Before the onset of BS facies metamorphism, some of these rocks equilibrated at conditions around 7Â kbar and 470Â°C. Once the oceanic crust starts subducting, most of its remnant porosity will be immediately lost by compaction and its pore fluids get expelled. Blueschists are formed in association with subduction and continental collision and reflect burial to high pressures at relatively low temperatures. Detrital zircons from an albitic micaschist were dated using the UâPb method and provided an age of 399Â±62Â Ma, interpreted as the maximum age for the HPâLT event (Peucat & CognÃ©, 1977; Peucat, 1986). Because of The subducting slab probably began to tear locally at depth from â¼10Â Ma to the present. High-pressure rocks of the blueschist and/or eclogite facies were metamorphosed at a low geothermal gradient, that is., small increase in temperature with depth (Figure 1). These sediments are therefore not strictly “accretionary prism” and the trench was not located at the NW Borneo Trough. Tentative tectonic model for the European Variscan Belt (compiled from geotectonic models after Franke, 2000; O’Brien, 2000; Faure et al., 2005; Schulmann et al., 2005). However, sodic amphiboles display a large range oin chemical compositions, due principally to the Fe 2+ Mg-1 and Fe 3+ Al-1 substitutions. Blueschist (pronounced /ËbluËÊÉªst/) is a rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius. Primarily from the eastern border of the Bohemian Massif and later during Alpine events, they were incorporated into the Alpine–Carpathian thrust-belt system (Paquette et al., 1989; Medaris et al., 2003; Guillot & Ménot, 2009). Lawsonite stability is displaced toward higher temperature as anorthite (Ca, Al), H, and C increase. UHP metamorphism is the first evidence of deep subduction of continental crust in the geologic record. Eclogite occurs as lenses in garnet-bearing glaucophane schist at Gemu, south of the Lungmuco–Shuanghu suture. The source granites could have originated in a magmatic arc or similar geotectonic environment, which could be related to the closure of the Lower- to Middle-Devonian oceanic basin. Most of the Zagros BS were formed during convergence at conditions near 11Â kbar and 520â530Â°C. The blue color of the rock comes from the presence of the mineral glaucophane. 750Â Â°C). These are close to the BS nearby in the Oman that are 5â10Â Ma older than those in the Zagros. Charles S Hutchison, in Geology of North-West Borneo, 2005. Earth and Planetary Science Letters 163: 361â379.) Those wonât make a blue rock (theyâll be greenish). As the southeastward subduction (Franke & Stein, 2000; O’Brien, 2000) continued during the Upper Devonian–Lower Carboniferous (365–350 Ma), the arc segment was involved in the subduction zone and produced HP/UHP metamorphic rocks (Figure 12.13). The addition of a CO2 component further complicates phase relations. Some authors have proposed that the metamorphic zonation is related to a gradient with PâT conditions varying continuously from west to east and where the metamorphic domains are separated by âisogradsâ (Triboulet, 1974; Carpenter, 1976; Quinquis, 1980; Djro et al., 1989; Schulz et al., 2001). et al., 2006). Li et al. In the Qulukatage Complex, these Neoarchean and Paleoproterozoic rocks underwent two metamorphic events at 1.9â1.8Â Ga and 1.1â1.0Â Ga, which are considered as having been related to the assembly of the Columbia and Rodinia supercontinents, respectively (Shu et al., 2011; Zhang et al., 2012). A. Feenstra, G. Franz, in Reference Module in Earth Systems and Environmental Sciences, 2015. Blueschist facies generally is considered to form under pressures of >0.6 GPa, equivalent to depth of burial in excess of 15â18 km, and at temperatures of between 200 and 500 °C. Westward subduction of the Moldanubian back-arc basin and underplating of Brunia resulted in exhumation of HP/UHP rocks in the Moldanubian Zone. Kent C. Condie, in Earth as an Evolving Planetary System (Second Edition), 2011. Shaded area represents PâT paths for slab surface from Arcay et al. Consistent with the paleogeographic reconstructions (Matte, 1986; Franke & Stein, 2000; Linnemann et al., 2007), the Rheic Ocean played a key role in the formation and exhumation of HP rocks in the western part of the European Variscan Belt. In the Moldanubian Zone of the Bohemian Massif, these rocks (HP felsic granulites, garnet peridotites, and HT eclogites) are part of the GfÃ¶hl Unit, which has also been considered to be a nappe overlaying the amphibolite facies Monotonous and Varied Units (Tollmann, 1982; Matte et al., 1990). Water contents of the bulk rock at the beginning of the blueschist facies are â¼6 wt.% (Figure 4). The presence of eclogites within the Monotonous and Varied Units suggests that the Moldanubian Zone was, during the Variscan orogeny, part of an accretionary wedge into which the HP/UHP were tectonically emplaced. Pyroxenite layers within peridotites are interpreted as the result from melt infiltration into the mantle above the subduction zone. The zeolite facies is the metamorphic facies with the lowest metamorphic grade. The peak P–T conditions for metamorphic suites discussed in this chapter, some representative exhumation P–T paths, the positions of the wet and dehydration solidi for partial melting of mafic and sedimentary lithologies, and peak metamorphic conditions of some other selected UHP suites (figure modified after Hacker, in press; also note the patterned field for “All eclogites,” and the graphite–diamond and quartz–coesite equilibria). Even in young collisional mountain chains such as the Himalayas, only a few minor occurrences of blueschist have not been removed by erosion. The last stage of extensional and strike-slip tectonics resulted in displacement of some crystalline fragments from the main massifs. The sodic amphibole glaucophane is generally considered as indicative of blueschist-facies metamorphism. Geological Magazine 148 (5â6), 692â725. The blueschist facies is at relatively low temperature but high pressure, such as occurs in rocks in a subduction zone. The similarity of ages (410–380 Ma) for eclogite facies metamorphism for both the eastern and western parts suggests that they could form by subduction and closure of one (Rheic?) shallower blueschist facies, exhumation slowed and became less uniform among blocks. Information and translations of blueschist in the most comprehensive dictionary definitions resource on the web. Metamorphic rock - Metamorphic rock - Zeolite facies: In the zeolite facies, sediments and volcanic debris show the first major response to burial. Uplift of blueschists led to recrystallization of lower pressure mineral assemblages. Nevertheless the Labuk Highlands area around Telupid must have been dramatically inverted and exhumed to their present outcrop position from a depth of around 20 km. Uplift and erosion after continental collisions may also remove blueschists. Definition of blueschist in the Definitions.net dictionary. 2.10 in Chapter 2), resulting in recrystallization of blueschist-facies assemblages to greenschist- or amphibolite-facies assemblages. The Bohemian Massif, which represents the most complex segment of the European Variscan Belt, exposes well-preserved HP/UHP lithologies, allowing us to decipher the relationships among different units and to localize possible Variscan sutures. K-feldspar, muscovite, biotite, and stilpnomelane are the only four important potassium-bearing minerals, besides the K-free Fe-Mg-Al-phases plotting at the AF-line. c. migmatite melting in a subduction zone. Beyond depths of â¼15 km the oceanic crust enters into the blueschist facies in which the major hydrous minerals are chlorite, sodium-rich, calcium-poor amphiboles (glaucophane to barroisite), phengite (white mica), lawsonite or zoisite, and paragonite (e.g., Sorensen, 1986). Blueschists are formed in association with subduction and continental collision and reflect burial to high pressures at relatively low temperatures. Bosse et al. A discontinuous oceanic suture with ophiolites and eclogites of MORB affinity begins in the Galician and continues through the Ligerian or southern Brittany, the French Massif Central to the northwest along the boundary between the Saxothuringian and Moldanubian Zones. The 40Ar–39Ar age of glaucophane associated with eclogite is 220 Ma, and the 40Ar–39Ar age of phengite is 222 Ma (Li et al., 2006b). Recent studies have suggested that epidoteeclogite can form at relatively high pressure, overlapping the quartzcoesite PT curve. Two main phases of deformation were described according to the metamorphic history (Quinquis, 1980; Quinquis & Choukroune, 1981). These rocks have not been sheared and the metamorphism to high-pressure low-temperature epidote-glaucophane facies took place under static conditions of strong loading without shearing. Data below 550 Â°C are based on natural blueschists and greenschists, all other data are based on experiments (compare with Figure 1). Blueschist, Ile de Groix Ile de Groix, situated about 10 km from the south coast of Brittany (Fig.1), represents the emerged part of NW-SE elongated metamorphic belt.This belt, covering an area of about 40 x 10 km, consists of rocks formed under high-pressure and low- or medium-temperature conditions (blueschist and eclogite facies). Prior to 2 Ga, steeper subduction geotherms may have prevented formation of high-pressure low-temperature rocks. In the assemblage K-feldsparâbiotiteâmuscoviteâquartz, the phengite content of muscovite expressed as Si per formula unit decreases with temperature but increases with pressure due to substitution of the Tschermak-molecule Mg+Â 1Si+Â 1AlâÂ 1AlâÂ 1. Occurrences of blueschist and eclogite form part of a discontinuous belt more than 1500 km long. Recently published seismic reflection profiles across the central high zone (Lu et al., 2009) reveal a north-dipping thrust zone suggesting that the lower crust in this region was affected by subduction of the Indian plate northward under Qiangtang. 1.3). High activity of Na in the salt diapirs provided a suitable mechanism to form blue amphiboles before the theoretical values of pressure and temperature. Two metamorphic phases have been recognized: a blueschist to eclogite facies metamorphism M1 related to subduction, and a greenschist facies overprint M2 related to the exhumation process (Triboulet, 1974; Carpenter, 1976; Djro et al., 1989; Barrientos, 1992; Bosse et al., 2002; BallÃ¨vre et al., 2003). Again feldspar, quartz, and H2O are used as projection phases. This event was followed by collision and large-scale crustal melting in the European Variscan Massifs. Fig. The facies is named for zeolites, strongly hydrated tectosilicates. Figure 9. Experimentally derived phase diagram for H2O-saturated mid-ocean ridge basalt (modified from Schmidt MW and Poli S (1998) Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Garnet peridotites are less common in the Massif Central, and garnet pyroxenites are known from the Iberian and Armorican Massifs. The Canadian part of the North American Cordillera was built onto a plate with a long history of contraction and extension prior to orogenic development in the late Paleozoic. Blue schist definition at Dictionary.com, a free online dictionary with pronunciation, synonyms and translation. The greatest prospects for survival of prograde reactions and geochemistry are found in mafic rocks that experienced relatively rapid, down-T exhumation paths. Felsic rocks and pelitic sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt. the KorlaâQuruqtagh area, the AksuâKeping area, the Tiekelike area and the AltunâDunhuang areas at the NE, NW, SW and SE margins of the Tarim Block, respectively (Fig. The Ile de Groix corresponds to the emerged part of an HPâLT metamorphic Hercynian blueschistâeclogite unit, located off the southern coast of Brittany (Audren & Lefort, 1977). Although the stability of lawsonite is shifted by only 30â80Â Â°C, this difference is relevant as the reaction has a slope parallel to most PâT paths, and small temperature differences control whether the rock is dry or volatiles are present in the rock. Most felsic granulite derived from granite (JanouÅ¡ek et al., 2004), and their present mineral assemblages show maximum pressures of 2.0â2.3Â GPa (RÃ¶tzler et al., 2004; VrÃ¡na et al., 2006; OâBrien, 2008). Contrary to clastic sediments, magmatites need to be hydrated to make metamorphic reactions possible. True Metamorphic rocks are the oldest known rocks. This area was mapped by Johnston and Walls (1974) but no detailed bulletin has been published. The facies is named after the schistose character of the rocks and the blue minerals glaucophane and lawsonite.
2020 blueschist facies indicate