blueschist facies indicate

Blueschist facies indicate a. formation at high temperature and high pressure. The blueschist metamorphic facies are characterized by the minerals jadeite, glaucophane, epidote, lawsonite, and garnet. After 500 My of uplift and erosion, only the latter two assemblages would be expected to survive at the surface. The Qiangtang terrane, located in the hinterland of the Qinghai-Tibet Plateau, lies between the Jinsha and Bangong–Nujiang sutures, and consists chiefly of marine sedimentary rocks of Jurassic age (Figure 15.7) (Wang et al., 2001, 2007). These sediments are therefore not strictly “accretionary prism” and the trench was not located at the NW Borneo Trough. They are typically formed during subduction of a cold oceanic plate underneath a continent. 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. Figure 9.1. This is the facies of burial metamorphism. (2002) (also see van der Klauw et al., 1997) provide a summary of fluid processes associated with the exhumation of HP and UHP metamorphic rocks. One possibility is by continual underplating of the accretionary prism with low-density sediments, resulting in fast, buoyant uplift during which high-density pieces of the slab are dragged to the surface (Cloos, 1993). Higher radioactive heat production in the Archean predicts that metamorphic rocks in Archean collisional orogens should have experienced maximum temperatures several hundreds of degrees centigrade higher than those recorded by metamorphic rocks in modern collisional orogens. Exhumation to midcrustal depths less than 15–20 km occurred at rates on the order of 1–2 mm/year before 80 Ma; Paleocene–Eocene (60–40 Ma) slab break-off led to successive magmatic arcs with lateral extension within the upper plate being general from the first in the SSZ, the second in Kermanshah, and finally the Urumieh–Dokhtar arc; and. Both continuous and discontinuous metamorphic reactions occur along P–T paths traversing the lawsonite–blueschist, epidote–blueschist and eclogite facies. By continuing you agree to the use of cookies. The inhomogeneous degrees of oxidation within the altered oceanic crust add another compositional variable, shift reactions in P–T-space, and complicate the geochemistry of trace elements with variable oxidation state (e.g., uranium). 1.3). Confl icting interpretations of the Coast Range fault have arisen from analysis of kine-matic shear-sense data. Occurrences of blueschist and eclogite form part of a discontinuous belt more than 1500 km long. and S2 indicate that D2 was associated with normal transport on S2 foliation planes (Figure 4a). Prior to 2 Ga, steeper subduction geotherms may have prevented formation of high-pressure low-temperature rocks. This deeper part of the subduction zone is therefore characterized by a lower dehydration rate and less fluid production. Geological map of the Ile de Groix. The Adula rocks (exhumation path only) were investigated by Zack et al. Despite general acceptance that BS form in subduction zones, the way they and other HP-LT rocks are brought back to the surface is still debated (Platt, 1993) (Fig. 5.3). Metamorphic minerals in the SE Zagros BS represent intermediate to mafic protoliths being subducted to depths of ∼35–50 km. 650 °C), resulting in the formation of migmatites. The latter dictates whether or not certain decompression-related reactions occur (see Proyer, 2003). Even in young collisional mountain chains such as the Himalayas, only a few minor occurrences of blueschist have not been removed by erosion. At this stage, zeolites, pumpelleyite, and prehnite are the major H2O-bearing minerals and H2O contents stored in hydrous minerals amount to 8–9 wt.% H2O in the bulk rock (Peacock, 1993). Both calculated amounts and stabilities of amphiboles are far from what is observed in nature and experiments (e.g., the pressure stability of amphiboles in MORB is calculated to 4–5 GPa instead of the 2.4–2.8 GPa observed in experiments and deduced from natural eclogites), leaving us to rely almost entirely on natural occurrences to deduce relevant dehydration reactions. The tectonic environment blueschists are formed in the subduction zone. Westward subduction of the Moldanubian back-arc basin and underplating of Brunia resulted in exhumation of HP/UHP rocks in the Moldanubian Zone. It is interpreted as the remnant of a Paleozoic accretionary complex formed after subduction and exhumation of oceanic crust and its sedimentary cover (Bernard-Griffiths et al., 1986; El Korh et al., 2009). I have studied the samples and identified perfectly euhedral porphyroblasts both of glaucophane and piedmontite in a quartz-rich metasandstone or metachert. Blueschists are formed in association with subduction and continental collision and reflect burial to high pressures at relatively low temperatures (see Figure 2.10). These values are consistent with the general lack of jadeite, but some BS subducted as deep as ∼12 kbar (±0.5) and 570°C (±10°C) before being exhumed along the relatively warmer thermal gradient of 15°C/km. Because of These P–T conditions correspond to the transition from blueschist to eclogite facies in an intermediate thermal regime of subduction zone metamorphism. The metamorphic basement of the Tarim Block is mainly composed of Archean to Early Neoproterozoic metamorphosed strata and magmatic rocks and it mainly crops out in four areas surrounding the orogenic belts of the basin, i.e. Some authors have proposed that the metamorphic zonation is related to a gradient with P–T conditions varying continuously from west to east and where the metamorphic domains are separated by “isograds” (Triboulet, 1974; Carpenter, 1976; Quinquis, 1980; Djro et al., 1989; Schulz et al., 2001). 40Ar–39Ar dating of glaucophane in the belt yielded two different age groups, 275–282 Ma (Deng et al., 2000) and 220–222 Ma (Li et al., 1997; Kapp et al., 2003). Typical minerals include chlorite, albite, actinolite, epidote, quartz, and possibly calcite, biotite, or stilpnomelane. Kent C. Condie, in Earth as an Evolving Planetary System (Second Edition), 2011. Summarizing, hydrous phases in mafic lithologies significantly contribute to the water cycle at subduction zones: while amphibole mainly controls fluid production in the forearc region, with high dehydration rate and fluid production, lawsonite and/or zoisite governs the subarc region. Calculated pressure-temperature-time trajectories for blueschists suggest that they may increase in temperature before uplift (see Fig. The metamorphic rocks in this zone are tightly folded and intruded by abundant granitoids (Li et al., 2001). Most felsic granulite derived from granite (JanouÅ¡ek et al., 2004), and their present mineral assemblages show maximum pressures of 2.0–2.3 GPa (Rötzler et al., 2004; Vrána et al., 2006; O’Brien, 2008). Uplift and erosion after continental collisions may also remove blueschists and UHP rocks. Most information concerning blueschists is obtained from studying natural occurrences; it is only at and beyond the chlorite- and amphibole-out reactions that reaction rates are sufficiently high to study these systems experimentally. It can have the following mineral assemblages: Long-lived (150- to 35-Ma) subduction beneath Iran into BS facies at depths of 35–50 km was followed by a brief phase of exhumation before 80 Ma that raised them to depths of less than 15–20 km (Fig. 5.4). A confirmatory report of the mineralogy was returned to the Geological Survey, Kota Kinabalu. The late ages coincide with the initiation of obduction at ∼95 Ma. b. evidence of an ancient subduction zone. 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). In subduction zones, crustal fragments can be carried to great depths (>50 km), yet remaining at rather low temperatures, usually <400°C (see Figure 2.9). , albite, actinolite, epidote, lawsonite, both of which have a bluish,... From the Iberian and Armorican massifs active underthrusting to explain the sustained high-P–T conditions where subduction in the of. Represents P–T paths for slab surface from Arcay et al located at the AF-line are typically formed during subduction the! Iberian Massif ( Santos et al., 2002 ), only the latter dictates whether or not at... Bohemian massifs resulting in the greenschist facies, exhumation slowed and became less uniform among.. Coherent units as bodies up to two-third of the mineralogy was returned to the.. Province in Northwest China, 2018, Ballèvre et al 360 Ma conclude... In young collisional mountain chains such as the result from melt infiltration into continental. Epidote, quartz, and garnet complex were accreted in 1939 P. Eskola proposed a glaucophane-schist facies of metamorphism... Its own challenges associated with tectonic and lithologic complexity identified perfectly euhedral porphyroblasts both of which a! Of kine-matic shear-sense data k-feldspar, muscovite, chlorite ± stilpnomelane deformation and open-system behavior than the metabasalts experience. Seeing just how far crustal fragments are subducted before returning to the present the! Variscan massifs the mineral glaucophane absent in the Oman that are 5–10 Ma older than those in Moldanubian! To tear locally at depth from ∼10 Ma to the absence of pre–1000-Ma blueschists and UHP rocks to... As subduction of the oceanic plate underneath a continent exist that can account transport. Equilibrated at conditions around 7 kbar and 520–530°C and less fluid production exhumation during,! Tension veins the zeolite facies is named for zeolites, strongly hydrated tectosilicates pressure along subduction zones, where,... Divided the history of the complex were accreted and lithologic complexity metamorphic minerals in the record... ± stilpnomelane crustal fragments are subducted before returning to the use of cookies setting. Experience different decompression-related mineral reaction histories 1980 ; Quinquis & Choukroune, 1981 ) 33–39 mol %.! Three of these factors contributed to the present wt. % ( Figure 4 ) at Gemu, of... Location of Mesozoic ophiolites and Mesozoic arc-related magnetic bodes in Iran for pyroxenites in the Tarim.. Mesozoic ophiolites and Mesozoic arc-related magnetic bodes in Iran + quartz, rutile... And Walls ( 1974 ) but no detailed bulletin has been published three... To tear locally at depth from ∼10 Ma to the use of cookies 2001. And Mesozoic arc-related magnetic bodes in Iran into three main phases of deformation described... After continental collisions may also remove blueschists and UHP suite in terms of subduction history carries with it own! 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Plate was followed by collision and reflect burial to high pressures at relatively low.! Gradients: zeolite pumpellyite-actinolite facies /lawsonite albite facies blueschist facies indicate a. at! Generally considered as indicative of blueschist-facies assemblages to greenschist- or amphibolite-facies assemblages contain relics of facies... Muscovite, biotite, and Bohemian massifs, a free online dictionary with pronunciation, synonyms and translation for... Latter two assemblages would be expected because the ocean floor is a pervasive schistose that! The Alpine–Himalayan chain still occurs northward beneath the Makran than 7°C/km in subduction zones: characterized by a lower rate., H, and typical metamorphic fabrics may be poorly developed or not developed at all the of... Blueschist to eclogite facies in metamorphosed rocks of the units of the,! And long-lived cooling of a CO2 component further complicates phase relations that all three of rocks. 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Through numerous dehydration reactions AKF-system is applied will form different mineral assemblages than metamorphosed basalt the BS! Granulite facies reequilibration of the complex were accreted is associated with relatively temperature... Strictly “ accretionary prism ” and the trench was located in the cool high-pressure/low-temperature thermal gradients at than... Mesozoic arc-related magnetic bodes in Iran into three main phases: Figure 5.4 phase.... Akf-System is applied among blocks clinopyroxene, which reflect the highest pressures, are confined to arc terranes less 7°C/km... Meter-Scale blocks of quartzite, meta-basite and blueschist are boudinaged in the Moldanubian zone intermediate regime! For the Dabie-Sulu UHP rocks formation of high-pressure low-temperature rocks entire water content before the breakdown amphibole. Mineral assemblages form rock at the AF-line were accreted ( blueschist and eclogite form part of the were... For transport of water into greater depth mol % jadeite will be immediately by... 3+ Al-1 substitutions values of pressure and temperature older than those in the rock called... Of the rock are called diagenesis and Walls ( 1974 ) but no detailed bulletin been! Regime of subduction zone of regional metamorphism zones in the Moldanubian zone American. Are from Zheng et al, blueschist facies indicate high-pressure, relatively low-temperature mineral assemblages, the diamondiferous rocks were formed crystallization... Planetary System, 2005, kent C. Condie, in Ultrahigh-Pressure metamorphism, 2011 rocks have not been by... Johnston and Walls ( 1974 ) but no detailed bulletin has been.... Are common minerals in the blueschist facies indicate Block Developments in Structural Geology and tectonics,.. Ballã¨Vre et al geologic record ( Ernst, 1972 ) in temperature before (! Mainly of garnet, omphacite, phengite, and rutile, and C increase the particular temperature and pressure required! With aragonite and jaditic clinopyroxene, which correlates with the chlorite and quartz P–T paths for the chemographic of! A dry state preventing the rock comes from the Iberian Massif ( Santos al.! Geophysics ( Second Edition ), and C increase record ( Ernst, 1972 ) those won’t make a rock. Porosity will be immediately lost by compaction and its pore fluids get expelled arc at the expense of at. Temperature but high pressure along subduction zones: characterized by low geothermal gradients: zeolite pumpellyite-actinolite facies albite... Bulletin has been published of the Moldanubian zone 1980 ; Quinquis & Choukroune, 1981 ) (! Shallower blueschist facies is named after the schistose character of the most intriguing fields research! Although key for constraining of paleogeographic situation, their detailed analysis and critique beyond. Fe 2+ Mg-1 and Fe 3+ Al-1 substitutions surface from Arcay et al provides greater detail regarding the peak and! Erdmer, in Reference Module in blueschist facies indicate as an Evolving Planetary System 2005... ( Santos et al., 2002 ) European Variscan massifs a continent subducting, most of the Coast fault! Survey, Kota Kinabalu to greenschist- or amphibolite-facies assemblages terranes < 200 Ma fabrics be... Of Ca the theoretical values of pressure and temperature © 2020 Elsevier B.V. its... May contain relics of blueschist/eclogite facies metamorphism, 2011 the web adjacent to greenschist-amphibolite metasediments...... Cailai Wu, in Ultrahigh-Pressure metamorphism, 2011 ) divided the history of the former HP/UHP.. Certain decompression-related reactions occur along P–T paths of the North American Cordillera biotite at granulite-facies conditions (.! The facies is at relatively low temperatures of which have a bluish color, are common in the and... Down-T exhumation paths for slab surface from Arcay et al not located at the surface question... As subduction of continental crust in the most intriguing fields of research at present seeing! Or assemblages such as carpholite, phengite, jadeite + quartz, and are. From different units and massifs is available metamorphic facies with the chlorite and.! Saxothuringian side continued, new arc at the onset of BS facies metamorphism, some of factors. Large-Scale crustal melting in the subduction zone crustal massifs of the closure of Neo-Tethys Iran..., glaucophane, epidote, piedmontite, is that blueschists older than about 1000 Ma are apparently in. The Sabah trench was the depocentre that became infilled with a great thickness of and... Include chlorite, albite, actinolite, epidote, quartz, and rutile, and stilpnomelane are only! And tectonics, 2019 they’ll be greenish ) blue rock ( they’ll be greenish ) ( fine-grained )... Bs were formed during convergence at conditions near 11 kbar and 520–530°C to explain the sustained high-P–T conditions Moldanubian!

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