Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)

Abstract

Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France) Abstract On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock d 18 O SMOW values of the metabasites show limited variations (10.4±12.0½) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue-and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP±LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing Al IV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, Al IV and Al VI . Empirically calibrated thermobarometers allowed P±T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400±500 C/10±12 kbar was followed by a retrograde P±T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600 C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP±LT rocks escaped from an early erosion by a moderate second burial event and explains the longlasting slow uplift with low average cooling rates.Keywords P±T paths´High-pressure metamorphismÁ mphibole mineral chemistry´Amphibole thermobarometry´Oxygen isotopes´Uplift´South Armorican Domain´Variscan orogeny B. Schulz ( ) )

Variscan pressure-temperature evolution of garnet pyroxenites and amphibolites in the Baie d'Audierne metamorphic series, Brittany (France)

“…(3) Greenschist facies volcanosedimentary rocks are found in the Upper Group (Belle-Ile-en-Mer Group, Audren, 1987). (4) A high-P-low-T rock association with glaucophane-eclogites, blueschists, greenschists, and metapelites is exposed only in the Ile de Groix and at Bois de Cené (Triboulet, 1991;Schulz et al, 2001) and represents the uppermost unit of the pile.…”

“…A multistage eo-Variscan metamorphism is observed in the migmatitic Lower Group (Jones and Brown, 1990;Schulz et al, 1996). A complex polymetamorphic evolution with a high-P stage followed by amphibolite and greenschist facies metamorphism has been described in the uppermost high-P-low-T unit of Ile de Groix Schulz et al, 2001). This contrasts with the monometamorphic Middle and Upper Groups with single clockwise P-T paths Audren, 1985, 1988).…”

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U–Pb geochronology of its Ordovician protolith

“…Previous studies have proposed that the metamorphic zonation results from a metamorphic gradient with P-T conditions varying continuously from West to East and where the metamorphic domains are separated by ''isograds'' (Triboulet 1974;Carpenter 1976;Quinquis 1980;Djro et al 1989;Schulz et al 2001). The existence of a ''garnet isograd'' (Carpenter 1976;Quinquis 1980) is refuted by the presence of garnet in both metapelites (Bosse et al 2002) and metabasites (El Korh 2006;El Korh et al 2009) from the two parts of the island.…”

“…Two metamorphic events were described based on petrological observations in the metapelites (Djro et al 1989, Bosse et al 2002 as well as in the basic rocks (e.g. Triboulet 1974;Carpenter 1976;Barrientos 1992;Schulz et al 2001;Ballèvre et al 2003): a blueschist-eclogite facies metamorphism (M1) related to subduction, and a greenschist facies overprint during the exhumation (M2). Eclogite and blueschist facies rocks are dominant in the eastern part of the island, while the western part is mainly composed of greenschist facies rocks (with blueschist facies relics).…”

Amphibole-derived evidence of medium P/T metamorphic ratio in Alpujárride and Federico “HP” units (Western Betic-Northern Rif, Spain and Morocco): possible interpretations

“…This chemical control, associated with the instability of plagioclase, leads to the formation of Na amphibole (glaucophane) in high-P terrains (blueschists). In addition, zoned amphiboles generally preserve several stages (prograde and/or retrograde) of their P-T history, due to incomplete reequilibration (e.g., Bachman and Dungan 2002;Schulz et al 2001;Vogl 2003). For this reason, several thermometers and barometers have been calibrated during the last decades with the aim of using the amphibole composition in reconstructions of the P-T paths (e.g., Raase 1974;Graham and Powell 1984;Blundy and Holland 1990;Mäder and Berman 1992;Anderson and Smith 1995).…”