Paleo-European crust of the Italian Western Alps: Geological history of the Argentera Massif and comparison with Mont Blanc-Aiguilles Rouges and Maures-Tanneron Massifs

Abstract

The "External Crystalline Massifs" of the Western Alps (Mont Blanc, Aiguilles Rouges, Grandes Rousses, Belledonne, Pelvoux, and Argentera) consist of a polymetamorphic Variscan basement, which was only marginally reworked during the Alpine tectonometamorphic cycle. These massifs experienced an early subduction event at peak metamorphic conditions of ~700°C and 1.5 GPa, followed by continental collision coupled with amphibolite-facies metamorphism, anatexis and exhumation to shallow crustal levels in the Carboniferous (see von Raumer et al., 1999 for a review).This contribution focuses on the magmatic and metamorphic history of the Argentera Massif, the southernmost and largest of the External Crystalline Massifs exposed in Italy. Its evolution from Ordovician to Early Permian is compared to that recorded in the Mont Blanc-Aiguilles Rouges Massif, the other External Crystalline Massif extensively exposed in the Italian Alps. Further 2 comparison is drawn between these large massifs and the Maures-Tanneron Massif of Provence, France, the area of Variscan Europe nearest to Argentera.

Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)

“…Nonetheless, recent U-Pb geochronological studies argue for a Carboniferous HP metamorphism recorded by zircons dated at 340.7 ± 4.2 Ma and 336.3 ± 4.1 Ma, within HP granulites that recorded peak PT conditions of ca. 1.4 GPa and 735 ± 15 °C (Ferrando et al, 2008;Rubatto et al, 2010). These Carboniferous ages in the Argentera Massif are in agreement with the D1/M1 event reported in the Belledonne Massif and can be related to the same crustal thickening.…”

“…To the south, in the Argentera Massif ( Fig. 1), in spite of the recognition of HP metamorphic rocks and migmatites (Compagnoni et al, 2010;Paquette et al, 1989;Rubatto et al, 2010), the Variscan deformation sequence is not clearly identified, owing to a strong Alpine overprint.…”

Variscan eclogites from the Argentera–Mercantour Massif (External Crystalline Massifs, SW Alps): a dismembered cryptic suture zone

“…The ECMs belong to the Helvetic-Dauphinois domain of the Alpine belt (Schmid et al 2004), where Alpine tectonic history has not significantly erased Variscan signatures (Von Raumer et al 1999Guillot and Menot 2009;Compagnoni et al 2010;Spalla et al 2014;Fréville et al 2018). The Argentera-Mercantour Massif, located at the French-Italian border, is a piece of European continental crust shortened in response to late Alpine collision (Polino et al 1990;Bogdanoff et al 2000;Schwartz et al 2007;Schreiber et al 2010).…”

“…Following the first investigations of Faure-Muret (1955), Schiavinato (1958, 1960) and Malaroda et al (1970), this massif was systematically divided into two main units (Bogdanoff et al 1991;Rubatto et al 2001;Compagnoni et al 2010;Carosi et al 2016) separated by a steeply dipping, NW-SE striking, km-scale shear zone, the «Ferriere-Mollières Shear Zone» (FMSZ, Fig. 2), also known as «Valletta Shear Zone».…”

Structure of lamprophyres: a discriminant marker for Variscan and Alpine tectonics in the Argentera-Mercantour Massif, Maritime Alps

“…Myers, 1978;Mørk, 1985;Williams, 1985;Spalla et al, 2000;Baletti et al, 2012;Gosso et al, 2015;Zanoni et al, 2016). In the Argentera-Mercantour Massif, a portion of deep Variscan crust reactivated during the Alpine collision, swarms of pre-Alpine lamprophyres are known for over sixty years (Faure-Muret, 1955;Bortolami and Sacchi, 1968;Malaroda et al, 1970;Compagnoni et al, 2010). Since these lamprophyres intruded into Variscan migmatites and early Permian granitoids and were subsequently deformed and metamorphosed, they are appropriate markers to distinguish between Variscan and Alpine syn-metamorphic structures in the basement rocks of the Argentera-Mercantour Massif.…”