutons (i.e., Town Mountain Granite), and most of these are upper intercept dates for multigrain zircon fractions. Far fewer dates are interpreted to represent the timing of metamorphism, and they don’t fully distinguish between the M₁ (~1147-1128 Ma) and M₂ (~1134-1102 Ma) events. Further, the metamorphic dates are published only in abstracts and lack context.

The Packsaddle domain has only two imprecise geochronologic constraints on metamorphism. Ten samples were collected from the Packsaddle domain at Enchanted Rock State Natural Area. The samples have gneissic to fine-grained schistose foliation and contain plagioclase, biotite, muscovite, hornblende, sillimanite, garnet, and/or quartz. Polished thin sections were prepared for petrographic analysis and high-resolution scanning (XPL, PPL) using an Epson Perfection v850 scanner. After locating geochronometers (e.g., zircon, monazite) and describing metamorphic and deformation textures (e.g., foliations, leucosomes, porphyroblasts), the high-resolution scans will be used to evaluate the usefulness of in situ geochronologic analyses. This work is done in preparation for further analysis, including elemental mapping for Y, Ce, and Zr and/or EDS point analyses to identify grains of interest; BSE imaging to locate zircon and monazite grains in their matrix at higher magnification; and, eventually, in situ geochronologic and/or petrochronologic analysis by LA-ICP-MS or LASS-ICP-MS.