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

Abstract

In the Baie d'Audierne of the South Armorican domain, an intermediate metabasic-ultrabasic complex with isolated garnet pyroxenite lenses is exposed between upper and lower metasedimentary units. Metabasites in all units show geochemical characteristics similar to mid-ocean ridge magmatites. Garnets in pyroxenites are zoned with increasing XMg from cores to rims, and coexist with clinopyroxene (Jd 10 to Jd 33 ) and plagioclase (An 17 to An 35 ) of a mineral assemblage M1. Maximal pressure-temperature (P-T) conditions of the M1 metamorphism range between 800 and 950 °C and 14 and 19 kbar in different garnet pyroxenite lenses. The high-P-high-T rocks were overprinted by an assemblage M2 with Ca-amphiboles. Brown-green tschermakitic Ti-hornblende is in some cores and there are core to rim zonations from actinolite to tschermakite. Increasing Al IV and Al VI toward the rims of green amphiboles signal increases of P and T from 3 to 9 kbar and < <500 to 680 °C. In the metabasites of the lower metasedimentary unit, the Ca-amphiboles display similar zonation trends from actinolite cores to tschermakite rims, and a similar prograde P-T path up to 630 °C/6 kbar. Amphibolites in the upper metasedimentary unit show browngreen amphibole, interpreted as magmatic relict. Zonations from actinolite to magnesio-hornblende surround these cores and recorded prograde P-T paths to 5 kbar and 550 °C. The high-P-high-T rocks were welded with the metasedimentary units at the end of the µrst decompression path M1-M2. Subsequently the units underwent new burial and heating along an amphibolite facies P-T cycle M2. The present metamorphic conµguration could be the consequence of thrusting at a late stage of M2. This two-stage metamorphic history predates late Variscan granite intrusions and differs from P-T paths of other Armorican high-P units. 89Lucks, H., Schulz, B., Audren, C., and Triboulet, C., 2002, Variscan pressure-temperature evolution of garnet pyroxenites and amphibolites in the Baie d'Audierne metamorphic series, Brittany (France), in Martínez Catalán,

Zoned Ca-amphibole as a new marker of the Alpine metamorphic evolution of phyllites from the Jubrique unit, Alpujárride Complex, Betic Cordillera, Spain

“…The maximal T and P determined are similar to the values observed by Zenk and Schultz (2004) in metabasites from the chlorite-to-garnet zones of Barrovian series of Scotland. The P-T estimates also agree with those determined by Lucks et al (2002) in metabasites from the Baie d'Audierne (South Armorican Domain), in the zones where amphiboles show zonation trends from actinolite to magnesiohornblende (from 550ºC/5 kbar), similar to sample JU-4Q, and from actinolite to FIG. 10.…”

Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation

“…The allochthonous units (Fig. 2) found in the Variscan nappe pile underwent an HP event well established around 390 Ma in the whole Variscan chain (Abati et al 2010;Ballèvre et al 2009;Faryad and Kachlík 2013;Godard 2009;Lardeaux et al 2001;Lucks et al 2002;Stampfli et al 2002). Diverging points of view regard that event either as an early collision between Gondwana and Laurussia (Matte 2002, and references therein;Arenas et al 2014), or as peri-Gondwanan (Stampfli et al 2013, and reference therein).…”

Palaeozoic history of the Armorican Massif: Models for the tectonic evolution of the suture zones

“…In the Armorican Massif, three ophiolitic complexes are recognized (Fig. 2), namely the Audierne Complex [18,89,128,154], the Drain-le Hâvre Unit in the Champtoceaux Complex [18,144] and potentially the Conquet metagabbro [68,70]. Geochemical data on the Audierne and Drain Complexes indicate that the mafic rocks are akin to mid-ocean ridge basalts.…”