INSIGHTS INTO THE PRECAMBRIAN LLANO UPLIFT FROM GEOCHRONOLOGY OF THE CAMBRIAN HICKORY SANDSTONE, CENTRAL TEXAS

Structural, petrographic, and geochronologic studies of the Mesoproterozoic Llano Uplift in Central Texas document significant complexity throughout one of the best-exposed records of the southern Grenville Orogeny. The Llano Uplift includes the Valley Spring Gneiss (VSG), Packsaddle Schist, and Coal Creek metamorphic domains, which were intruded by the Town Mountain Granite (TMG) following the local orogeny. The Cambrian Hickory Sandstone (HS) unconformably overlies these Precambrian rocks. We completed U-Pb age dating of detrital minerals in the HS to provide a geochronologic record that characterizes the overall Llano Uplift.

We sampled the VSG, TMG, and HS, with the goal of isolating zircon, titanite, monazite, and apatite for LA-ICPMS U-Pb age dating. We recovered zircon from all samples, apatite from the VSG and TMG samples, titanite from the VSG sample, and monazite from one of two HS samples. Zircon ages for the VSG have a bimodal distribution, with a large age mode at ca. 1375 Ma, a smaller mode at ca. 1660 Ma, and a few older ages. These ages are consistent with a sedimentary protolith and predate reported age ranges for the VSG. The apatite and titanite U-Pb ages for the VSG are 1090±37 Ma and 1081±13 Ma respectively, and likely record mineral growth and/or resetting with emplacement of the TMG. The TMG zircons yield a best age estimate of 1088±15 Ma, consistent with other U-Pb dates of post-tectonic Llano granites. TMG apatite was not dated due to insufficient uranium. HS zircon and monazite ages have similar bimodal distributions with modes at ca. 1084 Ma and ca. 1375 Ma.

Detrital mineral geochronology of the two geographically and temporally distinct HS samples is consistent with local provenance in the Llano Uplift. Furthermore, the uniformity of HS results indicates that the HS age spectra effectively sample the diverse Llano Uplift region and can thus be used to characterize the overall Mesoproterozoic system. Our results also reveal the limitations of less-refractory minerals in provenance studies, given their apparent susceptibility to alteration, weathering, diagenesis, and compositional variations. These challenges highlight the complexity of the Llano Uplift, even as U-Pb age data clarifies the region’s evolution.