HIGH-PRESSURE METAMORPHISM IN THE LLANO UPLIFT: MESOPROTEROZOIC SUBDUCTION OF THE SOUTHERN LAURENTIAN CONTINENTAL MARGIN

Collisional orogenesis in the Llano Uplift of central Texas during the late Mesoproterozoic drove metamorphism that comprised both an initial high-pressure (HP) phase (610-775 °C at 1.4-2.4 GPa) and a subsequent moderate-pressure (MP) phase (~700 °C at ~0.7 GPa). A later low-pressure overprint (525-625 °C at 0.3 GPa) took place under largely static conditions. Evidence for HP metamorphism is geographically widespread, but confined principally to boudins of mafic eclogite encased in felsic gneisses. The geographic distribution of P-T conditions for HP metamorphism inferred from the eclogites, combined with evidence from the relative degrees of homogenization of growth zoning in garnet from both eclogites and felsic gneisses, suggests a general increase from northeast to southwest in depths of burial during HP metamorphism. Exhumation of HP rocks to shallower depths prior to MP metamorphism appears to have been rapid: ages for the two phases of metamorphism overlap, on the basis of the very limited data now available. The region's early metamorphic history is best explained by southwestward subduction of the Laurentian continental margin during collision with a still-unidentified continental mass, followed by buoyancy-driven uplift to lower crustal levels while collisional contraction continued.