INVESTIGATING THE P-T HISTORY OF THE PACKSADDLE DOMAIN, LLANO UPLIFT, VIA PHASE EQUILIBRIA MODELING

The Llano Uplift of central Texas exposes polymetamorphosed supracrustal rocks that record a protracted, Grenville-aged orogeny along Laurentia’s southern margin. The sedimentary and igneous protoliths of the Packsaddle Domain, which includes the Honey, Sandy, Rough Ridge, and Click formations, are thought to represent forearc basin deposits. These units record dynamothermal metamorphism at upper amphibolite facies conditions (~700 °C, ~7 kbar) and were subsequently overprinted by primarily static metamorphism (525-625 °C, ~3 kbar) associated with the emplacement of syn- to post-tectonic granites. Both metamorphic events were preceded by eclogite facies metamorphism recorded by retrogressed eclogites in the Valley Spring Domain.

Current pressure-temperature (P-T) constraints for the Packsaddle Domain are based on observed mineral assemblages, interpreted metamorphic reactions, recrystallization textures, and traditional quantitative geothermobarometry; however, widespread overprinting, units lacking metamorphic assemblages appropriate for geothermobarometry, and a lack of textural relationships hinders further development of the P-T history. Metasedimentary samples from the Honey and Rough Ridge formations are analyzed using XRF to determine sample bulk composition and petrographic analysis of thin sections to identify metamorphic assemblages and reaction histories. Phase equilibria (pseudosection) modeling is employed to investigate the extent to which samples 1) are unsuitable for the development of useful metamorphic assemblages or 2) have been overprinted by late, low pressure metamorphism. This work will also assess the further usefulness of phase equilibria modeling for improving constraints on the P-T history of the Packsaddle Domain.