Sapphirine as a Breakdown Product of Garnet in a Variscan UHP/HT Peridotite from the Vosges Mountains (France) – An Indication of Near-Isothermal Decompression

Altherr, Rainer; Soder, Christian

doi:10.1093/petrology/egy096

Cited by 5 publications

(4 citation statements)

References 81 publications

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“…In the Schwarzwald, the metabasites with a HP-UHT granulite heritage from north of Hohengeroldseck Castle can easily be connected with felsic HP-UHT metagranulites from the CSGC and SSGC described by Marschall et al (2003). In the Vosges Mts, such rocks occur in close association with UHP-UHT garnet peridotites and such an association is typical for the 'Granulite (Gföhl) Unit' of the Moldanubian part of the BM (Altherr & Soder 2018). The fact that granulites from these two areas suffered severe retrogression has limited the interest to discover UHP phases in these rocks.…”

Section: Possible Equivalents Of the Investigated Metabasites In Other Parts Of The European Variscidesmentioning

confidence: 98%

“…The uppermost 'Granulite (Gföhl) Unit' or 'Upper Gneiss Unit (UGU)' in the FMC consists of relic HP-UHT granulites with intercalations of UHP-UHT garnetbearing peridotites and eclogites (e.g. Fuchs 1976;Thiele 1984;Rey et al 1992;Gardien et al 1997;Berger et al 2010;Faryad et al 2013;Altherr and Soder 2018;Sorger et al 2020;Vanderhaeghe et al 2020). Not only the garnet peridotites and eclogites but also the granulites of this unit locally contain relic UHP minerals such as coesite and diamond (Lardeaux et al 2001;Naemura et al 2011;Perraki and Faryad 2014;Jedlicka et al 2015;Thiéry et al 2015).…”

Section: Possible Equivalents Of the Investigated Metabasites In Other Parts Of The European Variscidesmentioning

confidence: 99%

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Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

Altherr

Hepp

Klein

et al. 2021

Int J Earth Sci (Geol Rundsch)

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In the Variscan Schwarzwald metabasic rocks form small bodies included within anatectic plagioclase-biotite gneisses. Many metabasites first underwent an eclogite-facies metamorphism at about 2.0 GPa and 670–700 °C, resulting in the assemblage garnet + omphacite + rutile + quartz ± epidote ± amphibole ± kyanite. Since these eclogites are nearly free of an OH-bearing phase, they underwent almost complete dehydration during subduction, suggesting formation along an average to warm top-of-the-slab geotherm of 10–13 °C/km. The age of the Variscan high-P/high-T metamorphism is > 333 Ma. After partial exhumation from ~ 65 to ~ 15 km depth, the eclogites were overprinted under increasing activity of H2O by a number of retrograde reactions. The degree of this overprint under amphibolite-facies conditions (0.4–0.5 GPa/675–690 °C) was very different. Up to now, only retrograde eclogites have been found, but some samples still contain omphacite. Kyanite is at least partially transformed to aggregates of plagioclase + spinel ± corundum ± sapphirine. On the other hand, there are amphibolites that are extensively recrystallized and show the assemblage amphibole + plagioclase + ilmenite/titanite ± biotite ± quartz ± sulphides. The last relic phase that can be found in such otherwise completely recrystallized amphibolites is rutile. After the amphibolite-facies metamorphism at ~ 333 Ma, the metabasites underwent a number of low-temperature transformations, such as sericitization of plagioclase, chloritization of amphibole, and formation of prehnite. The intimate association of metabasite bodies with gneisses of dominantly meta-greywacke compositions suggests derivation from an active plate margin. This view is corroborated by bulk-rock geochemical data. Excluding elements that were mobile during metamorphism (Cs, Rb, Ba, K, Pb, Sr, U), the concentrations of the remaining elements in most of the metabasites are compatible with a derivation from island-arc tholeiites, back-arc basin basalts or calc-alkaline basalts. Only some samples have MORB precursor rocks.

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Section: Possible Equivalents Of the Investigated Metabasites In Other Parts Of The European Variscidesmentioning

confidence: 98%

Section: Possible Equivalents Of the Investigated Metabasites In Other Parts Of The European Variscidesmentioning

confidence: 99%

Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

Altherr

Hepp

Klein

et al. 2021

Int J Earth Sci (Geol Rundsch)

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“…The Lalaye-Lubine Fault Zone separates the low-grade Lower Paleozoic meta-sedimentary rocks of the Northern Vosges domain (Piqué et al, 1994;Skrzypek et al, 2014) from the HP-HT Eclogite-Granulite-Migmatite complex of the Central Vosges (Fluck, 1980;Rey et al, 1989;Latouche et al, 1992;Altherr and Kalt, 1996;Skrzypek et al, 2012b;Altherr and Soder, 2018). Despite polyphase reworking of its initial structure, it was generally considered to localize the Mid-Upper Devonian (ca.…”

Section: Outline Of the Geological Evolution Of The Variscan Vosges Mmentioning

confidence: 99%

Zircon age of vaugnerite intrusives from the Central and Southern Vosges crystalline massif (E France): contribution to the geodynamics of the European Variscan belt

Guillot

Averbuch

Dubois

et al. 2020

BSGF - Earth Sci. Bull.

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To provide a better definition of the geodynamical processes active at the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on specific mafic Mg-K magmatic rocks (the vaugnerites), intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20- 40 % vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of mafic to intermediate, metaluminous to slightly peraluminous rocks of calc-alkaline affinity. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircons were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre- Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2s) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data thus give consistent results suggesting a relatively early emplacement of the vaugnerites regarding the Late Variscan collisional history, i.e. during Middle Visean times. These results are in line with previously published ages within the Southern Vosges volcano-sedimentary sequences (Oderen- Markstein) and the nearby Mg-K granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a Mg-K magmatic event of regional significance. Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here the major Mg-K magmatic pulse at 340-335 Ma to be the marker of a significant evolution into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, it could traduce the first effects of a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian- Moldanubian continental block). This process would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.

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“…1-a), small ultramafic mantle bodies, including garnet peridotites, spinel peridotites and pyroxenites, are found in the metamorphic allochthonous units that form exhumed portions of the lower-to-mid-crustal orogenic root, mainly in the Bohemian massif (Kubeš et al, 2022;Medaris et al, 2005Medaris et al, , 2015, the Vosges -Black Forest massif (Altherr, 2021;Altherr & Kalt, 1996) and the French Massif Central (Gardien et al, 1990;Godard, 1990). In detail, these mantle bodies are found in exhumed lower to mid-crustal units from different tectonic domains, which record different metamorphic evolutions (Altherr & Soder, 2018;Lardeaux et al, 2014;Medaris et al, 2005). In the Bohemian massif, where the largest number of these ultramafic bodies have been found, a distinction has been recognized between peridotite-bearing units exhumed close to the plate interface (Saxo-Thuringian suture) and those exhumed further south, between the former Devonian subduction arc and the Brunia continental backstop (Moldanubian zone, Schulmann et al, 2009).…”

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confidence: 99%

Discovery of Variscan orogenic peridotites in the Pelvoux Massif (Western Alps, France)

Jacob¹,

Janots²,

Cordier³

et al. 2023

BSGF - Earth Sci. Bull.

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Small bodies of mantle-derived peridotites and other ultramafic rocks are commonly found in exhumed lower crustal units of collisional orogens. They provide a direct record of the complex evolution of the upper mantle before and during an orogeny, and are therefore key markers of the geodynamic evolution of an orogen. We report here the discovery of such mantle-derived peridotites in high-grade Variscan lower crust exposed in the Pelvoux massif (External Western Alps), which occur as fragmented enclaves in migmatites. A wide petrographic diversity has been observed, from very fertile, garnet-bearing lherzolites, to more depleted spinel/chromite-bearing harzburgites. Thermobarometric calculations on a garnet lherzolite indicate an initial stage at 3.0 ± 0.5 GPa and 973 ± 50°C, followed by exhumation to 0.8-1.3 GPa and 800-850°C, while the harzburgites do not show any evidence of equilibration in the garnet field and sample shallower mantle (<2.0 GPa). Petrological observations, whole-rock geochemistry and in-situ mineral compositions suggest the peridotites have undergone a complex history prior to their incorporation in the lower crust during the Variscan Orogeny. They derive from refractory mantle domains, which have experienced variable degrees of melt depletion, and have then been extensively refertilized. Cryptic metasomatism is observed in all samples. It is characterized by an enrichment in fluid-mobile incompatible elements relative to immobile ones (LILE vs HFSE), leading to the development of pronounced negative anomalies Nb and Ta, and is presumably related to percolation of hydrous fluids in the mantle. In addition, strong enrichment in incompatible LREE relative to HREE is observed in some samples, commonly associated with modification of the modal composition (crystallization of phlogopite and/or pargasite with accessory chromite and apatite). This modal metasomatism is attributed to percolation by a K2O-P2O5-Cr2O3-rich silicate melt, which might be at the source of syn-collisional ultrapotassic magmas (durbachites) emplaced in the crust during the middle-upper Carboniferous. These geochemical characteristics are in line with whole-rock Nd isotopic compositions, which indicate enrichment of the mantle by a continental crust component, presumably related to Variscan subductions. This evolution is consistent with that of other Variscan peridotites in the eastern Alps (Ulten) and the Bohemian massif, where multiple metasomatic episodes related to melts or fluids released in Variscan subduction zones have been documented.

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Sapphirine as a Breakdown Product of Garnet in a Variscan UHP/HT Peridotite from the Vosges Mountains (France) – An Indication of Near-Isothermal Decompression

Cited by 5 publications

References 81 publications

Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

Zircon age of vaugnerite intrusives from the Central and Southern Vosges crystalline massif (E France): contribution to the geodynamics of the European Variscan belt

Discovery of Variscan orogenic peridotites in the Pelvoux Massif (Western Alps, France)

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