Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central

Abstract

The southwestern French Massif Central (Najac-Laguépie area) displays an inverted metamorphic sequence with high grade units at the top of the nappe stack. These high pressure and/or high temperature units are often grouped into the ill-defined leptyno-amphibolite complex forming the base of the Upper Gneiss unit. This paper presents new data on the pressure-temperature-time evolution of the Najac and Laguépie units and discuss the implications of the results for the structure and evolution of the Variscan French Massif central during the Upper Devonian.
 The Najac unit consists of phengite-garnet metapelites, a basic-ultrabasic intrusion containing retrogressed eclogites and phengite-orthogneisses. P-T estimates on micaschists with syn-kinematic garnets yielded peak pressure at 16 kbar for 570°C, followed by retrogression in the greenschist facies. According to recently published data, the age of high pressure metamorphism is constrained between 383 and 369 Ma. The Laguépie amphibolite unit is a mafic migmatite comprising garnet-free and garnet-bearing amphibolite with felsic leucosomes. Thermobarometry and phase diagram calculation on a garnet amphibolite sample yielded peak P-T conditions at 710°C, 10 kbar with retrogression and deformation under greenschist facies conditions. New U-Pb analyses obtained on zircon rims from a leucosome yield an age of 363 ± 3 Ma, interpreted as the timing of medium-temperature metamorphism.
 Considering the different metamorphic evolutions and the contrasted age-deformation relationships in the two localities, it is suggested that Najac and Laguépie rocks depict two distinct tectono-metamorphic units. The Najac LT-HP domain recorded the subduction of a continental margin during Upper Devonian. It is tentatively correlated to a Middle allochthon, sandwiched between the Lower Gneiss unit and the Upper gneiss unit. Such an intermediate unit is still poorly defined in the French Massif Central but it can be a lateral equivalent of the Middle Allochthon (encompassing the Groix blueschist units) in the Armorican massif. The Uppermost Devonian MT-MP metamorphism recorded by the Laguépie amphibolite correlates with the P-T-t evolution of the Upper Gneiss unit in the Western French Massif Central and the Upper Allochthon in the Armorican massif. This event is contemporaneous with active margin magmatism in the Western European Variscan belt confirming that the Upper Gneiss unit, encompassing the Laguépie amphibolite, once belonged to the continental upper plate of an ocean-continent subduction system just before the stacking of Mississippian nappes.

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

“…However, some of these eclogitic occurrences were also interpreted as the remnants of thickened orogenic crust (e.g. Benmammar et al, 2020; Jacob et al, 2021; Roger et al, 2020; Whitney et al, 2015, 2020). 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.…”

Carboniferous high‐P metamorphism and deformation in the Belledonne Massif (Western Alps)

“…400-420 Ma) inferred for the HP stage in eclogites have recently been re-estimated at ca. 360-385 Ma by Lu-Hf dating and in situ U-Pb dating of zircon (Benmammar et al, 2020;Lotout et al, 2018Lotout et al, , 2020. Many eclogites in the Variscan Belt still lack recent petrological investigation in order to solidly anchor geodynamic models that are based on possibly outdated constraints (e.g.…”

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

“…The dykes of La Bosse stockwork are of alkaline affinity and may be linked to the calk-alkaline to alkaline series described elsewhere in northern Massif Central and known as the "Limousin tonalite belt" [61,62]. This magmatism was dated at 360-380 Ma [30], which is syn HP metamorphism recorded by the nappe package in the western FMC [18,20,21]. It is considered to represent the melting of a dual source, consisting of a subduction-modified mantle and the underplated crust, during the northward subduction of the Rheic ocean [63].…”

“…The FMC is part of the internal zone of the Variscan belt that exposes a complexly deformed nappe pile [13][14][15][16][17][18]. Late Devonian HP relics attest to the deep burial and exhumation of oceanic and continental units during an early stage of tectonic accretion to the south of the Armorica continental ribbon [19][20][21][22]. Early to middle Carboniferous pervasive MP/MT Barovian metamorphism has been attributed to the transition from continental subduction to collision [22,23] and the late Carboniferous exhumation of large migmatite domes associated with LP/HT metamorphism is considered to represent the gravitational collapse of the Variscan belt [24][25][26][27].…”

“…PT-path is constrained by metamorphic assemblages [23,36], fluid inclusion microthermotetric data from [35], and compilation of geochronological data compiled in the Figure 5. During the early Carboniferous, ongoing plate convergence and orogenic evolution (Figure 6B) is marked by the outward propagation of the Variscan front [38], by the activation of orogenic scale transcurrent shear zones [75,76] and detachments [77] as well as by widespread MP/MT Barrovian metamorphism [21,23,78]. This period is followed at Visean by the emplacement of both crustal-derived and mantle-derived magmas in the form of plutons and volcanic tuffs, preferentially localized along transcurrent shear zones and pull-apart basins [33,42,79].…”