Geochemical and isotopic characteristics of blueschist facies rocks from the Île de Groix, Armorican Massif (northwest France)

Bernard-Griffiths, J.; Carpenter, M.S.N.; Peucat, Jean-Jacques; Jahn, Bor‐ming

doi:10.1016/0024-4937(86)90025-3

Cited by 46 publications

(19 citation statements)

References 46 publications

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“…Sanchez Carretero et al 1990), through the southern Massif Central (Pin and Marini 1993;Briand et al 1995) and Armorican Massif (Bernard-Grif®ths et al 1986) to the West Sudetes of the Bohemian Massif (e.g. Nare Îbski et al 1986;Furnes et al 1994;Nare Îbski 1994;Winchester et al 1995;Floyd et al 1996).…”

Section: Introductionmentioning

confidence: 97%

Middle/Late Cambrian intracontinental rifting in the central West Sudetes, NE Bohemian Massif (Czech Republic): geochemistry and petrogenesis of the bimodal metavolcanic rocks

Dostál

Patočka

2001

Geological Journal

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The Early Palaeozoic East Krkonos Ïe Complex (EKC) situated in the central West Sudetes, NE Bohemian Massif, is a volcanosedimentary suite containing abundant ma®c and felsic volcanics metamorphosed to greenschist facies.The trace element distribution patterns and Nd isotope signatures (E Nd500 3.1 to 6.6) of the metabasites (metabasalts) indicate that they may be related to a rising mantle diapir associated with intracontinental rifting. At the early stage, limited melting of an upwelling asthenosphere produced alkali basalts and enriched tholeiites which compositionally resemble oceanic island basalts. A later stage of rifting with larger degrees of melting at shallower depths generated tholeiitic basalts with E-MORB to N-MORB characteristics.The values of ( 87 Sr/ 86 Sr) i 0.706 and E Nd500 À5 AE 1 of the porphyroids (metarhyolites) as well as the lack of rocks with intermediate compositions suggest that the felsic rocks were formed by a partial melting event of continental crust triggered by mantle melts.The geochemistry of the EKC bimodal metavolcanics and their association with abundant terrigenous metasediments suggest that the felsic±ma®c volcanic suite was generated during intracontinental rifting. This process, widespread in Western and Central Europe during the Early Palaeozoic, is evidence of large-scale fragmentation of the northern margin of the Gondwana supercontinent.

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Section: Introductionmentioning

confidence: 97%

Middle/Late Cambrian intracontinental rifting in the central West Sudetes, NE Bohemian Massif (Czech Republic): geochemistry and petrogenesis of the bimodal metavolcanic rocks

Dostál

Patočka

2001

Geological Journal

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“…The metabasites of the Ile de Groix are interlayered with pelitic sediments, whose trace element composition is similar to the upper continental crust (UCC) or global subducting sediment (GLOSS), with high LILE and LREE contents (Bernard-Griffiths et al 1986;Bosse et al 2002;El Korh et al 2013). Most metabasites have (La/Sm) PM ratios typical of MORB (up to 1.5; John et al 2010) ( Fig.…”

Section: Effect Of Hydrothermal Alteration On Fe Isotope Fractionationmentioning

confidence: 99%

“…1) (see Bosse et al 2002;Ballèvre et al 2003;El Korh et al 2013; and references therein). The metabasites correspond to former hydrothermally altered enriched mid-ocean ridge basalts (E-MORB), interlayered with pelitic micaschists (Bernard-Griffiths et al 1986;Bosse et al 2002;El Korh et al 2009. Because of the lack of ultrabasic and gabbroic rocks, the high-pressure rocks were interpreted as remnants of the uppermost part of an oceanic lithosphere or of basaltic lavas, emplaced during the Cambro-Ordovican rifting widely recognised along the northern Gondwana margin (El Korh et al 2012;von Raumer et al 2015).…”

Section: Geological Setting and Investigated Samplesmentioning

confidence: 99%

Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)

Korh

Luais

Deloule

et al. 2017

Contrib Mineral Petrol

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1during metamorphism: newly-formed Fe-rich minerals allowed preserving bulk rock Fe compositions during metamorphic reactions and hampered any Fe isotope fractionation. Greenschists have δ 56 Fe values (+0.17 ± 0.01 to +0.27 ± 0.02‰) similar to high-pressure rocks. Hence, metasomatism related to fluids derived from the subducted hydrothermally altered metabasites might only have a limited effect on mantle Fe isotope composition under subsolidus conditions, owing to the large stability of Fe-rich minerals and low mobility of Fe. Subsequent melting of the heavy-Fe metabasites at deeper levels is expected to generate mantle Fe isotope heterogeneities.Keywords Fe isotopes · Metabasites · Subduction · HP-LT metamorphism · Blueschists · Eclogites · Greenschists · Basaltic protoliths IntroductionHigh-pressure/low-temperature (HP-LT) rocks are remnants of ancient subduction zones. They provide information on geochemical processes and deep fluid-rock interactions occurring at present-day active convergent margins. Fluid infiltration during high-and lowtemperature hydrothermal alteration of the oceanic crust at mid-ocean ridges and on the seafloor is responsible for the hydration of basic rocks and serpentinisation of the lithospheric mantle. During subduction, the hydrothermally altered oceanic crust dehydrates continuously and releases large amounts of H 2 O at a relatively shallow level in the subduction zone (50-80 km) (Schmidt and Poli 1998;Rüpke et al. 2004). Deserpentinisation of the lithospheric mantle occurs at deeper levels (100-200 km), where fluids are expected to be the source of arc melting (Ulmer and Abstract Characterisation of mass transfer during subduction is fundamental to understand the origin of compositional heterogeneities in the upper mantle. Fe isotopes were measured in high-pressure/low-temperature metabasites (blueschists, eclogites and retrograde greenschists) from the Ile de Groix (France), a Variscan high-pressure terrane, to determine if the subducted oceanic crust contributes to mantle Fe isotope heterogeneities. The metabasites have δ 56 Fe values of +0.16 to +0.33‰, which are heavier than typical values of MORB and OIB, indicating that their basaltic protolith derives from a heavy-Fe mantle source. The δ 56 Fe correlates well with Y/Nb and (La/Sm) PM ratios, which commonly fractionate during magmatic processes, highlighting variations in the magmatic protolith composition. In addition, the shift of δ 56 Fe by +0.06 to 0.10‰ compared to basalts may reflect hydrothermal alteration prior to subduction. The δ 56 Fe decrease from blueschists (+0.19 ± 0.03 to +0.33 ± 0.01‰) to eclogites (+0.16 ± 0.02 to +0.18 ± 0.03‰) reflects small variations in the protolith composition, rather than Fe fractionation

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“…These exhibit high strain characterized by WNW-directed shear and a large extensional component, interpreted to reflect gravitational spreading by Vauchez et al (1987). Blueschists derived from rocks of oceanic affinity (Bernard-Griffiths et al, 1986) are exposed at ile de Groix and at Bois de CenC ; lpwer-grade tectonic units are not distinguished by separate ornaments; blueschist occurrences are identified by fivepoint stars (Ile de Groix (G) and Bois de Cene (BC)); and eclogite localities are indicated by five-armed crosses (Champtoceaux Complex (C) and Essarts Complex (E)). SASZ is the South Armorican Shear Zone and numbered boxes represent 40Ar/39Ar sample localities.…”

Section: Tectonic Frameworkmentioning

confidence: 99%

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

Brown

Dallmeyer

1996

Journal Metamorphic Geology

114

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The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with 40Ar/39Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303-298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306-305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new 40Ar/3'Ar data suggest a minimum time-integrated average cooling rate between c. 725 "C and c. 125 "C of c. 14+4"C Ma-', although below 600 'C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800-750 "C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakor during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.

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Geochemical and isotopic characteristics of blueschist facies rocks from the Île de Groix, Armorican Massif (northwest France)

Cited by 46 publications

References 46 publications

Middle/Late Cambrian intracontinental rifting in the central West Sudetes, NE Bohemian Massif (Czech Republic): geochemistry and petrogenesis of the bimodal metavolcanic rocks

Middle/Late Cambrian intracontinental rifting in the central West Sudetes, NE Bohemian Massif (Czech Republic): geochemistry and petrogenesis of the bimodal metavolcanic rocks

Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

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