The Llano uplift of central Texas exposes Mesoproterozoic crystalline rocks affected by a Grenvillian orogeny between 1.2 and 1.1 Ga. We report four new U-Pb zircon protolith ages of 1247 ± 4, 1257 ± 3, 1272 +8/−5, and 1366 ± 3 Ma for four quartzofeldspathic rocks from the regionally defined Packsaddle Schist and Valley Spring Gneiss in the southeastern part of the uplift. The 1366 ± 3 Ma gneiss also yields a metamorphic age of 1325 ± 5 Ma. These new U-Pb ages, in conjunction with previous U-Pb age data, demonstrate that several regionally defined map units contain constituents with widely disparate ages.
We have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the Llano uplift that appear to represent rock packages of distinct tectonic origins: (1) the northern, 1366–1272 Ma felsic gneisses of volcanic, plutonic, and continentally derived sedimentary origin (Valley Spring Gneiss and an older gneiss component), (2) the geographically intermediate, ca. 1257–1247 Ma (and possibly older) basinal sequence that formed along a continental shelf and slope near an arc (Packsaddle Schist), (3) the southern, 1326–1275 Ma remnant of an exotic, ensimatic arc complex (Big Branch Gneiss and Coal Creek Serpentinite and plutonic complex), and (4) the 1239–1232 Ma tectonized felsic rocks that intruded the first two suites. The first three suites of rocks were structurally imbricated during Grenville orogenesis, and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the Packsaddle Schist.
In addition, these new data show that, contrary to previous reports, younger Packsaddle Schist lies in structural contact above older components of the Valley Spring Gneiss. Moreover, one of the dated meta-igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most Llano uplift rocks. Its 1366 ± 3 Ma protolith age is coeval with rocks of the Western Granite-Rhyolite terrane. Age and geochemical similarities link this unit of unknown regional extent with the Western Granite-Rhyolite terrane of known Laurentian affinity. Thus, this component of the Llano uplift may be the basement on which the younger components of the Valley Spring Gneiss and Packsaddle Schist formed in a tectonic setting proximal to Laurentia and south of the Western Granite-Rhyolite terrane.