Zircon age of vaugnerite intrusives from the Central and Southern Vosges crystalline massif (E France): contribution to the geodynamics of the European Variscan belt

Abstract

To provide a better definition of the geodynamical processes active at the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on specific mafic Mg-K magmatic rocks (the vaugnerites), intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20- 40 % vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of mafic to intermediate, metaluminous to slightly peraluminous rocks of calc-alkaline affinity.
 Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircons were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre- Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2s) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data thus give consistent results suggesting a relatively early emplacement of the vaugnerites regarding the Late Variscan collisional history, i.e. during Middle Visean times. These results are in line with previously published ages within the Southern Vosges volcano-sedimentary sequences (Oderen- Markstein) and the nearby Mg-K granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a Mg-K magmatic event of regional significance.
 Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here the major Mg-K magmatic pulse at 340-335 Ma to be the marker of a significant evolution into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, it could traduce the first effects of a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian- Moldanubian continental block). This process would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.

3‐D Structure of the Variscan Thrust Front in Northern France: New Insights From Seismic Reflection Profiles

“…Borehole legend: ANN: Annappes; CAL: Calais; CES: Condé-sur-l'Escaut; DV: Douvrain; EPY: Epinoy; FOC: Focant; FRE: Frevillers; GHL: Ghlin; GZT: Gouzeaucourt; HAV: Havelange; JEU: Jeumont; LZ: Leuze; MTY: Montigny-en-Cambrésis; ROS: Rosières; SGH: Saint-Ghislain; TOU: Tournai; WEP: Wépion. c. Sketch map of the Variscan orogenic system in NW Europe (modified from Guillot et al, 2020 Cazes et al, 1985;Franke et al, 2017;Golonka, 2002;Golonka & Gaweda, 2012) (Figure 1). Starting from the middle Visean times onwards (ca 340 Ma), this ultimately led to the collision of Laurussia with the Armorica-Gondwana accretionary complex (e.g., Schulmann et al, 2002;Guillot et al, 2020).…”

“…During the Late Devonian and Mississippian, a general N–S convergence caused the gradual closure of the Rheno‐Hercynian Ocean by southward subduction beneath the Armorican‐Saxothuringian block (Averbuch & Piromallo, 2012; Cazes et al., 1985; Franke et al., 2017; Golonka, 2002; Golonka & Gaweda, 2012) (Figure 1). Starting from the middle Visean times onwards (ca 340 Ma), this ultimately led to the collision of Laurussia with the Armorica‐Gondwana accretionary complex (e.g., Schulmann et al., 2002; Guillot et al., 2020). The Variscan collision stage is characterized by an overall NNW‐SSE shortening (e.g., Corfield et al., 1996; Oncken et al., 2000; Averbuch et al., 2004; Averbuch & Piromallo, 2012) that was responsible for the tectonic inversion of the southern Laurussian margin.…”

“…Borehole legend: ANN: Annappes; CAL: Calais; CES: Condé‐sur‐l’Escaut; DV: Douvrain; EPY: Epinoy; FOC: Focant; FRE: Frevillers; GHL: Ghlin; GZT: Gouzeaucourt; HAV: Havelange; JEU: Jeumont; LZ: Leuze; MTY: Montigny‐en‐Cambrésis; ROS: Rosières; SGH: Saint‐Ghislain; TOU: Tournai; WEP: Wépion. c. Sketch map of the Variscan orogenic system in NW Europe (modified from Guillot et al., 2020). NVF: Northern Variscan deformation front; MS: Moldanubian suture; RHS: Rhenohercynian suture; SAS: South Armorican suture; AM: Armorican Massif; Ard.…”

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

“…Schaltegger and Corfu, J o u r n a l P r e -p r o o f 1992; Debon et al, 1994Debon et al, , 1998Guerrot and Debon, 2000;Rubatto et al, 2001) show similar geochemical compositions and are also interpreted as a mixing between crustal and mantle sources (Debon and Lemmet, 1999 and reference therein). Granite emplacement with a major mantle component and dated between 340-330 Ma is also known in other Variscan massifs (e.g., Vosges: Guillot et al, 2020;Tabaud et al, 2015;Corsica: Paquette et al, 2003;Li et al, 2014). These similarities suggest that a mantle thermal anomaly may have affected a large part of the Variscan belt and may have contributed into the bulk heat budget of the continental crust.…”

A Deformed Wedge‐Top Basin Inverted During the Collapse of the Variscan Belt: The Permo‐Carboniferous Lorraine Basin (NE France)

“…Transects indicate the location of the ECORS and DEKORP seismic profiles as well as the Paris Basin cross‐section represented below (Gély & Hanot, 2014). The inset map shows the location of the Lorraine‐Saar‐Nahe Basin (LSNB) within the main tectonic domains of the Variscan orogen (compiled from Faure, 1995; Franke, 2014; Guillot et al., 2020; Schneider & Romer, 2010).…”