The study of blueschists also demonstrates that many of their minerals contain significant amounts of Fe3+ (e.g., Maruyama et al., 1986; Brown and Forbes, 1986). These features suggest that the Archean and Paleoproterozoic rocks had undergone the same tectono-metamorphic event in the Paleoproterozoic Era because the low-grade metamorphic Mesoproterozoic unconformably overlies on the Archean and the Paleoproterozoic rocks (Xijiang BGMR, 1993). However, Fig. Also, several new high-pressure hydrous minerals have been identified in these assemblages, indicating that some water is recycled into the mantle and that not all water is lost by dehydration to the mantle wedge. In water-feel conditions, orthopyroxene may form at the expense of biotite at granulite-facies conditions (ca. However there is definite glaucophane in one restricted locality. As a result, the bulk composition exerts a fundamental role on phase stabilities, and therefore, experiments performed in certain tholeiitic compositions cannot be simply extrapolated to the wide range of compositions known to exist in basaltic oceanic crust. It started during the prograde path and the HP–LT event (M1), and lasted up to the beginning of the retrograde metamorphism (M2). In many cases, they occur adjacent to greenschist-amphibolite facies metasediments that may contain relics of blueschist/eclogite facies metamorphism. Although BS were probably exhumed along most of the length of the Zagros Mountains, the only known outcrops are in the Hajiabad–Esfandageh area (usually shortened to the Hajiabad BS) of the SSZ (Agard et al., 2006). The HP peak metamorphic conditions were estimated to be 1.6–2.5 GPa and 550–600°C in eclogites, and 1.7–2.3 GPa, 450–525°C in blueschist facies metabasites from the eastern part of the island using THERIAK-DOMINO (El Korh et al., 2009). The blueschist-facies metamorphic rocks provide critical evidence for paleo-subduction zones. Earth and Planetary Science Letters 443, 48–58. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780123852274000080, URL: https://www.sciencedirect.com/science/article/pii/B9780128150481000056, URL: https://www.sciencedirect.com/science/article/pii/B9780123851444000084, URL: https://www.sciencedirect.com/science/article/pii/B0080437516030346, URL: https://www.sciencedirect.com/science/article/pii/B9780124095489090497, URL: https://www.sciencedirect.com/science/article/pii/B9780128128725500015, URL: https://www.sciencedirect.com/science/article/pii/B9780123851444000114, URL: https://www.sciencedirect.com/science/article/pii/B978044453802400052X, Earth as an Evolving Planetary System (Second Edition), Tectonic and Structural Framework of the Zagros Fold-Thrust Belt, Developments in Structural Geology and Tectonics. In contrast to the ages obtained by the SHRIMP method, most available U–Pb ages on zircon do not consider the effect of inherited relics from magmatic or sedimentary protoliths or a stepwise growth during later stages of recrystallization in the granulite and amphibolite facies conditions. Older (Devonian) metamorphic rocks are exposed in the central part of the terrane, where they form an east-west striking “central high zone” about 500 km long and 10 km wide in the west-central part of Qiangtang. • Metamorphism of mafic rocks first evident in the greenschist facies, which correlates with the chlorite and biotite zones of associated pelitic rocks. Thereby, these rocks do not appear blue overall in color. Interpreting each HP and UHP suite in terms of subduction history carries with it its own challenges associated with tectonic and lithologic complexity. Neoarchean to Paleoproterozoic rocks in the Tarim Block mostly outcrop along its eastern and northern margins which are mainly exposed in the Quruqtagh and Dunhuang complexes, and include the Neoarchean tonalitic granitic rocks and the Paleoproterozoic amphibolite to granulite facies paragneiss, most of which were emplaced in the period 2.60–2.50 Ga (Lu, 1992; Long et al., 2010, 2011; Shu et al., 2011; Zhao and Gawood, 2012; Zhang et al., 2012). After 500 My of uplift and erosion, only the latter two assemblages would be expected to survive at the surface. The first event (D1) corresponds to the progressive deformation responsible for the blueschist facies foliation S1. Figure 9 provides greater detail regarding the peak P–T and varying prograde P–T paths of the units of the Catalina Schist. and S2 indicate that D2 was associated with normal transport on S2 foliation planes (Figure 4a). Detrital zircons from an albitic micaschist were dated using the U–Pb method and provided an age of 399±62 Ma, interpreted as the maximum age for the HP–LT event (Peucat & Cogné, 1977; Peucat, 1986). 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). Another possibility is that blueschists are thrust upward during later collisional tectonics. Figure 6. P. Fumagalli, S. Klemme, in Treatise on Geophysics (Second Edition), 2015. Typical minerals include chlorite, albite, actinolite, epidote, quartz, and possibly calcite, biotite, or stilpnomelane. Glaucophane and lawsonite indicate the blueschist facies in metamorphosed rocks of mafic (MAY-fic) composition, like basalt. Contrary to clastic sediments, magmatites need to be hydrated to make metamorphic reactions possible. Kent C. Condie, in Earth as an Evolving Planetary System (Second Edition), 2011. The high-pressure metamorphic rocks occur in coherent units as bodies up to several hundred meters thick. Additionally, counterintuitively, the stability of lawsonite is slightly enhanced by the addition of CO2, extending its stability to higher temperature (Poli et al., 2009). Uplift of blueschists led to recrystallization of lower pressure mineral assemblages. Minor amounts of K2O may stabilize phengite that, with its large stability field well beyond the stability of epidote group minerals, controls most of melting relations and geochemical signature of first partial melts (Okamoto and Maruyama, 1999; Schmidt, 1996). … In the assemblage K-feldspar–biotite–muscovite–quartz, 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+ 1Si+ 1Al− 1Al− 1. In general, the univariant line separating lawsonite from epidote in the blueschist facies is defined by a positive Clapeyron slope (Tsujimori and Ernst, 2014). The presence of blueschist metavolcanics indicate that these processes occurred at considerable depth. The Osayama blueschists indicate that the ‘cold’ subduction type (Franciscan type) metamorphism to reach eclogite‐facies and subsequent quick exhumation took place in the northwestern Pacific margin in Carboniferous time, like some other circum‐Pacific orogenic belts (western USA and eastern Australia), where such subduction metamorphism already started as early as the Ordovician. Discoveries of coesite (high-pressure silica phase) and diamond inclusions in pyroxenes and garnet from eclogites from high-pressure metamorphic rocks in eastern China record astounding pressures of 4.3 GPa (about 150-km burial depth) at 740°C (Schreyer, 1995; Zheng, 2008). Zagros orogeny: a subduction-dominated process. Considering the subduction zone system, the high‐grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Subduction zones: Characterized by low geothermal gradients: zeolite pumpellyite-actinolite facies /lawsonite albite facies blueschist facies type C eclogites. Most of these groups are considered to have formed in Andean-type continental margins, which were deformed and metamorphosed at 1.0–0.9 Ga, probably related to the assembly of Rodinia (Zhang et al., 2003; Lu et al., 2008). They are typically formed during subduction of a cold oceanic plate underneath a continent. In this case, the ages of <350 Ma may date granulite facies reequilibration of the former HP/UHP rocks. Agard et al. between blueschist-facies rocks in the Fran-ciscan Complex and overlying greenschist (or lower)-facies rocks of the ancestral forearc crust. 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. I have examined all these localities and confidently conclude that there are no blueschists in Sabah. 2004). Blueschist (pronounced /ˈbluːʃɪst/) 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. Those with aragonite and jaditic clinopyroxene, which reflect the highest pressures, are confined to arc terranes less than 200 Ma. 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?) In the Qulukatage Complex, these Neoarchean and Paleoproterozoic rocks underwent two metamorphic events at 1.9–1.8 Ga and 1.1–1.0 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). Late Mesoproterozoic to Early–Middle Neoproterozoic metamorphosed strata are exposed on the peripheral margins of the Tarim Block, represented by the Bowamu, Aierjigan and Aksu groups on the northern margin, the Kalakashi (Sailajiajitage) and Ailiankate groups on the southern margin, the Bulunkule Group on the southwestern margin, and the Altyn-Tagh Group on the southeastern margin. Thermodynamic modeling of phase relations in MORB is generally accurate, however, it becomes difficult when incorporating the amphibole solid solution. Recent studies have suggested that epidoteeclogite can form at relatively high pressure, overlapping the quartzcoesite PT curve. Water contents of the bulk rock at the beginning of the blueschist facies are ∼6 wt.% (Figure 4). Besides slight discrepancies, mainly related to different experimental setup and starting materials, all experimental studies suggest that phase relations in mafic systems are dominated by solid solutions and complex continuous reactions, with significant changes in mineral chemistry with P and T and phase abundances rather than changing phase assemblages. et al., 2006). Shufeng Yang, ... Xing Yu, in The Early Permian Tarim Large Igneous Province in Northwest China, 2018. The Lower Devonian southeastward subduction of the oceanic plate was followed by arc subduction around 360 Ma. Figure 9.1. In metasedimentary rocks, the indicator minerals are phengite, chlorite and quartz. 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. Blueschists are formed in association with subduction and continental collision and reflect burial to high pressures at relatively low temperatures. Metamorphic rock - Metamorphic rock - Zeolite facies: In the zeolite facies, sediments and volcanic debris show the first major response to burial. Shah Wali Faryad, in Ultrahigh-Pressure Metamorphism, 2011. (2003); also see P–T path for the Shuange UHP locality in Dabie. The zeolite facies was first described from southern New Zealand, but similar rocks … 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). Bosse et al. Confl icting interpretations of the Coast Range fault have arisen from analysis of kine-matic shear-sense data. 750 °C). If you look at the facies diagram, you’ll see that the blueschist facies result from high-pressure (P >6 kbar) low-temperature (T ~300˚ Celsius) conditions and correspond to depths of ~20 to 40 km. Exhumation of HP/UHP rocks, including HP granulites on the Moldanubian side, could occur along sutures and channels related to westward subduction of the Moldanubian Ocean or to a back-arc basin (Franke & Stein, 2000; Schulmann et al., 2005; Medaris et al., 2006; Faryad et al., 2009) that was situated between the Brunia and Moldanubian. Talbot, in Developments in Structural Geology and Tectonics, 2019. Figure 4. Reactions are often not complete, and typical metamorphic fabrics may be poorly developed or not developed at all. The high dehydration rates in conjunction with a—close to the trench—thin mantle wedge should lead to rapid, full serpentinization of the cold corner (i.e., within 0.8–3.0 Ma) if fluids pass pervasively through the mantle wedge (see also Gerya et al., 2002). Blueschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form blueschist. Laser probe 40Ar-39Ar radiometric age constraints on phengite yield cooling ages between 95 and 85 Ma, with a scatter of up to 115 Ma. The Eastern Belt of the Franciscan includes large tracts of lawsonite-albite facies siliciclastic rocks with a fairly coherent structure, some substantial bodies of mélange, and a number of large sheets or slabs of blueschist-facies metasediment and metabasalt (Suppe, … However, there is no evidence for this and the Archean record is dominated by ordinary P-T conditions and crustal melting at relatively low temperatures (Brown, 2007). Albite breaks down at high pressure by reaction to jadeite (a pyroxene) quartz Definition of blueschist in the Definitions.net dictionary. It has long been recognized that blueschists older than about 1000 Ma are apparently absent in the geologic record (Ernst, 1972). The facies is named for zeolites, strongly hydrated tectosilicates. At these conditions, the H2O content of the rock was estimated to be around 6 wt.% (Schmidt and Poli, 2014). Metamorphism along a medium P/T-gradient leads to only a few metamorphic minerals, such as almandine-rich garnet at lower amphibolite facies conditions – and partial melting takes place at the temperatures of the granite minimum in H2O-saturated conditions (ca. Therefore, the whole-rock composition (namely its Na 2 O and FeO* content, and the Fe 2+-Fe 3+ ratio), strongly control the … The late ages coincide with the initiation of obduction at ∼95 Ma. 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. High-Pressure Metamorphism Blueschist-facies metamorphism is important in subduction zones, where high-pressure, relatively low-temperature mineral assemblages form. The trench was the depocentre that became infilled with a great thickness of Trusmadi and Crocker Formation sediments. Three general ideas have been proposed for the absence of pre–1000-Ma blueschists and UHP metamorphic rocks: (1) steeper geotherms beneath pre–1000-Ma arcs prevented rocks from entering the blueschist and UHP stability fields, (2) uplift of blueschists and UHP rocks led to recrystallization to lower pressure mineral assemblages, and (3) erosion has removed old blueschists and UHP rocks. One of the most intriguing fields of research at present is seeing just how far crustal fragments are subducted before returning to the surface. They record metamorphism in the cool high-pressure/low-temperature thermal gradients at less than 7°C/km in subduction zones in the last 1 billion years. In 1939 P. Eskola proposed a glaucophane-schist facies of regional metamorphism. What does blueschist mean? After 500 Ma of uplift and erosion, only the latter two assemblages would be expected to survive at the surface. Data below 550 °C are based on natural blueschists and greenschists, all other data are based on experiments (compare with Figure 1). The black, unfilled oval region is for the Parigi UHP locality of the Dora Maira Massif (see discussion in Hermann et al., in press; Sharp et al., 1993), and the black, unfilled rectangular region is for the Erzgebirge UHP rocks studied by Massone and Kopp (2005). (2000, 2003) considered it to have formed by uplift and extensional detachment of the Jinsha suture subducting southward below the Qiangtang terrane. A similar model is accepted for pyroxenites in the Iberian Massif (Santos et al., 2002). Talbot, in Developments in Structural Geology and Tectonics, 2019. Seven Metamorphic Facies . The P–T peak conditions in the blueschist facies were determined to be 1.6–1.8 GPa, 450–500°C in the eastern part of the island (Upper Unit), and 1.4–1.6 GPa, 400–450°C in the western part (Lower Unit) using the NFMASH system for metapelites (Bosse et al., 2002). The general conclusion, made by Hutchison et al. 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). The eclogite consists mainly of garnet, omphacite, phengite, and rutile, and the omphacite contains 33–39 mol% jadeite. For a number of HP and UHP suites, metasedimentary rocks experience far greater overprinting during exhumation than adjacent metabasaltic rocks (Reinecke, 1998; van der Klauw et al., 1997; Fitzherbert et al., 2005). The blue color of the rock comes from the presence of the mineral glaucophane. Such paths are commonly attributed to exhumation during continued, active underthrusting to explain the sustained high-P–T conditions. Textural and compositional relations of minerals formed during the HP stage (e.g., garnet, which may even be used to date the prograde stages of metamorphism) are therefore essential to unravel the age relationships of the HP metamorphic terranes. These rocks are not “blueschists” for the small porphyroblasts cannot be discerned nor suspected in outcrop and are seen only in thin section. Copyright © 2020 Elsevier B.V. or its licensors or contributors. The P–T for the Western Alps is for the internal units (see Chalot-Prat et al., 2003), and peak metamorphic P–T estimates for Tianshan HP rocks are from Gao and Klemd (2001). From the Oligocene onward (∼30–0 Ma): As ocean closure migrated to the SW, like the closure of a zip fastener, it led to the progressive migration of deformation to the SW with successive topographic buildup of the SSZ (20–15 Ma) the High Zagros (∼12–8 Ma), and the still actively advancing simply folded belt (5–0 Ma). Perhaps the most exciting aspect of these findings is that for the first time we have direct evidence that crustal rocks (both felsic and mafic) can be recycled into the mantle. However, the chemical composition of muscovite changes significantly with T and particularily with P and may be used as a geobarometer. In addition, there have been recent advances in the study of geothermobarometry of very high-pressure rocks using different calibrations of geothermobarometers and PT pseudosections. 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). It may be that all three of these factors contributed to the absence of pre-1000-Ma blueschists. Once the oceanic crust starts subducting, most of its remnant porosity will be immediately lost by compaction and its pore fluids get expelled. Charles S Hutchison, in Geology of North-West Borneo, 2005. These sediments are therefore not strictly “accretionary prism” and the trench was not located at the NW Borneo Trough. The contribution of the mafic, basaltic, and gabbroic, part of the slab to the transport and release of fluids during subduction, has been extensively investigated both at H2O-saturated conditions, that is, in the presence of water (Forneris and Holloway, 2003; Litasov and Ohtani, 2005; Okamoto and Maruyama, 2004; Pawley and Holloway, 1993; Poli, 1993; Poli and Schmidt, 1995; Schmidt and Poli, 1998), in the presence of a CO2 fluid (Molina and Poli, 2000; Yaxley and Green, 1994), and with variable C–O–H fluids (Poli et al., 2009). (2003) have estimated peak conditions for the blueschist facies rocks (1.8–2.0 GPa, 450°C) using THERMOCALC, based on the presence of lawsonite pseudomorphs. In general, the geochronological data from HP/UHP rocks in the European Variscan Belt are classified into two sets of 380–410 Ma and around 340 Ma that are assumed to date two HP metamorphic events and hence closure of at least two oceanic basins (Franke, 2000; Linnemann et al., 2007; Keppie et al., 2009). Blueschists are formed in association with subduction and continental collision and reflect burial to high pressures at relatively low temperatures (see Figure 2.10). The hydrated oceanic crust loses up to two-third of the entire water content before the breakdown of amphibole through numerous dehydration reactions. These new results indicate that the anomalously high-T, high-P/T amphibolite facies rocks of the Catalina Schist had been deposited, accreted, and metamorphosed at peak grade conditions ~15-20 m.y. 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. Because of Figure 9. From Fig. 5 by Agard, P., Omrani, J., Jolivet, L., Whitechurch, H., Vrielynck, B., Spakman, W., Wortel, R., 2011. Most of the Zagros BS were formed during convergence at conditions near 11 kbar and 520–530°C. This must be the most attractive Malaysian rock in thin section, because of the blue–violet pleochroism of glaucophane and the yellow–pink pleochroism of piedmontite, set in a mosaic of normal quartz. I have studied the samples and identified perfectly euhedral porphyroblasts both of glaucophane and piedmontite in a quartz-rich metasandstone or metachert. At pressures of 2.2–2.4 GPa, i.e., the maximum pressure stability of amphibole in MORB (65–70 km), dehydration reactions are numerous and their orientations in P–T-space are mostly oblique to a typical subduction-type P–T path, resulting in high dehydration rates. Initially abundant chlorite has high H2O contents (12 wt.% H2O) and decomposes completely in the depth range to 70 km through various continuous and discontinuous reactions. 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. 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. 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). Geological map showing the spatial distribution of Precambrian rocks in the Tarim Block. before the lawsonite-blueschist and lower-temperature, high-P/T rocks of the complex were accreted. Shaded area represents P–T paths for slab surface from Arcay et al. Copyright © 2020 Elsevier B.V. or its licensors or contributors. 2.10 in Chapter 2), resulting in recrystallization of blueschist-facies assemblages to greenschist- or amphibolite-facies assemblages. UHP metamorphism is the first evidence of deep subduction of continental crust in the geologic record. Experimental results on mixed fluid (C–O–H) MORB compositions suggest however that, as CO2 strongly fractionates into carbonates leaving a coexistent H2O-rich fluid, amphibole stability is not significantly affected. As a reference, ocean crust is generally only 6-10 km thick. This is the rock name to remember when you find a hard, nondescript rock that looks like it … The zeolite facies is the metamorphic facies with the lowest metamorphic grade. However, sodic amphiboles display a large range in chemical compositions, owing principally to the Fe2+Mg–1and Fe3+Al–1substitutions. We use cookies to help provide and enhance our service and tailor content and ads. Our findings suggest The HP rocks are integral to all crustal massifs of the European Variscan Belt. Based on their geochemistry and geochronological data (Wendt et al., 1994; Janoušek et al., 2004; Schulmann et al., 2005), the granulite precursors were Middle- to Upper Devonian (370–380 Ma) granitic rocks. The ages of ca. Although the stability of lawsonite is shifted by only 30–80 °C, this difference is relevant as the reaction has a slope parallel to most P–T paths, and small temperature differences control whether the rock is dry or volatiles are present in the rock. The younger age (340–336 Ma) of UHP metamorphism, documented in diamond-bearing gneisses, is similar to the ages of the granulite to amphibolite facies reequilibrium of former HP/UHP rocks. The manganese epidote, piedmontite, is to be expected because the ocean floor is a region of manganese enrichment. Subduction and relatively cool and rapid exhumation of BS with fragmentation of the upper plate may have been triggered by obduction when the convergence velocities doubled (to 5–6 cm/year) during the Cretaceous (115–85 Ma). Figure 5.3. In the Moldanubian Zone of the Bohemian Massif, these rocks (HP felsic granulites, garnet peridotites, and HT eclogites) are part of the Gföhl Unit, which has also been considered to be a nappe overlaying the amphibolite facies Monotonous and Varied Units (Tollmann, 1982; Matte et al., 1990). The older ages within this range (383–365 Ma) from felsic granulites are assumed to date the formation of their protoliths (Wendt et al., 1994; Schulmann et al., 2005). However, granitoids often remain in a dry state preventing the rock from complete metamorphosis. 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). 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). The subducting slab probably began to tear locally at depth from ∼10 Ma to the present. Otherwise, incipient metamorphism is demonstrated by phengitic muscovite, chlorite ± stilpnomelane. Lawsonite blueschist and epidote blueschist facies are characterized by lawsonite + glaucophane and epidote + glaucophane paragenesis, respectively (Evans, 1990). (2002) have defined two metamorphic units separated by a ductile thrust dividing the island into the Upper Unit, or high-pressure zone, in the eastern part of the island and the Lower Unit, or lower-pressure zone, in the west. Blueschist facies generally is considered to form under pressures of >0.6 GPa, equivalent to depth of burial in excess of 15–18 km, and at temperatures of between 200 and 500 °C. The facies is named after the schistose character of the rocks and the blue minerals glaucophane and lawsonite.

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