In … blueschist facies [′blü‚shist ′fā‚shēz] (petrology) High-pressure, low-temperature metamorphism associated with subduction zones which produces a broad mineral association including glaucophane, actinolite, jadeite, aegirine, lawsonite, and pumpellyite. In the circum‐Pacific orogenic belts, the incipient subduction of the paleo‐Pacific plate took place during the Early–Middle Paleozoic, as indicated by the blueschist‐facies metamorphic rocks from the Klamath Mountains, western USA (ca 450 Ma, Cotkin et al. The Eastern Belt is the oldest, shows widespread high-P/low-T (lawsonite-albite and blueschist-facies) metamorphism, and a relatively coherent structural style. Blueschist-facies metamorphism, which is associated with high pressure and low temperature subduction zone metamorphism, is usually recognized by the presence of the sodic-amphibole glaucophane. Using the graph below, determine what facies a rock would be found in if it was buried 30km deep and heated to a temperature of approximately 200 degrees C. blueschist During metamorphism, mineral grains will align themselves as a result of forces applied to the rock. The blueschist‐facies metamorphic rocks provide critical evidence for paleo‐subduction zones. blueschist metamorphism together with the other blueschist blocks. Heat is the most important agent of metamorphism because it provides the energy necessary for chemical reactions to occur within the rock. Blueschist facies: phengite + chlorite + quartz (albite, jadeite, lawsonite, garnet, chloritoid, paragonite) Eclogite facies: phengite + garnet + quartz Mafic rocks (basalt, gabbro, diorite, tonalite etc.) Blueschist Blueschist is a regional metamorphic rock formed under high-pressure (HP) low-temperature (LT) conditions. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. The best studied example of this type of metamorphism occurs within the Cretaceous Franciscan Complex of California. Metamorphism along low geothermal gradients results in a series of rocks that pass through the Zeolite, Prehnite-Pumpellyite, Blueschist, and Eclogite Facies of Regional Metamorphism. Blueschist and Eclogite Facies • The blueschist facies is characterized in metabasites by the presence of a sodic blue amphibole stable only at high pressures (notably glaucophane, but some solution of crossite or riebeckite is possible) • The association of glaucophane + lawsonite is diagnostic. ____ facies are expected at the core of mountains formed at a continental-continental convergent plate boundary. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a … INTRODUCTION. yield a different set of minerals at the same P/T conditions, as follows: Phengite K–Ar ages of 16 ALBITE-EPIDOTE-HORNFELS FACIES (LP/LT-MT) 11/11/2012 The albite-epidote-hornfels facies is a facies at low pressure and relatively low temperatures. The low temperatures recorded by blueschist‐facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15–50 km. It is named for the two minerals albite and epidote, though they are stable in more facies. The mineralogic and paragenetic features of the Osayama blueschists are compatible with a hypothesis that they were derived from a coherent blueschist-facies metamorphic sequence, formed in a subduction zone with a low geothermal gradient (~10°C/km). The character and boundaries of these belts are loosely defined and somewhat controversial, but they provide a useful framework for discussion of the internal structure of the Franciscan.