scholarly journals The Erzgebirge, Germany, a subducted part of northern Gondwana: geochemical evidence for repetition of early Palaeozoic metasedimentary sequences in metamorphic thrust units

1998 ◽  
Vol 135 (6) ◽  
pp. 785-801 ◽  
Author(s):  
BIRGIT MINGRAM
Keyword(s):  
Granulite Facies ◽  
Passive Margin ◽  
Radiometric Dating ◽  
Chemical Compositions ◽  
Low Grade ◽  
Granulite Facies Metamorphism ◽  
Margin Setting ◽  
New Finding ◽  
Early Palaeozoic ◽  
Metamorphic Terranes

One of the major metamorphic terranes of the Bohemian Massif, the Erzgebirge, is interpreted to record a subducted part of a Palaeozoic margin of Gondwana. A geochemical study on non-calcareous metasediments from the various metamorphic units from lower greenschist to granulite facies metamorphism supports a recently established thrust model. Geochemical discrimination and correlation from the metasediments of the Erzgebirge suggest repetition of an early Palaeozoic metasedimentary sequence in metamorphic thrust units. This new finding is in line with recent radiometric dating of intercalated metarhyolitic rocks, which yielded ages of around 480 Ma. It is furthermore supported by correlation with a low-grade standard section in Thuringia, which represents the transition from an orogenic belt to a passive margin setting, with highly mature sediments. Significant geochemical signatures have been identified in three different lithotypes, which reappear in at least three metamorphic units of the Erzgebirge. Geochemical correlation of these units was established using simple comparison of averages and with statistical techniques. The identification of significant geochemical signatures from different lithotypes in metamorphic suites has important implications for terrane analysis and reconstruction of ancient tectonic settings.The repetition of lithologies and their distinct chemical compositions in progressively metamorphosed units is useful for examining element mobility during Barrovian metamorphism. Statistical comparison implies that Li is progressively depleted from the greenschist to amphibolite facies, whereas Ca exhibits some enrichment. All the other elements studied are considered to be immobile.

Extensional exhumation of a high-pressure granulite terrane in Payer Land, Greenland Caledonides: structural, petrologic, and geochronologic evidence from metapelites

2002 ◽  
Vol 39 (8) ◽  
pp. 1169-1187 ◽  
Author(s):  
Jane A Gilotti ◽  
Synnøve Elvevold
Keyword(s):  
High Pressure ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Low Grade ◽  
Granulite Facies Metamorphism ◽  
High Pressure High Temperature ◽  
High Pressure Granulite ◽  
Two Samples ◽  
Facies Metamorphism ◽  
Southern Half

The Payer Land gneiss complex is unique among the mostly amphibolite-facies, mid-crustal gneiss complexes in the East Greenland Caledonides due to its well-preserved, regional high-pressure (HP) granulite-facies metamorphism. High-pressure – high-temperature (HP–HT) assemblages are recognized in mafic, ultramafic, granitic, and metasedimentary lithologies. Anatectic metapelites contain the assemblage garnet + kyanite + K-feldspar + antiperthite (exsolved ternary feldspar) + quartz ± biotite ± rutile and record approximately the same peak metamorphic conditions (pressure (P) = 1.4–1.5 GPa, temperature (T) = 800–850°C) as those of the neighboring mafic HP granulites. The HP granulite-facies metamorphism is Caledonian based on in situ U–Th–Pb electron microprobe dating of monazite from two samples of the aluminous paragneiss. The monazites are found along garnet–kyanite phase boundaries, as inclusions in garnet and kyanite, and within small leucocratic melt pods (K-feldspar + plagioclase + kyanite ± garnet) within the HP–HT paragneisses. Mylonitic equivalents of the metapelites contain a detrital monazite age signature that suggests the Payer Land paragneisses correlate with other Mesoproterozoic metasedimentary sequences in the area. The gneisses form a metamorphic core complex that is separated from the overlying low-grade sedimentary rocks of the Neoproterozoic Eleonore Bay Supergroup by an extensional detachment. This newly recognized Payer Land detachment is part of a system of prominent extensional faults located in the southern half of the Greenland Caledonides (i.e., south of 76°N). The HP granulites preserve the deepest level of crust exposed in this southern segment of the orogen and attest to significant crustal thickening.

Granulite-facies metamorphism of the Palaeoproterozoic – early Palaeozoic gneiss domains of NE Mozambique, East African Orogen

2016 ◽  
Vol 154 (3) ◽  
pp. 491-515 ◽  
Author(s):  
A. K. ENGVIK ◽  
B. BINGEN
Keyword(s):  
High Pressure ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Northwestern Part ◽  
Granulite Facies Metamorphism ◽  
Medium Pressure ◽  
Nappe Complex ◽  
High Pressure Granulite ◽  
Facies Metamorphism ◽  
Early Palaeozoic

AbstractGranulite-facies metamorphism recorded in NE Mozambique is attributed to three main tectonothermal events, covering more than 1400 Ma from Palaeoproterozoic – early Palaeozoic time. (1) Usagaran–Ubendian high-grade metamorphism of Palaeoproterozoic age is documented in the Ponta Messuli Complex by Grt-Sil-Crd-bearing metapelites, estimated to pressure (P) 0.75 ± 0.08 GPa and temperature (T) 765 ± 96°C. The post-peak P-T path is characterized by decompression followed by near-isobaric cooling. (2) Irumidian medium- to high-pressure granulite-facies metamorphism is evident in the Unango and Marrupa complexes of late Mesoproterozoic – early Neoproterozoic age. High-pressure granulite-facies is documented by Grt-Cpx-Pl-Rt-bearing mafic granulites in the northwestern part of the Unango Complex, with peak conditions up to P = 1.5 GPa and T = 850°C. Medium-pressure granulite-facies conditions recording P of c. 1.15 GPa and T of 875°C are documented by Grt-Opx-Cpx-Pl assemblage in mafic granulites and charnockitic gneisses of the central part of the Unango Complex. (3) Tectonothermal activity during the Ediacaran–Cambrian Kuunga Orogeny is recorded in the Mesoproterozoic gneiss complexes as amphibolite facies to medium-pressure granulite-facies metamorphism. Granulite facies are documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses, reporting P = 0.99 ± 13 GPa at T = 738 ± 84°C in the Unango Complex and P = 0.92 ± 18 GPa at T = 841 ± 135°C in the Marrupa Complex. This metamorphism is attributed to crustal thickening related to overriding of the Cabo Delgado Nappe Complex, and shorthening along the Lurio Belt during the early Palaeozoic Kuunga Orogeny.

Geochronological studies of the Bohemian massif, Czechoslovakia, and their significance in the evolution of Central Europe

1982 ◽  
Vol 73 (2) ◽  
pp. 89-108 ◽  
Author(s):  
O. van Breemen ◽  
M. Aftalion ◽  
D. R. Bowes ◽  
A. Dudek ◽  
Z. Mísař ◽  
...  
Keyword(s):  
Central Europe ◽  
Granulite Facies ◽  
Temporal Separation ◽  
European Continent ◽  
Granulite Facies Metamorphism ◽  
Moldanubian Zone ◽  
Tectonic Model ◽  
Field Evidence ◽  
Early Palaeozoic ◽  
Tectonic Style

ABSTRACTU–Pb zircon and Rb–Sr whole-rock analyses from various gneisses and plutonie rocks of the Moldanubian and Moravo-Silesian zones and the stable foreland of the Hercynian (Variscan) orogenic belt indicate that most of the crust in Central Europe was first formed during the Cadomian orogeny which straddles the Precambrian–Cambrian boundary. Zircons, however, have a memory of older ages which correspond with those of events known in Fennoscandia. The new radiometrie data are consistent with the stratigraphie record in that they do not provide any evidence for a major early Palaeozoic tectonothermal event between the Cadomian and Hercynian orogenies.Granulites from two localities in the Moldanubian zone yield U–Pb zircon ages of 345 ± 5 Ma; discordant zircon data points indicate that the granulite facies metamorphism was not of long duration. Tectonic units containing these high grade rocks were emplaced amongst amphibolite facies rocks during an event of widespread shearing which has been dated at 341 ± 4 Ma on the basis of a lower U–Pb zircon intercept age from one of the sheared gneisses and 338 ± 3 Ma U–Pb ages from monazites. Rb–Sr muscovite ages of 331 ± 5 Ma from pegmatites axial planar to asymmetrical folds date the last stage of SE-directed simple shear. A Rb–Sr whole-rock isochron of 331 ± 4 Ma from a principal magmatic type of the Central Bohemian pluton confirms the field evidence that the large NE-trending plutons of the Moldanubian zone were emplaced during a late stage of the deformation. The strong disturbance of the U–Pb zircon isotopic system in the sheared gneisses suggests U loss while a high U/Th ratio in monazite from one of these tectonised rocks suggests the simultaneous passage of hydrothermal fluids. Thus a crustal source is indicated for the uranium deposits of the Moldanubian zone.Critical to any plate tectonic model for the development of the Middle European Hercynides was the existence of an ocean in Early Devonian times which separated a North European continent from a South European continent(s). The northward movement of the South European continent over a shallowly-dipping subduction zone and subsequent continental collision can explain the high T–low P metamorphism and the imbricated tectonic style of the Moldanubian zone and adjacent Moravo-Silesian zone along the southeastern Hercynian foreland. The temporal separation of granulites and granites implies distinct conditions of formation and it has been suggested that the plutonism, following on from the imbrication of the Cadomian crust, was initiated by the subduction of wet oceanic sediments.

Dating of igneous and metamorphic events in the Fiskenaesset region of southern west Greenland

1978 ◽  
Vol 15 (12) ◽  
pp. 2021-2025 ◽  
Author(s):  
R. T. Pidgeon ◽  
F. Kalsbeek
Keyword(s):  
Direct Measurement ◽  
Granulite Facies ◽  
Amphibolite Facies ◽  
Low Grade ◽  
Granulite Facies Metamorphism ◽  
West Greenland ◽  
Facies Metamorphism

U–Pb measurements on zircons from gneisses, which closely resemble Amitsoq gneisses, from nunataks from the Frederikshåbs Isblink show that they are in fact much younger, probably ca. 3000 Ma old. A direct measurement of the age of the granulite facies metamorphism in the northern part of the Fiskenaesset area, of [Formula: see text], has been achieved by U–Pb analyses on: (1) a large euhedral zircon from an ultrabasic pod considered to have crystallized during the granulite facies metamorphism; (2) zircons from the Ilivertalik granite, which is considered to have been emplaced shortly before the end of the granulite facies metamorphism; and (3) zircons from an anthophyllite–gedrite rock considered to have grown during the amphibolite facies retrogression following granulite facies metamorphism.A maximum age of 2660 ± 20 Ma for the end of the low grade amphibolite facies overprinting in the southern part of the Fiskenaesset area is given by a zircon U–Pb measurement on intrusive granitic sheets.

How did the Archean crust evolve? Insights from the structure and petrology of the Lewisian of Scotland

2021 ◽  
Author(s):  
Sophie Miocevich ◽  
Alex Copley ◽  
Owen Weller
Keyword(s):  
Mass Transfer ◽  
Central Region ◽  
Vertical Motion ◽  
Granulite Facies ◽  
High Grade ◽  
Near Surface ◽  
Granulite Facies Metamorphism ◽  
Dry Conditions ◽  
Archean Crust ◽  
Metamorphic Terranes

<p>High-grade Archean gneiss terranes expose mid to lower crustal rocks and are generally dominated by tonalite-trondhjemite-granodiorite (TTG) gneisses. Occurrences of mafic-ultramafic bodies and garnet-bearing felsic gneisses within these environments have been interpreted as supracrustal or near-surface rocks requiring a tectonic process involving mass transfer from the near-surface to the mid-crust. However, there is significant uncertainty regarding the nature of this mass transfer, with suggestions including a range of uniformitarian and non-uniformitarian scenarios.  One non-uniformitarian scenario, ‘sagduction’, has been proposed as a possible mechanism (Johnson <em>et al.,</em> 2016, and references therein), although the dynamics of sagduction are still relatively unexplored.</p><p>This study focuses on mafic, ultramafic and garnet-bearing felsic gneiss bodies in the central region in the Lewisian Gneiss Complex of northwest Scotland as test cases to investigate the behaviour of possibly supracrustal rocks in a mid-crustal environment. Existing datasets of TTGs (Johnson <em>et al.,</em> 2016), mafic gneisses (Feisel <em>et al.,</em> 2018) and ultramafic gneisses (Guice <em>et al.,</em> 2018) from across the central region were utilised in addition to felsic and mafic gneiss samples obtained in this study from the ~10 km<sup>2</sup> Cnoc an t-Sidhean (CAS) suite. The CAS suite is the largest reported supracrustal in the Lewisian, and dominantly comprises garnet-biotite felsic gneiss assemblages and an associated two-pyroxene mafic gneiss. Field mapping was undertaken to collect samples representative of the observed heterogeneity of the suite, and to assess field associations between possible supracrustals and surrounding TTGs. Phase equilibria modelling was conducted on all lithologies to ascertain peak pressure-temperature (<em>P-T</em>) conditions, and to calculate the density of the modelled rocks at peak conditions.</p><p>The results obtained in this study indicate peak metamorphic conditions of 950 ± 50 °C and 9 ± 1 kbar for the CAS suite, consistent with the central region of the Lewisian Complex (Feisel <em>et al.,</em> 2018). Density contrasts at mid-crustal conditions of 0.12–0.56 gcm<sup>-3</sup> were calculated between TTGs and the other lithologies and used to estimate the buoyancy force that drives density-driven segregation. This allowed us to investigate the rates of vertical motion that result from density contrasts, as a function of the effective viscosity during metamorphism. Independent viscosity estimates were attained using mineral flow-laws and our estimated <em>P-T</em> conditions, and from examination of modern-day regions of crustal flow. We were therefore able to estimate the conditions under which sagduction could have been a viable mechanism for crustal evolution in the Lewisian and similar high-grade metamorphic terranes. We conclude that sagduction was unlikely to have operated in the Lewisian under the dry conditions implied by preserved mineral assemblages.</p><p> </p><p> </p><p>Feisel, Y., et al. 2018. New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland. Journal of Metamorphic Geology. <strong>36</strong>, 799-819</p><p>Guice, G.L., et al. 2018. Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland. Geosciences, <strong>8, </strong>338.</p><p>Johnson, T.E., et al. 2016. Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland. Precambrian Research, <strong>283</strong>, 89-105.</p>

Pétrologie des niveaux trondhjémitiques de haute pression associés aux éclogites et amphibolites des complexes leptyno-amphiboliques du Massif Central français

1978 ◽  
Vol 15 (5) ◽  
pp. 696-707 ◽  
Author(s):  
C. Nicollet ◽  
A. Leyreloup
Keyword(s):  
High Pressure ◽  
Partial Melting ◽  
Granulite Facies ◽  
Massif Central ◽  
Granulite Facies Metamorphism ◽  
High Pressure Granulite ◽  
Greenstone Belts ◽  
Metamorphic Terranes ◽  
Archean Greenstone Belts ◽  
Initial Mineral

High pressure trondhjemitic rocks interbedded with eclogites and high pressure amphibolites have been recognized in the leptyno-amphibolitic series of the Rouergue and Marvejols metamorphic terranes. These rocks appear to have been derived by partial melting of the surrounding amphibolites (or amphibole-gabbro) at [Formula: see text] (<7 × 105 kPa). The initial mineral assemblage of these layers (Q, Pl, Ky, Ga, Zo, etc.) is suggested by microstructural evidence to have crystallized directly under high pressure granulite facies conditions (12.5 kbar <PT <20 kbar (12.5 × 105 kPa < PT <20 × 105 kPa); 750 °C<T<840 °C) from the trondhjemitic liquid. The partial melting has taken place before the well-known Barrovian metamorphism that has affected the whole crystalline basement of the French Massif Central. This older metamorphic event is considered to be equivalent to the high pressure granulite facies metamorphism recognized in the Variscan of Europe. This acid–basic high pressure bimodal association argues for a close similarity between Archean greenstone belts and the leptyno-amphibolitic series.

Metamorphic P–T conditions and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland

2013 ◽  
Vol 151 (3) ◽  
pp. 559-571 ◽  
Author(s):  
K. AOKI ◽  
B. F. WINDLEY ◽  
S. MARUYAMA ◽  
S. OMORI
Keyword(s):  
High Pressure ◽  
Granulite Facies ◽  
Metamorphic Rocks ◽  
Published Data ◽  
Chemical Compositions ◽  
Garnet Amphibolite ◽  
Granulite Facies Metamorphism ◽  
High Pressure Granulite ◽  
Mineral Assemblages ◽  
Facies Metamorphism

AbstractThe metamorphic P–T conditions and associated relationships of the Barrovian zones near Glen Muick were re-examined by focusing on the petrology and thermodynamics of rocks at Cairn Leuchan, where garnetite lenses and layers occur in surrounding garnet amphibolite in the highest-grade sillimanite zone. The representative mineral assemblages in the garnet-rich lenses and garnet amphibolite are garnet + quartz + clinopyroxene + plagioclase + amphibole ± epidote, and garnet + amphibole + quartz + plagioclase ± clinopyroxene ± epidote, respectively. The chemical compositions of constituent minerals are the same in both garnetite and garnet amphibolite. The metamorphic P–T conditions of these rocks were estimated by thermodynamic calculations. The results show that the rocks underwent high-pressure granulite facies metamorphism at P = c. 1.2–1.4 GPa and T = c. 770–800°C followed by amphibolite facies metamorphism at P = c. 0.5–0.8 GPa and T = c. 580–700°C. Integration of our new results with previously published data suggests that the retrograde P–T trajectory of the highest-grade Barrovian metamorphic rocks marks a temperature decrease during decompression from a crustal depth of the high-pressure granulite facies, which is much deeper than previously considered.

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

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