Mesoproterozoic High-Pressure Metamorphism in the Llano Uplift, Central Texas, USA
High-pressure (HP) terranes of Precambrian age are rare, and their rarity may reflect secular changes in the nature of subduction-zone processes. The Llano Uplift of central Texas contains evidence of HP metamorphism that dates back to the late Mesoproterozoic, documenting subduction and exhumation of the southern margin of the Laurentian continent. Collisional orogenesis in the Llano Uplift has been dated at ca. 1147-1116 Ma by U-Pb (zrc, ttn, mnz, rut) and Lu-Hf (grt-rut) geochronometry. This orogenic event drove metamorphism comprising both an initial high- pressure (HP) phase (610-775 C at 1.4-2.4 GPa) and a subsequent moderate-pressure (MP) phase (ca. 700 C at ~0.7 GPa). A low-pressure overprint (525-625 C at 0.3 GPa) took place under largely static conditions at ca. 1093-1070 Ma. 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, 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 tectonometamorphic history shares numerous characteristics with well-documented examples of Phanerozoic continental-margin subduction, including: the presence of disrupted ophiolites; a paucity of coeval island-arc volcanics and plutonics; the involvement of precursors representing continental basement and supracrustal rocks; rapid exhumation to lower crustal levels with significant Barrovian-style overprinting; the preservation of the highest pressure record only in eclogitic pods and lenses within quartzofeldspathic gneisses; and the presence of post-collisional and late-stage anorogenic granitic plutons. HP metamorphism in the Llano Uplift 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.
Significance of Ribbon Continents for HP Metamorphism and Lithospheric-Scale Extension in Collisional Orogenic Belts
The modern Anatolian-African plate boundary is represented by a north-dipping subduction zone that is part of a broad domain of regional convergence between Eurasia and Afro-Arabia since the latest Mesozoic. High- pressure metamorphic rocks occur along suture zones and in core complexes in the upper plate (Eurasia) of this convergence zone. A series of collisions between Gondwana-derived ribbon continents and trench- rollback systems in the Tethyan realm produced nearly E-W-trending, subparallel mountain belts with high elevation and thick orogenic crust in this region. Ophiolite emplacement, terrane stacking, high-P and Barrovian metamorphism, and crustal thickening occurred during the attempted subduction and accretion of these microcontinents into the upper plates of Tethyan subduction rollback systems during the late Cretaceous-early Eocene. Lithospheric-scale shortening associated with the accretion of ribbon continents displaced thick slices of crustal rocks deep into the mantle where they formed HP lithologies. Continued convergence and oceanic lithospheric subduction within the Tethyan realm were punctuated by slab breakoff events following the microcontinental accretion episodes. Slab breakoff resulted in asthenospheric upwelling and partial melting, which facilitated post-collisional magmatism along and across the suture zones. Resumed subduction and slab rollback-induced upper plate extension triggered a tectonic collapse of the thermally weakened orogenic crust in Anatolia in the late Oligocene-Miocene. This extensional phase resulted in exhumation of high-P rocks and medium- to lower-crustal material leading to the formation of metamorphic core complexes in the hinterland of the young collision zones. Thus, subduction rollback-driven distension of the collisional mountain belts in the Mediterranean region played a major role in the juxtaposition of tectonic sheets with different PTt trajectories (i.e. eclogite-blueschist slices, garnet peridotites, ophiolite sheets, quartzo-feldspathic gneiss) and in the exhumation of high-grade metamorphic rocks from mantle depths. This model does not call for significant buoyancy contrast for the exhumation of HP rocks in Tethyan orogenic belts, as in channel flow, corner flow, and/or diapiric motion models. Collision, attempted subduction, and accretion of ribbon continents in Tethyan subduction-rollback systems played a more significant role during the evolution of collisional mountain belts in the Alpine-Himalayan orogenic system than previously thought.