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

François Guillot,Pierre Lanari,Arnaud Gauthier,Olivier Averbuch,Cyril Durand,Michel Dubois,O Averbuch,J.M Lardeaux

doi:10.1051/bsgf/2020027

Abstract

To provide a better picture of the active geodynamics along the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on vaugnerites, which consist of mafic ultra-potassic magmatic rocks, 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 ultra-potassic mafic to intermediate, metaluminous to slightly peraluminous rocks. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircon grains 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 (± 2σ) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data suggest a relatively early emplacement during the Late Variscan collisional history (i.e. Middle Visean times). These results are in line with previously published ages from the Southern Vosges volcano-sedimentary sequences (Oderen-Markstein) and the nearby ultra-potassic granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a 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 that the major ultra-potassic magmatic pulse at 340–335 Ma is a consequence of a significant change 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, magmatism could result from 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 delamination 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.

Highlights

The European Variscan orogenic belt developed during Upper Paleozoic times (i.e. Mid-Devonian-Upper Carboniferous) in response to the broad scale collision of the megacontinents Laurussia and Gondwana and the closure of intervening oceanic realms
The data presented in this study for four Vosges vaugnerite yield additional constraints on the time-space distribution of the well-developed, ultra-potassic, mantle-derived, magmatic suites, typical of the late collisional evolution of the Variscan suture zones
The vaugnerites appear either as a basic rim of the Ballons granite pluton in Southern Vosges, or as isolated dykes cutting across the granite-gneiss complex (GGC), a large heterogeneous migmatite unit of the Central Vosges

Summary

Introduction

The European Variscan orogenic belt developed during Upper Paleozoic times (i.e. Mid-Devonian-Upper Carboniferous) in response to the broad scale collision of the megacontinents Laurussia and Gondwana and the closure of intervening oceanic realms. The subduction dynamics of involved oceanic plates and associated continental margins are still highly debated mainly due to a major late orogenic lithospheric reworking including pervasive magmatism, high temperature metamorphism and extensional deformations, that obliterated the initial collisional structure of the belt (Ledru et al, 1989; Burg et al, 1994; Costa and Rey, 1995; Faure, 1995; Gardien et al, 1997; Janousek et al, 2012; Schulmann et al, 2014) Such event, often referred to as late orogenic extensional collapse, included extensive melting of the crustal units involved in the Variscan orogenic wedge in response to a lithospheric-scale thermal imprint. They have been generally considered to originate from partial melting of a metasomatized mantle, variably enriched by crustal contamination, presumably associated to former or possibly still on-going continental subduction (e.g., Sabatier, 1980; Montel and Weisbrod, 1986; Rossi, 1986; Michon, 1987; Banzet, 1987; Turpin et al, 1988; Sabatier, 1991; Foley, 1992; Holub, 1997; Debon and Lemmet, 1999; Gerdes et al, 2000; Bonin, 2004; Solgadi et al, 2007; Scarrow et al, 2008; Parat et al, 2010; von Raumer et al, 2013; Couzinié et al, 2016; Moyen et al, 2017; Laurent et al, 2017; Förster et al, 2019)

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