Early Variscan I-type pluton in the pre-Alpine basement of the Western Alps: The ca. 360 Ma Cogne diorite (NW-Italy)

Abstract

Located at the internal border of the Grand-Saint-Bernard Zone, the diorite and its aureole lie on top of intensively studied Alpine eclogitic units but this pluton, poorly studied yet, has kept locally almost undeformed. The pluton intruded, at ~ 360 Ma, country-rocks mostly composed of dark shales with Na₂O > K₂O and minor mafic intercalations of tholeiitic basalt affinity. This association is characteristic of the Vanoise (France) basement series, where available age determinations suggest an Early Paleozoic age. Parts of the pluton, and of its hornfels aureole that is evidenced here for the first time, in the Punta Bioula section of Valsavaranche valley (NW-Italy), have been well-preserved from the Alpine deformation. Syn-emplacement hardening, dehydration-induced, probably prevented strain-enhanced Alpine recrystallization. Magmatic rock-types range continuously from subordinate mafic types at SiO₂ ~ 48%, of hornblendite with cumulative or appinite affinities, to the main body of quartz diorite to quartz monzonite (SiO₂ up to 62%). P–T estimates for the pluton emplacement, based on the abundance of garnet in the hornfelses, using also zircon and apatite saturation thermometry and Al-in-hornblende barometry, suggest T ~ 800–950 °C and minimum P in the 0.2–0.5 GPa range, with records of higher pressure conditions (up to 1–2 GPa?) in hornblendite phlogopite-cored amphibole. The high-K, Na > K, calc-alkaline geochemistry is in line with a destructive plate-margin setting. Based on major element data and radiogenic isotope signature (εNd_{360 Ma} from − 1.2 to + 0.9, ⁸⁷Sr/⁸⁶Sr_{360 Ma} from 0.7054 to 0.7063), the parental magmas are interpreted in terms of deep-seated metabasaltic partial melts with limited contamination from shallower sources, the low radiogenic Nd-content excluding a major contribution from Vanoise tholeiites. There is no other preserved evidence for Variscan magmatism of similar age and composition in the Western Alps, but probable analogs are known in the western and northern parts of French Massif Central. Regarding the Alpine tectonics, not only the age of the pluton and its host-rocks (instead of the Permo-Carboniferous age previously believed), but also its upper mylonitic contact, suggest revisions of the Alpine nappe model. The Cogne diorite allegedly constituted the axial part of the E-verging “pli en retour [backfold] du Valsavaranche”, a cornerstone of popular Alpine structural models: in fact, the alleged fold limbs, as attested here by field and geochemical data, do not belong to the same unit, and the backfold hypothesis is unfounded.

Introduction

Assessing the tectonic setting and P–T conditions of emplacement of granitoid bodies relies on a wide choice of methods. For fresh rocks, the study of mineral assemblages, mineral zoning and mineral chemistry may be privileged upon the consideration of “blind” whole-rock geochemical indicators. In case of secondarily metamorphosed plutons, this approach is not applicable any more, and it is necessary to use more indirect methods, based on immobile element contents at the whole rock scale, and refractory minerals. Here we attempt to characterize the Cogne diorite, a plutonic body that belongs to the polycyclic, pre-Triassic basement of the Western Alps, with special effort to separate the pre-Alpine heritage from the Alpine overprinting.

The Cogne diorite pluton crops out in the basement of the Middle Penninic Zone. Its igneous emplacement occurred near the Devonian–Carboniferous boundary, as documented by U–Pb zircon dating (Bertrand et al., 2000b) at ~ 360 Ma, so that this pluton is a rare, if not the only, piece of evidence of the early stage of the Variscan events in the Western Alps. Here we present new geochemical and petrological data for the Cogne diorite as an opportunity to assess the Early Carboniferous, or older, tectonic setting of yet poorly known pre-Alpine domains. In a context of Alpine high pressure and subduction-related nappes, tracing the origin of the units requires to take into account their protolith ages. A number of protolith age determinations have been obtained during the last decades on such pre-Alpine basement units, often with unexpected results, leading to various paleogeographic inferences (Bertrand et al., 2005, Michard and Goffé, 2005, Ring et al., 2005). It seems timely to unravel the pre-Alpine geological history of some of those recently dated units.