LOW-PRESSURE DEFORMATION AND METAMORPHISM OF THE PROTEROZOIC LLANO UPLIFT, CENTRAL TEXAS

The Proterozoic Llano Uplift of central Texas retains evidence of two main metamorphic events, an early dynamothermal event at pressures as high as the eclogite facies, and a later metamorphic event at relatively low pressures. This later event was previously considered to be “static”, but it is actually a complex interaction of deformation, metamorphism and intrusion spanning at least 50 million years. Low-pressure metamorphism began prior to 1118 Ma and contact metamorphism around plutons continued until at least 1070 Ma. GASP barometry, exchange-based thermometry, and assemblage-based thermobarometry all give pressures of 2-4 kbar and temperatures of 475-650°C for pelitic samples.

New compilation and mapping of aluminum silicate occurrences in the uplift helps constrain the metamorphic and deformational history of this low-pressure event. Previously unreported occurrences of andalusite in the western region confirm that late metamorphism occurred at low pressure throughout the uplift. In the eastern and western parts of the uplift, andalusite occurs both with and without sillimanite. In the central uplift, sillimanite pseudomorphs after andalusite are present, but no surviving andalusite has been found. Several explanations exist to explain this pattern, including heterogeneous Paleozoic uplift and thermal anomalies related to granitic intrusion.

Timing of the aluminum silicates relative to the deformation is variable, but not in any geographic pattern. Aluminum silicates that are pre-, syn-, and postkinematic to the final phase of deformation have all been identified. In the western uplift, muscovite pseudomorphs after andalusite are wrapped by the dominant foliation. In the central uplift, adjacent to the syn-folding Wolf Mountain intrusion, sillimanite pseudomorphs after andalusite are prekinematic to the final phase of deformation and prismatic sillimanite is synkinematic. In the southeastern uplift, several occurrences of andalusite remote from intrusions are pre- to synkinematic to the final phase of deformation. Some andalusite occurrences adjacent to plutons are pre- to synkinematic to intrusion-related deformation. Much of this low-pressure deformation is overprinted by static metamorphism, perhaps related to fluid mobilized during intrusion of the younger granitic bodies.