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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 9 (1991), S. 0 
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Abstract Aegirine–jadeite clinopyroxene (〉60 mol% jadeite) locally occurs within blueschists of the ‘Lower Allochthon’exposed in the Trás-os-Montes region of northern Portugal. Peak conditions attained during blueschist facies metamorphism are estimated to have been c. 420° C and 〉11 kbar. Porphyroblastic white mica (paragonite/phengite) within the blueschist assemblage records a 36Ar/40Ar versus 39Ar/40Ar isotope correlation age of 329.4 ± 1.6 Ma. In view of the relatively low-T nature of the metamorphism, the c. 330-Ma age is interpreted to date closely the high-P recrystallization. This tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex (‘Upper Allochthon/Ophiolite Nappe’) onto Iberian protoliths of the Lower Allochthon during terminal stages of the Hercynian orogeny.
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 8 (1990), S. 0 
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian (c. 370 Ma). Variably recrystallized amphibole (iron-rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south-westerly younging 40Ar/39Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron-rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post-metamorphic cooling. The 40Ar/39Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high-grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    International journal of earth sciences 71 (1982), S. 657-686 
    ISSN: 1437-3262
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Abstract Mineral chemistry and crystal growth-deformation relationships in metapelites, orthogneises and metabasic rocks from the Muxia-Finisterre region show that this area has been affected by plurifacial regional metamorphism during the Hercynian Orogenesis. Paragenetic evolution seems to be related to a change of the metamorphic gradient towards relatively lower Ps conditions in the mesozone, while this character is less evident in catazonal rocks. In both cases there was a continuous increase in T and decrease in {ie657-02}. The metamorphic climax was reached during the second metamorphic episode (M2). This event took place during and even after the second deformation phase (P2). The latter gave rise to the main foliation observed in studied rocks (S2) which almost practically erased signs of previous schistosity. Relationships between metamorphic evolution and different geotectonic models are discussed.
    Abstract: Résumé Les compositions chimiques des minéraux ainsi que leur relation croissance-déformation dans les métapélites, ortho-gneiss et métabasites de la région Muxia-Finisterre, suggèrent que cette secteur a subi un métamorphisme régional plurifacial lors de l'Orogenèse Hercynienne. L'évolution des paragenèses semble être liée à un changement du gradient métamorphique vers des conditions de Ps relativement plus basses dans la mésozone, ceci étant moins évident dans les roches catazonales. Dans les deux cas il y a eu une augmentation continue de la T et une diminution de la {ie657-03}. Le paroxisme du métamorphisme a été atteint lors du deuxième épisode métamorphique (M2). Cet événement a lieu généralement pendant ou légèrement après la deuxième phase de déformation (P2). Celle-ci donne la foliation principale de la roche (S2), laquelle a presque entièrement effacé les marques d'une schistosité pré-existante. Des relations entre l'évolution métamorphique et différents modèles géotectoniques sont discutées.
    Notes: Zusammenfassung Mineralchemismus und die Beziehungen Kristallwachstum-Deformation in Metapeliten, Orthogneisen und metabasischen Gesteinen aus der Muxia-Finisterra-Region, zeigen eine Beeinflussung dieses Gebietes durch eine plurifazielle Regionalmetamorphose während der herzynischen Orogenese. Die paragenetische Entwicklung scheint mit einer Veränderung des metamorphen Gradienten in Richtung niedrigerer Ps-Bedingungen in der Mesozone einherzugehen, während dies in katazonalen Gesteinen weniger deutlich ist. In beiden Fällen gab es eine kontinuierliche Zunahme der Temperatur (T) bzw. Abnahme des {ie657-01}. Der Höhepunkt der Metamorphose wurde während der zweiten metamorphen Episode (M2) erreicht. Dieses Ereignis fand statt während und sogar nach der zweiten Deformationsphase (P2). Letzteres verursachte die Hauptschieferung (S2) in den untersuchten Gesteinen, die die Merkmale einer älteren Schieferung nahezu ausgelöscht hat. Beziehungen zwischen metamorpher Entwicklung und verschiedener geotektonischer Modelle werden diskutiert.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Contributions to mineralogy and petrology 80 (1982), S. 14-24 
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The core of the El Tormes thermal dome, situated in the central part of one of the main metamorphic belts of the Iberian Peninsula, is formed by garnet-cordierite-biotite-sillimanite pelitic gneisses. These rocks, that very often are cut by minor intrusions of Al-rich S-type granites, are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite±plagioclase±biotite. The garnet, mantled and corroded mainly by cordierite, has never been found to occur in contact with the prismatic sillimanite of the matrix, thus indicating that the continuous reaction Gr+Sill+Q = Cd has taken place. The presence of corroded biotite inside the garnet-rimming cordierite of the aggregates as well as inside the cordierite of the matrix, which usually includes remmants of sillimanite, indicates that the continuous reaction Bi+Sill+Q = Cd+FK+H2O has occurred too. Therefore, a realistic net reaction for these aggretates should be represented by the univariant, at a given $$X_{{\text{H}}_{\text{2}} {\text{O}}} $$ , equilibrium: Biotite+Sillimanite+Garnet+Quartz = Cordierite+K-feldspar+H2O (1) The important garnet resorption near the anatectic granitic veins implies that this process is favoured by a decrease in $$X_{{\text{H}}_{\text{2}} {\text{O}}} $$ , this factor being otherwise buffered by the reaction (1) assemblage. The most probable P-T path, assuming these conditions, consistent with the AFM projection of the former (inferred) and present assemblages in the aggregates and in the matrix, implies a decrease in P coeval with a decrease in T (Fig. 4, path 2). The most reliable P-T determination for the final stage of garnet breakdown through reaction (1), based on the coexistence of the seven phase assemblage garnet — cordierite — biotite — sillimanite — plagioclase — potash feldspar — quartz plus melt, gives 695° C, 4.3 kbar, $$X_{{\text{H}}_{\text{2}} {\text{O}}} $$ = 0.5, The maximum pressure for this process, obtained from the garnet — plagioclase equilibrium, is 6.5±1 kbar at the same temperature. The estimates of the T for the garnet core-garnet included biotite pairs are consistently lower, ca. 550° C, than those obtained for the garnet rim-biotite in aggregates, ca. 645° C, or garnet rim-adjacent cordierite pairs, ca. 695° C. It may, therefore, be supposed that, during their evolution these rocks underwent first an increase in T and then, during the last stages, as garnet and biotite brokedown, a decrease in P and T. This represents an uplift of the core of El Tormes dome under high grade amphibolite to low pressure granulite facies conditions, accompanied by a process of partial melting with local decrase in $$X_{{\text{H}}_{\text{2}} {\text{O}}} $$ . It is suggested, from mineral growth-deformation relationships, that this process took place during the late hercynian deformation phases, P-3 or doming stage.
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  • 5
    Publication Date: 2014-12-20
    Description: El volcán Las Herrerías (Bolaños de Calatrava; Región Volcánica del Campo de Calatrava) se caracteriza por una gran cantidad y variedad de depósitos originados a partir de fuentes de lava. Todos estos depósitos son composicionalmente idénticos, estando constituidos por melanefelinitas fuertemente sódicas (Na₂O = 2.24–3.81%), ricas en magnesio (MgO = 11.58–14.19%), pobres en aluminio (Al₂O₃ = 9.64–10.99%) y con elevados contenidos en tierras raras (10x–200x condrito), fundamentalmente más en tierras raras ligeras que en pesadas [(La/Lu) N = 32–35]. Los contenidos en Ni (233–286 ppm), Cr (393–520 ppm) y el número de magnesio (Mg* = 45–54) indican que no son magmas primarios, a diferencia de la mayor parte de las melanefelinitas equivalentes de esta región volcánica. Por otro lado, las ligeras variaciones composicionales mostradas por las muestras estudiadas responderían al efecto de la irregular distribución del clinopiroxeno en el magma durante la erupción. Los datos obtenidos son coherentes con el desarrollo de estas fuentes de lava en un ambiente de intraplaca continental.
    Print ISSN: 0367-0449
    Electronic ISSN: 1988-3250
    Topics: Geosciences
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  • 6
    Publication Date: 2014-08-02
    Description: Iron-Mn phosphate minerals are common accessory phases in pegmatites, some granites and, rarely, in some hydrothermal quartz-rich dikes. A comprehensive textural and geochemical characterization of primary phosphate minerals belonging to the triplite-zwieselite, triphylite-lithiophilite, and graftonite-beusite series has been done. Chemical data, including major and trace elements contents, have been obtained by electron microprobe (EMP) and laser-ablation inductively coupled-plasma mass-spectrometry (LA-ICP-MS) techniques. The concentration of trace elements shows important differences for the three series of primary Fe-Mn phosphate minerals. Members of the triphylite-lithiophilite series are the poorest in trace elements, with Zn as the only element that may occur in important amounts. This low concentration in trace elements could be related to structural constrains. Graftonite-beusite members are the richest in REE, most probably due to the substitution of Ca by the rare earths. Members of the triplite-zwieselite series are the richest in certain HFSE, such as Nb and Ta. There is an important difference in the shape of multi-element diagrams of triplite-zwieselite samples associated with pegmatites, all of them showing a strong negative Eu anomaly, and those of samples from a quartz-rich dike, with no Eu anomaly. This difference is interpreted as the result of the Eu fractionation in pegmatites due to the previous crystallization of plagioclase. No correlation has been found between the Fe/(Fe+Mn) ratio and the different trace element contents of the primary phosphate series. Moreover, the concentrations of trace elements are not simply related to the pegmatite type, its paragenesis or its geological setting. Further investigation of mineral association, textural relationships, partition coefficients, and geology and history of crystallization of the pegmatitic bodies appears necessary in order to determine the factors controlling the variability in the trace elements content for the different primary phosphate series.
    Print ISSN: 0008-4476
    Topics: Geosciences
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  • 7
    Publication Date: 2017-05-24
    Description: Recently discovered Albian submarine diatremes sample tectonites from the Basque-Cantabrian Basin concealed basement. Garnet-sillimanite gneiss xenoliths provide petrologic, petrofabric, and radiometric evidence on a so-far unidentified early Permian (ca. 280–275 Ma) high-grade regional metamorphism. It is interpreted as a syntectonic event recorded at middle to lower crustal settings and related to the Variscan orogeny. This metamorphism is 25–30 m.y. younger than the Variscan high-grade regional metamorphism preserved in the axial zone of the Pyrenees and 15–30 m.y. younger than low-pressure plutonometamorphism documented in the core of the Ibero-Armorican arc. By contrast, it is 15–20 m.y. older than Variscan regional and contact metamorphism unraveled in basement outcrops around the Basque-Cantabrian Basin; it can be thoroughly discriminated from Proterozoic high-grade metamorphism preserved in lower crustal segments of the submerged north Iberian continental margin, and from low-pressure metamorphism related to mid-Cretaceous hyperextension predating the Alpine orogeny. The early Permian metamorphism reveals protracted and diachronic Variscan tectonomagmatic activity in the core of the Ibero-Armorican arc due to orocline buckling processes triggering lithosphere delamination, asthenosphere upwelling, and eventually heat and mass transfer through the continental crust.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
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  • 8
    Publication Date: 2009-01-01
    Print ISSN: 0022-1376
    Electronic ISSN: 1537-5269
    Topics: Geosciences
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  • 9
    Publication Date: 2007-06-01
    Print ISSN: 0954-4879
    Electronic ISSN: 1365-3121
    Topics: Geosciences
    Published by Wiley
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  • 10
    Publication Date: 1991-09-01
    Print ISSN: 0016-7649
    Topics: Geosciences
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