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

Abstract

Mafic eclogites are found in many orogens as lenses embedded in quartzofeldspathic migmatites. These high‐pressure relics are interpreted either as remnants of ancient sutures and thus formed during oceanic subduction or as fragments of lower crust exhumed from the root of orogenic thickened crust. It is critical to distinguish between these two endmember scenarios as the resulting palaeogeographic and geodynamic reconstructions may significantly differ. In this contribution, we investigated eclogite relics from Lac Cornu in the Aiguilles‐Rouges massif, one of the External Crystalline Massifs of the Western Alps. Phase equilibrium modelling suggests that these mafic rocks were buried along a prograde path (M1) from ~600°C/1.2 to 1.6 GPa to peak conditions of ~630–775°C and >1.6 GPa. Zircon rims, with a rare earth element signature typical of eclogite facies zircon (no Eu anomalies, flat HREE spectrum), and rutile were dated by U–Th–Pb laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) at c. 335–330 Ma. Prograde deformation has not been identified in the field and is only recognized thanks to the crystallographic preferred orientation (CPO) of inclusions of omphacite and rutile in garnet. Peak pressure conditions were followed by a decompression stage (M2) from ~760°C/1.4 GPa to ~600–650°C/0.9 GPa supported by the breakdown of omphacite into plagioclase–clinopyroxene symplectite and the crystallization of plagioclase–amphibole corona around garnet. The M2 retrogression stage is associated with the development of a main sub‐horizontal planar fabric and the CPO of minerals composing symplectite. This deformation stage is interpreted as the result of horizontal lower crustal flow. The final stage of exhumation (M3) is characterized by the replacement of symplectite and garnet by plagioclase and large euhedral amphibole and by the breakdown of rutile and ilmenite into titanite dated at c. 300 Ma. The CPO of titanite and amphibole are consistent with the development of vertical dextral shear zone in a transpressive regime. The combination of field observations and petrological, microtextural and geochemical analyses suggests that the mafic eclogites preserved in migmatitic rocks of the Aiguilles‐Rouges massif are remnants of a continental lower crust exhumed and juxtaposed with lower‐grade migmatites in crustal‐scale vertical transpressive shear zones.

Deformation, crustal melting and magmatism in the crustal-scale East-Variscan Shear Zone (Aiguilles-Rouges and Mont-Blanc massifs, Western Alps)

“…In the Lac Cornu area (see Fig. 1B for location), Vanardois et al (2022) interpreted an early sub-horizontal foliation as the result of horizontal flow of the partially molten crust.…”

“…Two metamorphic events are distinguished. The first, related to collision J o u r n a l P r e -p r o o f and crustal thickening, consists of a mid-to high-pressure metamorphism recorded as garnetkyanite bearing assemblage in micaschists and paragneisses, and as omphacite-bearing eclogitic assemblage within tholeiitic metamafic lenses (von Raumer and Schwander, 1985;von Raumer and Bussy, 2004;Liégeois and Duchesne, 1981;Vanardois et al, 2022). Highpressure conditions have been estimated at 1.6-1.85 GPa and 680-750°C for the Lac Cornu eclogites (Vanardois et al, 2022), and 1.3-1.5 GPa and 650-750°C for the Val Bérard eclogites von Raumer, 1993, 2011) (see Fig.…”

“…Recently, LA-ICPMS U-Th-Pb analyses on zircon and rutile from a Lac Cornu eclogite specimen has dated the HP metamorphism at 340-330 Ma and the emplacement age of the protolith at ca. 460 Ma (Vanardois et al, 2022). The AR and MB massifs were subsequently metamorphosed at pressures between 0.2 and 0.6 GPa and temperatures at ~600-750°C (Dobmeier, 1996(Dobmeier, , 1998von Raumer, 1993, 2011;von Raumer et al, 1996;Chiaradia, 2003;Marshall et al, 1997;Genier et al, 2008).…”

“…Partial melting in the AR and MB massifs started during the HT-HP metamorphism, at near peak pressure conditions at ~1.3-1.85 GPa at ca. 340-330 Ma (Vanardois et al, 2022). Ongoing partial melting occurred during decompression and subsequent HT-LP metamorphism (Genier et al, 2008).…”

Constraining the Timing of Evolution of Shear Zones in Two Collisional Orogens: Fusing Structural Geology and Geochronology

“…They are mainly mafic eclogites embedded within migmatites. HP metamorphism occurred at ~341 Ma in the Argentera Massif (U-Pb on zircon [90,91]) and around ~345 Ma (U-Pb on zircon [92]) down to ~335-330 Ma (U-Pb on zircon and rutile [93]) in the Aiguilles Rouge Massif.…”