Fluid inclusions of the high-pressure granulites from the Namche Barwa complex of the eastern Himalayan syntaxis, Tibet: Fluid composition and evolution in the continental subduction-zone
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Early Precambrian rock units in the Urals are present in several polymetamorphic complexes, which are exposed in the Urals in the form of small (<1500 km2) tectonic blocks. Their ages are Archaean (as old as 3.5 Ga) and Palaeoproterozoic. During the formation of these complexes in the early Precambrian, two stages of ultra-high-temperature (granulite) metamorphism occurred. The maximum age of the early Neoarchaean stage of metamorphism is 2.79 Ga. Evidence of this metamorphic event includes the dating of the Taratash gneiss-granulite complex of the South Urals. Gneiss-migmatite complexes, which dominate the lower Precambrian section of the Urals, were formed in the Palaeoproterozoic during the sequential appearance of granulite facies metamorphism followed by amphibolite facies metamorphism and accompanying granitization. The maximum age of the Palaeoproterozoic stage of granulite metamorphism in the Alexandrov gneiss-migmatite complex, the most well-studied complex in the South Urals, is 2.08 Ga.
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The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km.
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On the basis of lithology and metamorphic grade,the Precambrian basement of the Trans-North China Orogen( TNCO) is divided into the high-grade metamorphic terrains and granite-greenstone belts. The former consist of the Taihua,Luliang,Fuping,Hengshan,Huai 'an and Xuanhua complexes,which underwent high-amphibolite to granulite faces metamorphism. The granitegreenstone belts include the Dengfeng,Zanhuang and Wutai domains,metamorphosed in greenschist to amphibolite facies. Their P-T estimates show that the granulites and / or retrograded eclogites in the Hengshan,Huai'an and Xuanhua domains record the maximum pressures of the peak metamorphism,and the granulites in the Hengshan,Fuping and Xuanhua complexes yield the maximum tempretures of the peak metamorphism. It was found that the high-pressure amphibolites and metapelites were exposed in the Zanhuang and Wutai metamorphic complexes. The basement rocks from the TNCO,regardless of their protolith and metamorphic grade,record a similar metamorphic history characterized by the prograde, peak, nearly isothermal decompression and retrogressive cooling metamorphic stages,defining near-isothermal decompressional clockwise P-T paths,which related to continental collision in the central section of the North China Craton. Geochronological data of the complexes in the TNCO indicate three group metamorphic ages,~ 1. 85 Ga,~ 1. 95 Ga and ~ 2. 5Ga. The ubiquitous metamorphic ages of ~ 1. 85 Ga retrieved by miscellaneous methods throughout the metamorphic domains of the TNCO, representing the peak metamorphism occurred at the Palaeoproterozoic, and ~ 1. 95 Ga metamorphic ages suggest a prograde segement of this metamorphic event. Those of ~ 2. 5Ga represent a regional granulite-facies metamorphic event related to the intrusion and underplating of large amounts of mantle-derived magma in some old block,occurred at the Late Neoarchean. However,the relationship between the metamorphic stages and the metamorphic ages is still unidentified.
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Metamorphic minerals are widely developed in Laoling Group of southern Jilin Province.Based on a detailed analysis of the metamorphic minerals in rock samples,the authors hold that the main minerals in lower sub-group of Laoling Group can be divided into two generations:M1 and M2,which represent two metamorphic stages.Researches on metamorphic assemblage,distribution characteristics and division of metamorphic facies show that M1 is characterized by low-to-moderate temperature regional dynamic heat flow metamorphism which can be divided into two metamorphic facies:lower greenschist facies and high greenschist facies.By contrast,M2 is represented by local thermal metamorphism which can be divided into three metamorphic facies:lower greenschist facies,high greenschist facies and lower amphibolitic facies.Comprehensive studies show that metamorphism of the sub-group of Laoling Group in southern Jilin Province is characterized by reduction of influencing area and decrease of intensity from early to late.This tendency suggests hat the earth's crust became stable in Paleo Proterozoic.
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The various models for the nature and origin of fluids in granulite facies metamorphism were summarized. Field and petrologic evidence exists for both fluid-absent and fluid-present deep crustal metamorphism. The South Indian granulite province is often cited as a fluid-rich example. The fluids must have been low in H2O and thus high in CO2. Deep crustal and subcrustal sources of CO2 are as yet unproven possibilities. There is much recent discussion of the possible ways in which deep crustal melts and fluids could have interacted in granulite metamorphism. Possible explanations for the characteristically low activity of H2O associated with granulite terranes were discussed. Granulites of the Adirondacks, New York, show evidence for vapor-absent conditions, and thus appear different from those of South India, for which CO2 streaming was proposed. Several features, such as the presence of high-density CO2 fluid inclusions, that may be misleading as evidence for CO2-saturated conditions during metamorphism, were discussed.
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