Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating

Abstract

Magma types and timing of intrusion in the Mont Collon gabbro 125 ABSTRACT We have selected and dated three contrasting rock-types representative of the magmatic activity within the Permian layered mafic complex of Mont Collon, Austroalpine Dent Blanche nappe, Western Alps. A pegmatitic gabbro associated to the main cumulus sequence yields a concordant U/Pb zircon age of 284.2 ± 0.6 Ma, whereas a pegmatitic granite dike crosscutting the latter yields a concordant age of 282.9 ± 0.6 Ma. A Fe-Ti-rich ultrabasic lamprophyre, crosscutting all other lithologies of the complex, yields an 40 Ar/ 39 Ar plateau age of 260.2 ± 0.7 Ma on a kaersutite concentrate. All ages are interpreted as magmatic. Sub-contemporaneous felsic dikes within the Mont Collon complex are ascribed to anatectic back-veining from the country-rock, related to the emplacement of the main gabbroic body in the continental crust, which is in accordance with new isotopic data. The lamprophyres have isotopic compositions typical of a depleted mantle, in contrast to those of the cumulate gabbros, close to values of the Bulk Silicate Earth. This indicates either contrasting sources for the two magma pulses -the subcontinental lithospheric mantle for the gabbros and the underlying asthenosphere for the lamprophyres -or a single depleted lithospheric source with variable degrees of crustal contamination of the gabbroic melts during their emplacement in the continental crust. The Mont Collon complex belongs to a series of Early Permian mafic massifs, which emplaced in a short time span about 285-280 Ma ago, in a limited sector of the post-Variscan continental crust now corresponding to the Austroalpine/Southern Alpine domains and Corsica. This magmatic activity was controlled in space and time by crustal-scale transtensional shear zones.

Lamprophyre dikes as tectonic markers of late orogenic transtension timing and kinematics: A case study from the Central Iberian Zone

“…Elsewhere in the Variscan orogen, away from Spain: Monjoie et al [2007] conclude that the late Variscan alkaline mafic magmatism at Mont Collon in the Western Swiss Alps was controlled in space and time by crustal‐scale transtensional shear zones. In the Lower Penninic domain, Central Alps, Visean (330–340 Ma) Variscan magmas characterized by high Mg/Fe ratios and K‐rich compositions were emplaced along major transcurrent fault zones [ Schaltegger and Corfu , 1992; Debon and Lemmet , 1999; Bussy et al , 2000].…”

“…Localized intrusion of lamprophyres followed Carboniferous, 324 Ma, granitoid intrusion in the French Massif Central [ Guthrie and Carpena , 1990]. The previously mentioned alkaline lamprophyre dikes from Mont Collo, western Swiss Alps, crosscut all other lithologies of an Early Permian, 285–280 Ma, mafic massif series [ Monjoie et al , 2007]. To the north in the Bohemian Massif, Seifert and Sandmann [2006] related the German Erzgebirge metallogenetic province to postcollisional lamprophyre dikes, likewise the Oberharz lead‐zinc deposit was linked to late Variscan lamprophyre dikes [ Schulze , 1968].…”

Kinematics and Age of Syn‐Intrusive Detachment Faulting in the Southern Alps: Evidence for Early Permian Crustal Extension and Implications for the Pangea A Versus B Controversy

“…Dextral shearing in the diagram for all ages (lower left) indicates age range of dextral shear zones in the Helvetic External Massives after Genier et al () and Simonetti et al (); Pangea B to A indicates age range of transformation from Pangea B to Pangea A after Muttoni et al (). Sources of U‐Pb zircon ages: Beltrando et al (); Bergomi et al (, ); Berra et al (); Bertrand et al (, ); Bossart et al (); Bussien Grosjean et al (); Bussien et al (, ); Bussy and Cadoppi (); Bussy et al (, , ); Capuzzo and Bussy (); Cavargna‐Sani et al (); Dallagiovanna et al (); Debon et al (); Eichhorn et al (, ); Galli et al (); Gretter et al (); Guerrot and Debon (); Hansmann et al (); Kebede et al (); Klötzli et al, ); Letsch et al (); Liati and Gebauer (); Liati et al (); Maino et al (); Mandl et al (); Manzotti et al (); Marocchi et al (); Marquer et al (); Masson et al (); Monjoie et al (); Paquette et al (); Pawlig (); Peressini et al (); Petri et al (); quick et al (); Ring et al (); Rubatto et al (); Schaltegger (); Schaltegger and Brack (); Schaltegger and Corfu (, ); Sergeev et al (); Veselá et al (, ); Visonà et al (); Von Quadt et al (). See supporting information Table S7 for details.…”

Pre‐Alpine (Variscan) Inheritance: A Key for the Location of the Future Valaisan Basin (Western Alps)

“…The Paleozoic history of the Sesia‐Dent Blanche nappes has been recently reviewed (e.g., Manzotti, Ballèvre, et al, ) and will not be reiterated here. Their pre‐Mesozoic basement consists of (i) lower crustal units, displaying a high‐grade (amphibolite‐ to granulite‐facies) metamorphism (e.g., Manzotti & Zucali, ) of Permian age (e.g., Kunz et al, ), and (ii) upper crustal units, essentially made of granitoids and a few gabbros, also of Permian age (Bussy, Venturini, et al, ; Manzotti, Ballèvre, & Dal Piaz, ; Manzotti, Rubatto, et al, ; Monjoie et al, ). The identification of pre‐Permian events is a matter of debate.…”