A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U\u2013Pb geochronology of Permian magmatism

Michel Ballèvre,Paola Manzotti,Marc Poujol,Audrey Camonin

doi:10.1186/s00015-020-00367-1

Abstract

The Briançonnais Domain (Western Alps) represented the thinned continental margin facing the Piemonte-Liguria Ocean, later shortened during the Alpine orogeny. In the external part of the External Briançonnais Domain (Zone Houillère), the Palaeozoic basement displays microdioritic intrusions into Carboniferous sediments and andesitic volcanics resting on top of the Carboniferous sediments. These magmatic rocks are analysed at two well-known localities (Guil volcanics and Combarine sill). Geochemical data show that the two occurrences belong to the same calc-alkaline association. LA-ICP-MS U–Pb ages have been obtained for the Guil volcanics (zircon: 291.3 ± 2.0 Ma and apatite: 287.5 ± 2.6 Ma), and the Combarine sill (zircon: 295.9 ± 2.6 Ma and apatite: 288.0 ± 4.5 Ma). These ages show that the calc-alkaline magmatism is of Early Permian age. During Alpine orogeny, a low-grade metamorphism, best recorded by lawsonite-bearing veins in the Guil andesites, took place at about 0.4 GPa, 350 °C in the External Briançonnais and Alpine metamorphism was not able to reset the U–Pb system in apatite. The Late Palaeozoic history of the Zone Houillère is identical to the one recorded in the Pinerolo Unit, located further East in the Dora-Maira Massif, and having experienced a garnet-blueschist metamorphism during the Alpine orogeny. The comparison of these two units allows for a better understanding of the link between the Palaeozoic basements, mostly subducted during the Alpine convergence, and their Mesozoic covers, generally detached at an early stage of the convergence history.

Highlights

In western and central Europe, crustal shortening associated with Variscan orogeny mainly took place during the Early and early Late Carboniferous
In a recent synthesis about the Western Alps, Ballèvre et al (2018) pointed out that Permian magmatism is widespread in the internal domains of the belt, i.e. in the basement of the Briançonnais paleogeographical domain representing the European passive margin towards the Piemonte-Liguria Ocean and in the Sesia-Dent Blanche and South-Alpine domains of the opposite passive margin
Concerning the Briançonnais Domain in Liguria some authors pointed to the existence of two post-Variscan magmatic cycles of Permian age (e.g. Cortesogno et al 1998)

Summary

Introduction

In western and central Europe, crustal shortening associated with Variscan orogeny mainly took place during the Early and early Late Carboniferous. In a recent synthesis about the Western Alps, Ballèvre et al (2018) pointed out that Permian magmatism is widespread in the internal domains of the belt, i.e. in the basement of the Briançonnais paleogeographical domain (referred to as Briançonnais Domain in the following) representing the European passive margin towards the Piemonte-Liguria Ocean and in the Sesia-Dent Blanche and South-Alpine domains of the opposite passive margin. It is very sparsely found in the external paleogeographic domains (External Massifs); these basement complexes are characterized by Carboniferous-age magmatism leading to the widespread intrusion of Carboniferous granitoids. This contribution investigates in detail the suspected Permian magmatism in the type-area of the Briançonnais Domain (referred to as External Briançonnais in the following) located along a continuous transect between the Pelvoux External Massif and the Dora-Maira Internal Massif (Fig. 1) with the aim to (i) further constrain extent and timing of such magmatism within the various paleogeographic domains, and (ii) determine to which of the two postulated Permian cycles this magmatism may belong

Methods

Findings

Discussion

Conclusion