GRANULITE- TO AMPHIBOLITE-FACIES METAMORPHISM AND PENETRATIVE DEFORMATION IN A DISRUPTED OPHIOLITE, KANGAROO MOUNTAIN AREA, KLAMATH MOUNTAINS, CALIFORNIA: A DEEP VIEW INTO THE BASEMENT OF AN ACCRETED, OCEANIC ISLAND ARC

Neogene doming in the north-central Klamath Mountains, California, tilted the Rattlesnake Creek terrane, chiefly an ophiolitic mélange, exposing an oblique cross section through disrupted and metamorphosed oceanic crust and mantle. The deepest section of the tilted terrane, in the Kangaroo Mountain area near Seiad Valley, contains tectonic slices of ultramafic, mafic, and sedimentary rocks that were penetratively deformed and metamorphosed under upper amphibolite- to granulite-facies conditions. Field and petrographic relations show post-kinematic replacement of pyroxene by amphibole in granulite-facies metagabbro, indicating that high-temperature mineral reactions continued after deformation ended. Geothermobarometric data from two samples of scarce garnet amphibolite within the ophiolitic suite yielded pressure and temperature conditions of ~7 kb, ~700 °C and ~5 kb, ~650 °C. Crosscutting, radiometrically dated plutonic bodies and the regional geologic context suggest that the timing of this metamorphism was ~172–167 Ma—at least 15 m.y. before the Late Jurassic Nevadan orogeny. This time interval broadly corresponds with contraction along several regional thrust faults and was immediately followed by the intrusion of a suite of calc-alkaline plutons (e.g., Ironside Mountain batholith and related intrusive bodies). A possible tectonic model for this mid-Jurassic orogeny is an oblique collision between an outboard oceanic arc and a westward-facing arc fringing the western North American continental margin. In this model, the amphibolite- to granulite-facies Rattlesnake Creek terrane is the basement for the outboard arc, and terranes to the east represent a complex accretionary amalgam trapped between the collided arcs. Renewed arc magmatism may have played a key role in the high-temperature thermal architecture that persisted after the termination of the inferred mid-Jurassic collisional event.