Lawsonite growth in the epidote blueschists from the Ile de Groix (Armorican Massif, France): a potential geobarometer

Abstract

The garnet blueschists from the Ile de Groix (Armorican Massif, France) contain millimetre-to centimetre-sized pseudomorphs consisting of an aggregate of chlorite, epidote and paragonite. The pseudomorphed phase developed at a late stage of the deformation history, because it overgrows a glaucophane-epidote-titanite foliation. Garnet growth occurred earlier than the beginning of the ductile deformation, and thus garnet is also included in the pseudomorphs. Microprobe analyses show that garnet is strongly zoned, with decreasing spessartine and increasing almandine and pyrope contents from core to rim. Grossular content is higher in garnet cores (about 35 mole%) compared to garnet rims (about 30 mole%). Blue amphibole has glaucophane compositions with a low Fe 3+ content and become more magnesian when inclusions in garnet (X Mg ¼ 0.62-0.65) are compared with matrix grains (X Mg ¼ 0.67-0.70). Matrix epidote has a pistacite content of about 50 mole%. On the basis of their shape and the nature of the breakdown products, the pseudomorphs are attributed to lawsonite. A numerical model (using THERMOCALC THERMOCALC) has been developed in order to understand the reactions controlling both the growth and the breakdown of lawsonite. Lawsonite growth could have taken place through the continuous hydration reaction Chl + Ep + Pg + Qtz + Vap ¼ Gln + Lws, followed by the fluid-absent reaction Chl + Ep + Pg ¼ Grt + Gln + Lws. Peak P-T conditions are estimated at about 18-20 kbar, 450°C. This indicates that lawsonite growth took place at increasing P and T, hence can be used as a geobarometer in the buffering assemblage garnet-glaucophane-epidote. The final part of the history is recorded by lawsonite breakdown, after cessation of the ductile deformation, and recording the earliest stages of the exhumation.

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

“…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.…”

“…Omphacite porphyroblasts are present in eclogites. Blueschist facies rocks often contain pseudomorphs after lawsonite, composed of epidote, phengite, paragonite, actinolite, and albite (Ballèvre et al 2003). The different mineral assemblages between blueschists and eclogites result from slight differences in the composition of the protolith and/or small variations in temperature (50-75 °C) for the peak metamorphic conditions (El Korh et al 2009).…”

Petrologic modeling of chloritoid–glaucophane schists from the NW Iberian Massif

“…Hence, the Basal Units of the NW Iberian Massif can be correlated with similar terranes with high-P and low-to intermediate-temperature metamorphism from Portugal to the northern areas of eastern Europe, such as the Kaczawa Complex in the Polish Sudetes of the Bohemian Massif (Kryza et al, 1990;Cymerman et al, 1997). In addition, similarities between certain geological units of the Armorican Massif (Brittany) and the Malpica-Tui Unit allows correlation of the latter with the ile de Groix (Kienast and Triboulet, 1972;5helley and Bossiere, 1999;Ballevre et al, 2003 ) and Champtoceaux Complex (Ballevre et al, 2009 ), particularly since the age of the high-P metamorphism is the same in both terranes (Bosse et al, 2005). This age has been tightly constrained to around 371 Ma in NW Iberia by 4°Arp9Ar geochronology on white micas from different lithologies, including eclogites (Van Calsteren et al, 1979;Rodrtguez et al, 2003 ), and byU-Pb zircon ages (Abati et aI., 2009 ).…”

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

“…2). Basic bodies, sometimes with isoclinal folds or boudinage, outcrop as centimetric to metric lenses to layers of several tens of metres Bosse et al 2002;Ballèvre et al 2003). Different studies have classified the metamorphic rocks of the Ile de Groix into different groups according to the nature of their protolith (Triboulet 1974;Bernard-Griffiths et al 1986;Audren et al 1993;Bosse et al 2002;El Korh 2010;El Korh et al 2009: massive metabasites, banded volcano-sedimentary rocks and metapelites.…”

“…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).…”