High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a … The blueschist‐facies metamorphic rocks provide critical evidence for paleo‐subduction zones. yield a different set of minerals at the same P/T conditions, as follows: The best studied example of this type of metamorphism occurs within the Cretaceous Franciscan Complex of California. INTRODUCTION. 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 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). blueschist metamorphism together with the other blueschist blocks. Blueschist Blueschist is a regional metamorphic rock formed under high-pressure (HP) low-temperature (LT) conditions. 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. 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. Phengite K–Ar ages of 16 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. Blueschist facies: phengite + chlorite + quartz (albite, jadeite, lawsonite, garnet, chloritoid, paragonite) Eclogite facies: phengite + garnet + quartz Mafic rocks (basalt, gabbro, diorite, tonalite etc.) As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. 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 Eastern Belt is the oldest, shows widespread high-P/low-T (lawsonite-albite and blueschist-facies) metamorphism, and a relatively coherent structural style. Heat is the most important agent of metamorphism because it provides the energy necessary for chemical reactions to occur within the rock. 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. 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. 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. 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. It is named for the two minerals albite and epidote, though they are stable in more facies. In … ____ facies are expected at the core of mountains formed at a continental-continental convergent plate boundary.

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