The Grand St Bernard‐Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U‐Pb Dating

Abstract

The continental crust involved in the Alpine orogeny was largely shaped by Paleozoic tectono‐metamorphic and igneous events during oblique collision between Gondwana and Laurussia. In order to shed light on the pre‐Alpine basement puzzle disrupted and reamalgamated during the Tethyan rifting and the Alpine orogeny, we provide sensitive high‐resolution ion microprobe U‐Pb zircon and geochemical whole rock data from selected basement units of the Grand St Bernard‐Briançonnais nappe system in the Western Alps and from the Penninic and Lower Austroalpine units in the Central Alps. Zircon U‐Pb ages, ranging from 459.0 ± 2.3 Ma to 279.1 ± 1.1 Ma, provide evidence of a complex evolution along the northern margin of Gondwana including Ordovician transtension, Devonian subduction, and Carboniferous‐to‐Permian tectonic reorganization. Original zircon U‐Pb ages of 371 ± 0.9 Ma and 369.3 ± 1.5 Ma, from calc‐alkaline granitoids of the Grand Nomenon and Gneiss del Monte Canale units, provide the first compelling evidence of Late Devonian orogenic magmatism in the Alps. We propose that rocks belonging to these units were originally part of the Moldanubian domain and were displaced toward the SW by Late Carboniferous strike‐slip faulting. The resulting assemblage of basement units was disrupted by Permian tectonics and by Mesozoic opening of the Alpine Tethys. Remnants of the Moldanubian domain became either part of the European paleomargin (Grand Nomenon unit) or part of the Adriatic paleomargin (Gneiss del Monte Canale unit), to be finally accreted into the Alpine orogenic wedge during the Cenozoic.

What drives Alpine Tethys opening? Clues from the review of geological data and model predictions

“…This correspondence enforces the interpretation of a regional extensional tectonics that characterized the lithosphere of Pangea during the Permo-Triassic, causing the asthenospheric upwelling and lithospheric thinning (Diella et al, 1992;Lardeaux & Spalla, 1991;Marotta et al, 2009;Muntener & Hermann, 2001;Schuster & Stüwe, 2008;Spalla et al, 2014 the Early Triassic (Picazo et al, 2016;Schuster & Stüwe, 2008). In this review, we demonstrate that the variation in the thermal state of the lithosphere from 290 to 220 Ma can be explained even by a model the Permian calc-alkaline magmatism with the lithospheric thinning and strike-slip tectonics (Bergomi, Dal Piaz, Malusà, Monopoli, & Tunesi, 2017;Dallagiovanna et al, 2009;Schaltegger & Brack, 2007).…”

“…Dextral strike–slip faulting with an extensional component is compatible with Permian tectonic history, although minor sinistral effects are recorded in the Dolomites. In any cases, there is general consensus in connecting the Permian calc‐alkaline magmatism with the lithospheric thinning and strike–slip tectonics (Bergomi, Dal Piaz, Malusà, Monopoli, & Tunesi, ; Dallagiovanna et al, ; Schaltegger & Brack, ).…”

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.…”

Low-Temperature Crystal Chemistry of Hingganite-(Y), from the Wanni Glacier, Switzerland

“…The mineral assemblage of metamorphic rocks, exposed in the Wanni glacier area, which is located in the Binn Valley, Valais (Wallis), Switzerland, is unique and is the locality of eight rare-earth element (REE) arsenates and arsenites [35][36][37][38][39][40][41][42]. Its geology has been studied in detail [43][44][45][46][47][48]. Hingganites from this area occur in fissures of an alpine-type hydrothermal mineralization [49] and form crystals of different colors (greenish, yellow, or colorless), various forms (prismatic, rose-like), and up to 2 mm in size [50].…”