Talbot, in Developments in Structural Geology and Tectonics, 2019. After that time, however, when geotherms were not much steeper than at present, the second two factors may have controlled blueschist preservation. Three general ideas have been proposed for the absence of pre-1000-Ma blueschists: Steeper geotherms beneath pre-1000-Ma arcs prevented rocks from entering the blueschist pressure-temperature stability field. The nature and origin of high-pressure metamorphic rock crust have a bearing on the deep crustal geometry of the mountain belt, and thus the understanding of accretionary orogens. C.J. Their histories are recorded in the few outcrops of HP-LT time minerals of the BS metamorphic facies, which are jade, lawsonite, glaucophane, or another bluish amphibole, aragonite, garnet, and the pyroxene omphacite (a variety of jade) (Fig. 5.1). 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). 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. As a consequence, dehydration rates are considerable within this P–T regime, where a fully hydrated MORB loses ∼4–6 wt.% H2O when passing through the blueschist stage within the fore-arc region. Look it up now! 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). The fraction of Fe3+/Fetot is generally highest at the onset of subduction-zone metamorphism and decreases with increasing grade. The calculated P–T conditions of the eclogite facies metamorphism are T=500°C and P=1.56–2.35 GPa (Li et al., 2006a), or 482–625°C, 2–2.5 GPa (Zhang K.J. Mineralogists of the U.S. Geological Survey also studied the samples at Menlo Park. c. migmatite melting in a subduction zone. For example, Jayko et al. 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. This event was followed by collision and large-scale crustal melting in the European Variscan Massifs. 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. Extents of exhumation-related overprinting are, in general, related to the degrees of heating during decompression, the duration of any heating (or cooling) during this decompression, the volatile content of the rocks prior to exhumation, and the bulk composition of the rocks. It may be that all three of these factors contributed to the absence of pre-1000-Ma blueschists. 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. Lawsonite blueschist and epidote blueschist facies are characterized by lawsonite + glaucophane and epidote + glaucophane paragenesis, respectively (Evans, 1990). This knowledge allows the interpretation that the Sabah Trench was located in the neighbourhood of Telupid. Blueschist Facies. Mafic protoliths best document such high-pressure conditions by their diagnostic mineral assemblages in blueschist (sodic amphibole bearing; the term ‘blueschist' derives from the blue sodic amphibole, glaucophane) and eclogite (dominated by garnet + omphacitic clinopyroxene). In quartzofeldspathic rocks, which have undergone eclogite-facies metamorphism, plagioclase is usually replaced by jadeite, zoisite, and quartz and biotite by garnet and phengite. Regional geological map showing the location of Mesozoic ophiolites and Mesozoic arc-related magnetic bodes in Iran. 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 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. Metapelitic rock compositions may develop diagnostic minerals or assemblages such as carpholite, phengite, jadeite + quartz, and talc + kyanite. Even in young collisional mountain chains such as the Himalayas, only a few minor occurrences of blueschist have not been removed by erosion. et al., 2006). Such paths are commonly attributed to exhumation during continued, active underthrusting to explain the sustained high-P–T conditions. Available data (Figure 10) report that at 700–750 °C, amphibole is stable up to a pressure of about 2.5 GPa. 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. 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). At lower temperature and pressure processes in the rock are called diagenesis. Metabasalts and metasandstones (classified as ‘Crocker Formation’) outcrop along the Sungai Lividoi, a tributary of the Labuk River, some 13 km NW of Telupid. Calculated P-T-t trajectories for blueschists suggest that they may increase in temperature prior to uplift (see Figure 2.10), resulting in recrystallization of blueschist-facies assemblages to greenschist- or amphibolite-facies assemblages. At blueschist facies conditions (i.e., deeper than ~ 15 km), basalts are transformed into rock composed of chlorite, amphibole, phengite, lawsonite or zoisite, and paragonite. Calculated pressure-temperature-time trajectories for blueschists suggest that they may increase in temperature before uplift (see Fig. A similar model is accepted for pyroxenites in the Iberian Massif (Santos et al., 2002). This area was mapped by Johnston and Walls (1974) but no detailed bulletin has been published. A confirmatory report of the mineralogy was returned to the Geological Survey, Kota Kinabalu. The basement consists of Neoarchean TTG gneisses with minor supracrustals, Paleoproterozoic mafic-felsic intrusions, high-grade supracrustals and minor anatectic granites, and Late Mesoproterozoic to Early–Middle Neoproterozoic meta-sedimentary and volcanic strata metamorphosed in greenschist and blueschist facies, which are together unconformably overlain by Late Neoproterozoic Sinian unmetamorphosed cover. 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. We use cookies to help provide and enhance our service and tailor content and ads. 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). Those with aragonite and jaditic clinopyroxene, which reflect the highest pressures, are confined to arc terranes less than 200 Ma. The blueschist metamorphic facies are characterized by the minerals jadeite, glaucophane, epidote, lawsonite, and garnet. 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). and at oxygen fugacity of approximately Ni-NiO buffer. In basic rocks of the eastern part of the island, Ballèvre et al. Much geochronological data on protoliths and metamorphism of eclogites and blueschists from different units and massifs is available. The contiguous outcrops of metabasalts (fine-grained amphibolite) also contain small amounts of glaucophane and piedmontite. Shufeng Yang, ... Xing Yu, in The Early Permian Tarim Large Igneous Province in Northwest China, 2018. Anhydrous minerals typically comprise 5–25 vol.% (e.g., Thurston, 1985; Okay, 1980) at 5–10 km depth and some 100 °C, grow in volume to ∼50 vol.% just before the amphibole-out reaction, and increase to >70 vol.% at pressures beyond amphibole stability. Our findings suggest 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. 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). Lawsonite (11 wt.% H2O) has a maximum abundance of 25 vol.% at the onset of blueschist-facies metamorphism and decreases to ∼10 vol.% at the amphibole-out reaction. It has long been recognized that blueschists older than about 1000 Ma are apparently absent in the geologic record (Ernst, 1972). Shaded area represents P–T paths for slab surface from Arcay et al. 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). Figure 10. (2007) with different water weakening effects: straight line represents a model reference for a ‘dry’ rock; dashed line refers to a moderate rock strength reduction due to water; narrow dashed line refers to a relevant strength reduction. The rock type at these depths is eclogite and the total amount of water that mafic eclogites are able to store in subarc regions is ~ 1.5 wt.% (Schmidt and Poli, 2014). Geological map of the Ile de Groix. Copyright © 2020 Elsevier B.V. or its licensors or contributors. 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. 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. Again feldspar, quartz, and H2O are used as projection phases. They are considered to possibly date an early stage of metamorphism. Blueschist, which contains the blue mineral glaucophane, forms in subduction zones under high pressures and low temperatures, and its rare occurrence in Precambrian rocks may indicate that temperatures in early subduction zones were too high for its formation. Blueschists are formed in association with subduction and continental collision and reflect burial to high pressures at relatively low temperatures. the Korla–Quruqtagh area, the Aksu–Keping area, the Tiekelike area and the Altun–Dunhuang areas at the NE, NW, SW and SE margins of the Tarim Block, respectively (Fig. The sodic amphibole glaucophane is generally considered as indicative of blueschist-facies metamorphism. Argillite. Stilpnomelane is only stable in lower greenschist- and blueschist-facies conditions whereas muscovite remains stable up to about 650–700 °C, then breaking down to K-feldspar, quartz, and an Al2SiO5-modification. Miller et al. 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.
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