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  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Pb, Nd and Sr isotope data are reported from two localities on mineral separates from Mg-rich metapelites and associated rocks that have been subducted to depths of at least 100 km, for which metamorphic conditions are estimated at 28–33 kilobars pressure and 700°–800° C, and then returned to the surface. Initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites. The initial ratios indicate predominantly recycled, aged granitic crustal materials for the sources of all of the samples. Five zircon samples, 4 from pyrope megacrysts and 1 from fine-grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304±10 and 38.0±1.4 Ma in a concordia diagram. Zircon from the country rock also plots along the chord. The zircon data, together with initial Nd and Sr data, indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic Hercynian rocks. Ellenbergerite from pyrope megacrysts and monazite from the fine-grained ground mass yield slightly younger ages of 30–34 Ma, apparently reflecting lower blocking temperatures than that of zircon. Sm−Nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 Ma, in agreement with the zircon date. A major question concerns the timing of the ultrahigh-pressure metamorphism. Experimental data suggest that pyrope and quartz/coesite as well as ellenbergerite formed by various metamorphic reactions. If, as generally assumed, the ultrahigh-pressure metamorphism occurred ca. 100 Ma ago, our data require that the zircon did not experience measurable lead loss at that time, but lost major amounts of lead 38 Ma ago during late Alpine metamorphism. Estimates of diffusion rates for Nd in pyrope further suggest that the apparent Sm/Nd age of 38 Ma for the megacryst is not consistent with that model. Those problems are resolved if the ultrahigh-pressure metamorphism occurred 38–40 Ma ago, but problems remain from Ar/Ar dates of 100 Ma on phengite, an inferred 120 Ma age for zircon lead loss from another study, and possibly by the very rapid uplift required if the metamorphism is that young.
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  • 2
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.
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  • 3
    Publication Date: 2006
    Keywords: TF IV ; Task Force IV ; Ultra-Deep Continental Crust Subduction (UDCCS)
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  • 4
    Publication Date: 2007
    Keywords: TF IV ; Task Force IV ; Ultra-Deep Continental Crust Subduction (UDCCS)
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  • 5
    Publication Date: 2007
    Keywords: TF IV ; Task Force IV ; Ultra-Deep Continental Crust Subduction (UDCCS)
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  • 6
    Publication Date: 2007
    Keywords: TF IV ; Task Force IV ; Ultra-Deep Continental Crust Subduction (UDCCS)
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  • 7
    Publication Date: 2014-12-09
    Description: At Qianliyan Island, about 80 km East of Qingdao in the northern part of the South Yellow Sea of China, lenses and layers of eclogite and two-mica epidote–plagioclase gneiss occur within granitic gneiss. From the eclogite, peak metamorphic conditions of 775°C, 2.6 GPa were defined using compositions of garnet mantle zones, omphacite inclusions, and homogeneous phengite cores with high Si contents. During exhumation, the eclogite underwent different stages of retrogression, i.e. at eclogite-facies (740–790°C, 1.60–1.75 GPa), amphibolite-facies (640–690°C, 0.8–1.0 GPa), and, finally, at greenschist-facies conditions. Relics of omphacite and garnet in pyrite of the two-mica epidote–plagioclase gneiss, although obliterated in the rock matrix, prove this rock also to have experienced high-pressure (H P ) metamorphic conditions. Similar H P metamorphic rocks occur at Sulu and Dabie in the west and at the Hongseong complex of South Korea in the east, enhancing the probability of a coherent H P /(UH P ) belt ranging from Qinlin in the west towards the Korean peninsula in the east.
    Print ISSN: 0935-1221
    Electronic ISSN: 1617-4011
    Topics: Geosciences
    Published by Schweizerbart
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  • 8
    Publication Date: 2011-04-19
    Description: Sodic pyroxene is reported from an Ordovician metatrachyte of the Kaczawa Mountains, SW Poland. Its composition ranges from Jd0.98Ae0.02 to Jd0.15Ae0.85. Relict jadeite and phengite (up to 3.75 Si atoms per fomula unit) belong to the peak-pressure assemblage of an early HP-LT event. Later greenschist-facies stages are represented by riebeckite, biotite, chlorite, low-Si potassic white mica and actinolite. P-T pseudosections calculated for the range 200-450{degrees}C, 3-13 kbar allow evaluation of the conditions formation of jadeite in the metatrachyte and derivaton of a P-T path. Considering the position of prograde, peak and retrograde metamorphic assemblages and respective mineral compositions, we can derive the following equilibration stages: 8.5{+/-}0.5 kbar, 270{+/-}20{degrees}C for the pressure maximum, 6.0{+/-}1.0 kbar, 310{+/-}20{degrees}C for the temperature maximum and 3.5{+/-}0.5 kbar, 280{+/-}20{degrees}C as well as
    Print ISSN: 0026-461X
    Electronic ISSN: 1471-8022
    Topics: Geosciences
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  • 9
    Publication Date: 2019
    Description: 〈p〉High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role for understanding the tectonic evolution of orogenic belts (〈cross-ref type="bib" refid="SP474-18-149C19"〉Johnson, M.R.W. & Harley, S.L. 2012〈/cross-ref〉. 〈i〉Orogenesis: The Making of Mountains〈/i〉. Cambridge University Press, Cambridge). They have typically experienced complex changes during subduction and exhumation processes, arising from recrystallization, deformation, element redistribution, fluid–rock interactions and even partial melting, and may therefore carry a valuable record of evolving geodynamic systems in an orogenic belt. Until now, more than 20 UHP metamorphic belts, i.e. belts which contain rocks that experienced pressure–temperature (〈i〉P–T〈/i〉) conditions exceeding the lower limit of the coesite stability field, have been identified all over the world (〈cross-ref type="bib" refid="SP474-18-149C1"〉Carswell, D.A. & Compagnoni, R. 2003〈/cross-ref〉. 〈i〉Ultrahigh Pressure Metamorphism〈/i〉. Eotvos University Press, European Mineralogical Union, Notes in Mineralogy, Budapest; 〈cross-ref type="bib" refid="SP474-18-149C4"〉Liou, J.G., Ernst, W.G., Zhang, R.Y., Tsujimori, T. & Jahn, B.M. 2009〈/cross-ref〉. Ultrahigh-pressure minerals and metamorphic terranes – the view from China. 〈i〉Journal of Asian Earth Sciences〈/i〉, 〈b〉35〈/b〉, 199–231; 〈cross-ref type="bib" refid="SP474-18-149C17"〉Zheng, Y.F., Zhang, L.F., McClelland, W.C. & Cuthbert, S. 2012〈/cross-ref〉. Processes in continental collision zones: preface. 〈i〉Lithos〈/i〉, 〈b〉136〈/b〉, 1–9). New and innovative studies from different geoscience disciplines have been invaluable in developing a better understanding of the geodynamic evolution of orogenic belts. This special issue contains 15 papers, most of which were presented as part of the session ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’, held at the 35th International Geological Congress in Cape Town, South Africa during 27 August to 4 September, 2016.〈/p〉
    Print ISSN: 0375-6440
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 10
    Publication Date: 2018
    Description: 〈p〉Many archaeological sites with jadeitite artefacts are known in the Caribbean region, but defining the source of the raw material is a major problem because of great mineralogical heterogeneity both in potential sources and in artefacts. The archaeological settlement site of Playa Grande on the northern coast of the Dominican Republic is particularly significant because it yielded evidence of on-site axe manufacture, and lies only 20–30 km NE of a recently discovered potential source area of serpentinite mélanges in the nearby Río San Juan Complex (RSJC). A suite of nine artefacts was chosen from a collection of over 100 excavated woodworking tools rich in jadeite, as well as two blueschist artefacts. Permission to perform destructive analysis allowed data on petrography, mineral chemistry and bulk-rock chemistry to be obtained. Seven of the nine artefacts are jadeitite 〈i〉sensu stricto〈/i〉 (〉90 vol% jadeite), which are identical to material known from the RSJC. Two artefacts are jadeite–lawsonite rocks. These and the two blueschists show only minor differences from corresponding rocks of the RSJC source. With this direct linking of source and site material, it is now possible to better define source discriminators for the Caribbean and to assess sampling bias.〈/p〉
    Print ISSN: 0375-6440
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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