FROM SILLIMANITE PSEUDOMORPHS TO CORDIERITE-SAPPHIRINE SYMPLECTITES: MULTI-STAGE GRENVILLE METAMORPHISM PRESERVED IN PACKSADDLE SCHISTS AT ENCHANTED ROCK, LLANO UPLIFT, TEXAS

Detailed petrography of Packsaddle domain schists collected adjacent to the Enchanted Rock batholith reveals the preservation of complex, multi-stage Grenville metamorphism in the Llano uplift of central Texas. Past studies of Grenville metamorphism identified three metamorphic events or stages (M₁-M₃), and five or six deformation events (D₁-D₅, D₆?). Previous workers suggested that most of the deformation (D₁-D₅), as studied in the western uplift, was synchronous with M₂, which occurred at upper-amphibolite facies conditions. Due to proximity to the batholith, M₂ in Packsaddle domain schists collected in Enchanted Rock State Natural Area was expected to be strongly overprinted by M₃, a static, fluid-rich event associated with post-tectonic intrusion of the Town Mountain Granite, Enchanted Rock batholith included. However, these samples contain a large number of phases (quartz, biotite, garnet, sillimanite, K-feldspar, cordierite, sapphirine, muscovite, chlorite) plus evidence for early andalusite and possible relict orthopyroxene. Phases tend to occur in domains, e.g., anhedral garnet or sillimanite with cordierite-sapphirine symplectite. Textures and phase relationships within these domains allow for interpretation of a lengthy metamorphic history, and suggest preservation of much of the prograde M₂ P-T path, as well as part of the retrograde path. The peak assemblage is interpreted as biotite + plagioclase ± fibrolite ± K-feldspar ± sapphirine ± cordierite ± melt, suggesting regional metamorphism to upper amphibolite or lower granulite facies during M₂, with cordierite-sapphirine symplectite possibly forming during isothermal decompression at peak T. Cordierite-sapphirine symplectites are also characteristic of high T contact metamorphism. The M₂ event is currently constrained to ~1134-1102 Ma, though there are only two imprecise dates from the Packsaddle domain. This work to identify phases, phase relationships, and petrologic domains, and interpret metamorphic history was done in preparation for thermodynamic phase modeling and in situ geochronology. Zircon is widely observed across samples in all phase domains. If zircon grew throughout M₂, then future work could better constrain timing of M₂ and improve understanding of the Grenville orogeny in the Llano uplift.