scholarly journals Metasediments Covering Ophiolites in the HP Internal Belt of the Western Alps: Review of Tectono-Stratigraphic Successions and Constraints for the Alpine Evolution

Minerals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 411
Author(s):  
Paola Tartarotti ◽  
Silvana Martin ◽  
Andrea Festa ◽  
Gianni Balestro
Keyword(s):  
High Pressure ◽  
Western Alps ◽  
Metamorphic Evolution ◽  
Alpine Orogeny ◽  
Sedimentary Evolution ◽  
Hp Metamorphism ◽  
Tethys Ocean ◽  
Depositional Setting ◽  
Oceanic Rocks ◽  
Outer Band

Ophiolites of the Alpine belt derive from the closure of the Mesozoic Tethys Ocean that was interposed between the palaeo-Europe and palaeo-Adria continental plates. The Alpine orogeny has intensely reworked the oceanic rocks into metaophiolites with various metamorphic imprints. In the Western Alps, metaophiolites and continental-derived units are distributed within two paired bands: An inner band where Alpine subduction-related high-pressure (HP) metamorphism is preserved, and an outer band where blueschist to greenschist facies recrystallisation due to the decompression path prevails. The metaophiolites of the inner band are hugely important not just because they provide records of the prograde tectonic and metamorphic evolution of the Western Alps, but also because they retain the signature of the intra-oceanic tectono-sedimentary evolution. Lithostratigraphic and petrographic criteria applied to metasediments associated with HP metaophiolites reveal the occurrence of distinct tectono-stratigraphic successions including quartzites with marbles, chaotic rock units, and layered calc schists. These successions, although sliced, deformed, and superposed in complex ways during the orogenic stage, preserve remnants of their primary depositional setting constraining the pre-orogenic evolution of the Jurassic Tethys Ocean.

Chromian jadeite, phengite, pumpellyite, and lawsonite in a high–pressure metamorphosed gabbro from the French Alps

1980 ◽  
Vol 43 (332) ◽  
pp. 979-984 ◽  
Author(s):  
C. Mevel ◽  
J. R. Kienast
Keyword(s):  
High Pressure ◽  
Low Temperature ◽  
Western Alps ◽  
Alpine Orogeny ◽  
French Alps ◽  
Cr Substitution

SummarySmall ophiolitic bodies are enclosed in the calcschists of the Piemont zone (western Alps). They have been metamorphosed in the blueschist facies during the alpine orogeny. One of them, the Roche Noire massif, includes gabbro breccias. The magmatic mineralogy of the gabbro was plagioclase + clinopyroxene + minor chromite. There was no chemical homogenization during metamorphism because of the lack of penetrative deformation and on the site of previous chromites, chromium-rich high-pressure and low-temperature minerals (jadeite, phengite, pumpellyite, and lawsonite) were formed. The Al ⇌ Cr substitution does not affect other P- and T-dependent substitutions.

Geodynamic significance of the Variscan eclogites in the External Crystalline Massifs (Western Alps): marker of a subduction or crustal thickening?

2020 ◽  
Author(s):  
Jean-baptiste Jacob ◽  
Stéphane Guillot ◽  
Daniela Rubatto ◽  
Emilie Janots ◽  
Jérémie Melleton ◽  
...  
Keyword(s):  
High Pressure ◽  
Western Alps ◽  
Late Carboniferous ◽  
Crustal Thickening ◽  
Garnet Growth ◽  
Variscan Orogeny ◽  
High Grade ◽  
Pressure Stage ◽  
Hp Metamorphism ◽  
High Pressure Stage

<p><span>The Paleozoic basement exposed in the External Crystalline Massifs of the Western Alps (ECM) contains numerous relics of Variscan eclogites and high pressure granulites preserved in high grade migmatitic gneisses. </span><span>These relics are taken to indicate</span><span> that the </span><span>ECM</span><span> underwent an early HP metamorphic stage during the Variscan Orogeny. However, due to the scarcity of recent thermobarometric and geochronological data, the geodynamic significance of this high pressure metamorphism remains unclear. Based on petrological similarities with other eclogite-bearing formations in the European Variscides (especially the “leptyno-amphibolic compex” in the French </span><span>Variscides</span><span>), it has been suggested that the high pressure rocks from the ECM mark a mid-Devonian subduction cycle, preceding the main Carboniferous Variscan collisional stage </span><span>(Fr</span>éville et al., 2018; Guillot and Ménot, 2009)<span>. This interpretation mostly relies on one mid-Devonian U-Pb zircon age (395</span>±<span>2 Ma) obtained in eclogites from the massif of Belledonne (Paquette et al., 1989), which has been interpreted as the age of eclogitization. However, dating of high pressure granulites in the Argentera Massif (Rubatto et al., 2010) yielded a Carboniferous age (ca. 340 Ma) for the high pressure stage, questioning the previous geodynamical interpretation. </span>We present here the results of a detailed petrological and geochronological investigation of the high grade formation of the Lacs de la Tempête in NE Belledonne, where some of the eclogites dated by Paquette et al. (1989) were sampled. This area exposes mostly high-grade migmatitic metasediments with intercalated lenses of orthogneiss and garnet-bearing amphibolites, preserving locally eclogitic assemblages. Thermobarometric estimations coupling forward pseudosection modelling, Zr in rutile thermometry and garnet growth modelling constrain the minimal P conditions during the high pressure stage at ca. 1.4-1.6 GPa and 700 °C. The early HP assemblage was then strongly overprinted by granulite facies metamorphism at ca. 1.0-1.2 GPa and 750 °C, also recorded in the surrounding metasediments. U-Pb dating of zircon reveals that the eclogites derived from Ordovician protoliths. Zircon overgrowth in the eclogites and the surrounding metasediments constrain the age of HP metamorphism between ca. 350-305 Ma, with no evidence for a Devonian event. Rutile dating in the eclogites supports the late Carboniferous age of metamorphism. The middle-late Carboniferous corresponds to the main period of Variscan nappe stacking in the ECM, following a period of arc magmatism during late Devonian-Tournaisian (ca. 360-350 Ma, <span>Fr</span>éville et al., 2018). We therefore suggest that the 350-305 Ma ages recorded in the HP units of the ECM do not correspond to a Devonian subduction, but rather represent the equilibration of orogenic lower crust at HP-MT conditions during the Variscan nappe stacking events, followed by re-equilibration at lower P during late Carboniferous. This evolution presents striking similarities with the high pressure units of the Moldanubian zone in the Bohemian massif (Schulmann et al., 2009). However, deciphering the exact meaning of U-Pb ages in retrogressed eclogites remains a challenge, and further field and petrological investigation is required to produce a consistent history of the Variscan collision in the ECM.</p>

Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts

2014 ◽  
Vol 185 (2) ◽  
pp. 93-114 ◽  
Author(s):  
Jean-Marc Lardeaux
Keyword(s):  
High Pressure ◽  
Low Temperature ◽  
Western Alps ◽  
Metamorphic Rocks ◽  
Temporal Data ◽  
Metamorphic Evolution ◽  
Metamorphic History ◽  
Alpine Metamorphism ◽  
Collision Processes

AbstractIn this paper we review and discuss, in a synthetic historical way, the main results obtained on Alpine metamorphism in the western Alps. First, we describe the finite metamorphic architecture of the western Alps and discuss its relationships with subduction and collision processes. Second, we portray the progressive metamorphic evolution through time and space with the presentation of 5 metamorphic maps corresponding to critical orogenic periods, namely 85-65 Ma, 60-50 Ma, 48-40 Ma, 38-33 Ma and 30-20 Ma. We underline the lack of temporal data on high-pressure/low-temperature metamorphic rocks as well as the severe uncertainties on the sizes of rock units that have recorded the same metamorphic history (i.e. coherent P-T-t/deformation trajectories). We discuss the role of subduction-driven metamorphism in ocean-derived protoliths and the conflicting models that account for the diachrony of continental subductions in the western Alps.

scholarly journals The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

2020 ◽  
Vol 113 (1) ◽  
Author(s):  
Alice Vho ◽  
Daniela Rubatto ◽  
Pierre Lanari ◽  
Daniele Regis
Keyword(s):  
High Pressure ◽  
Central Area ◽  
Western Alps ◽  
Early Permian ◽  
Metamorphic Stage ◽  
Local Factors ◽  
Icp Ms ◽  
Hp Metamorphism ◽  
Metamorphic Terranes ◽  
High Temperature Metamorphism

AbstractMicroscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism.

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