Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U–Pb dates from migmatite‐hosted eclogites? (Montagne Noire, France)

Abstract

Eclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) have an oceanic protolith and recorded a prograde P-T path from ~19.5 kbar, 700°C to the pressure peak at ~21 kbar, 750°C (pseudosection modelling), suggesting metamorphism in a subduction setting. Subsequent exhumation to low-P high-T (LP-HT) conditions (~6 kbar, 730°C) is constrained by the equilibration conditions of the embedding migmatite. In samples of a fresh and a retrogressed eclogite, all zircon crystals but one display similar REE patterns (no Eu anomaly, flat HREE), usually ascribed to crystallization under eclogite-facies conditions. Yet, the U-Pb apparent ages of zircon crystals from both eclogites spread from c. 360 Ma to a dominant data cluster at c. 315 Ma. The c. 315-310 Ma zircon U-Pb dates obtained from the embedding migmatite are interpreted as the age of crystallization of the partial melt during the LP-HT metamorphic stage. First-order geological evidence, in particular the sedimentary record, excludes the existence of a subduction zone in the region at this period. Unless calling upon a major reappraisal of the tectonics of the European Variscan orogen, this suggests an ambiguous relation between REE patterns and U-Pb dates in the zircon population. Various scenarios that could account for the observations are discussed. Combining our data with the results of a previously published Sm-Nd dating of garnet, and regional considerations, we consider that 360 Ma is the best approximation of the minimum age of the eclogite-facies event. We hypothesize that the eclogites formed farther north and were transferred to their present location by lower-crustal flow. It is inferred that during the subsequent exhumation, eclogite-facies zircon grains recrystallized and underwent partial to total resetting of their U-Pb system, whereas the REE system remained mostly unmodified. These results caution against the use of REE patterns as the only criterion to associate a specific zircon age with HP metamorphism in eclogites occurring in migmatitic domes.

Exhumation of deep continental crust in a transpressive regime: The example of Variscan eclogites from the Aiguilles‐Rouges massif (Western Alps)

“…Compared with other ECMs, the peak pressure conditions of HP metamorphism in the ARM are similar to the ~1.8 GPa recorded in the Gothard massif (Abrecht et al, 1991; Biino, 1994), but slightly higher than the ~1.5 GPa estimated in the Argentera massif (Ferrando et al, 2008; Jouffray et al, 2020) or the 1.4 GPa estimated in the NE Belledonne massif (Jacob et al, 2021) (Figure 15b). Our P – T path of the ARM is similar to those calculated for the eclogites from the Montagne Noire gneiss dome (Pitra et al, 2021; Whitney et al, 2015, 2020) and the Sardinia‐Corsica massifs (Cortesogno et al, 2004; Cruciani et al, 2011, 2012, 2015, 2021; Giacomini et al, 2005; Libourel & Vielzeuf, 1988) (Figure 15b) and also for pre‐alpine eclogites occurrences in the internal Alps in the Adula, Tambo and Suretta nappes (see Biino et al, 1997).…”

“…These results are also consistent with the timing of HP prograde metamorphism in the Maures-Tanneron massif that ended at c. 331 Ma (monazite EPMA; Oliot et al, 2015). The timing of the HP in the Montagne Noire massif is still debated: c. 315 Ma estimated by U-Pb on zircon (Whitney et al, 2015(Whitney et al, , 2020 or c. 360 Ma estimated by Sm-Nd on garnet and U-Pb on zircon (Faure et al, 2014;Pitra et al, 2021).…”

“…315 Ma estimated by U–Pb on zircon (Whitney et al, 2015, 2020) or c . 360 Ma estimated by Sm‐Nd on garnet and U–Pb on zircon (Faure et al, 2014; Pitra et al, 2021).…”

“…In the Variscan belt, the eclogite relicts were often interpreted as remnants of suture zones (e.g. Guillot & Ménot, 2009; Jouffray et al, 2020; Lardeaux et al, 2001; Pitra et al, 2021; Rubatto et al, 2010). However, some of these eclogitic occurrences were also interpreted as the remnants of thickened orogenic crust (e.g.…”

“…Although most recent studies on eclogites integrate a wide variety of data (structural, petrological, geochemical, geochronological and numerical modelling), the interpretation regarding the significance and origin of HP metamorphism remains in some cases controversial (i.e. Pitra et al, 2021;Whitney et al, 2015Whitney et al, , 2020. This uncertainty regarding the tectonic setting associated with HP metamorphism has led to a debate about the number and location of suture zones within the Variscan belt (Franke et al, 2017;Kroner & Romer, 2013;Regorda et al, 2020).…”