blueschist facies
Blueschists are generally considered to be diagnostic of former subduction zones, because they imply relatively high pressure conditions relative to the temperature (compared to normal geothermal gradients). This low temperature, high pressure prograde metamorphic path is exposed in the US westcoast Franciscan facies series. (image of section of Ward Creek blueschist, courtesy of Andrew Alden, geology.about.com, reproduced under educational fair use: wallpapers: lo-res, hi-res) Blueschists appear blue, black, gray, or blue-green in outcrop, and approximate basaltic elemental composition. When blue, these rocks are tinted by sodium-bearing blue amphiboles glaucophane or crossite rather than by calcium-bearing green or black amphiboles actinolite or hornblende, which are developed in the more commonly encountered greenschist facies or amphibolite facies metamorphism. Glaucophane is azure-blue, lavender-blue or bluish-black mineral. Felsic rocks and pelitic sediments subjected to blueschist facies conditions form different mineral assemblages than does metamorphosed basalt.
(blueschist thin-sections at right - top, blueschist facies basalt, Sivrihisar, Turkey; middle, blueschist facies quartz sediment, Sivrihisar, Turkey; bottom, garnet-lawsonite-glaucophane blueschist from Sivrihisar, Turkey)
Ultramafic rocks subjected to blueschist metamorphism contain the minerals serpentine/lizardite +/- talc +/- zoisite. Pelites yeild kyanite + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnets 
Granites yield kyanite +/- paragonite +/- chlorites +/- albite +/- quartz +/- pargasite or phengite Blueschists terranes are typically found within young orogenic belts in faulted contact with rocks of the higher-temperature greenschist facies (rarely eclogite facies). The rocks are typically metamorphosed oceanic sediments and basaltic oceanic crust. Blueschists and other high-pressure subduction zone rocks are probably exhumed rapidly (and thus preserved) by flow and/or faulting in accretionary wedges or the upper parts of subducted crust. They may also be exhumed partly due to isostatic buoyancy when the metabasaltic rocks are associated with low-density continental crust (marble, metapelite, and other rocks of continental margins). Evidence of many blueschist belts has probably been lost because the hydrous minerals of blueschist facies metamorphism are overprinted by facies resulting from normal geothermal gradients. Surface geothermal gradients of the order of 30°C/km are common in continental crust and in thick sedimentary basins, whereas a steady-state surface gradient of only about 15 to 20°C/km are sufficient for blueschist metamorphism. Overprinting occurs when circulating fluids convert blueschist minerals to greenschist and amphibolite facies mineral assemblages. In dehydrating reactions, the rocks become denser and lose volatiles. In situ eclogitic schist lenses occur in the coherent low-grade epidote-zone Ward Creek metabasite unit of the Central Franciscan belt.[r] In addition to the circum-Pacific Franciscan complex, Japanese chains, and New Caledonia, blueschists are also well exposed in Alpine-Himalayan chains in Greece, Turkey. The islands of Sifnos and Syros in the Cycladic archipelago in Greece and the Tianshan Mountains in NW-China are examples of previous subduction zones that may be relics of subduction channels with outcrops that conform to tectonic melanges. Hydrous reactions occured during retrogressive metamorphism from the eclogite facies back to the blueschist facies. These rocks of the blueschists, some show relics of omphacite and garnets indicating eclogitic facies.[wp] |
links: images: hand-specimens: blueschist, 2, 3, 4, 5, 6, 7, blueschist with eclogite, close-up; close-ups: blueschist with stubby needles of glaucophane (metamorphosed volcanic rock of basic composition, part of the old ocean floor of Tethys); lineated schistose foliation produced by the lining up of elongate amphibole (var. glaucophane) crystals; seen trending lower left and upper right in this blueschist; glaucophane schist, 2, blueschist, 2, 3; eclogite-facies pillow basalt, partly overprinted to blueschist; eclogite surrounded by blueschist (Syros, Greece); subduction zone: fragment of blueschist containing coarse, white lawsonite (CaAl2Si2O8.H2O) in a vug; metamorphosed under blueschist conditions towards the base of the subducted oceanic slab - compare with dull green, tectonically polished, lower greenschist/blueschist metamorphic rocks mainly composed of chlorite and epidote - note vein of glaucophane and phengite (muscovite) in the material; see: blueschist formations: thin-sections: Blueschist in the Dalradian of Achill Island; green chlorite, blue glaucophane, clear white mica flakes in blueschist xp, pl, epidote-clinozoisite wp; webpages: metamorphic facies; field trip CA; Closeup Rock Photos, Geology Wallpaper Index; Pitt |
