In situ U–Pb dating of upper triassic magmatic flows (Spilite) in the External Western Alps (Pelvoux massif): A peripheral CAMP activity?

Abstract

Magmatic activity during the Triassic-Jurassic transition coincided with the breakup of Pangea, marking a pivotal period in Western European tectonic evolution. However, this activity remains poorly documented in the External Western Alps. Dating the emplacement and alteration of Triassic magmatic rocks such as the spilites in the Alps has long been challenging due to the complex alteration history and the scarcity of suitable mineral phases. This study employs in situ U-Pb dating of carbonate to constrain the timing of hydrothermal alteration of spilites from the Pelvoux massif (France), offering a new temporal framework for these processes. Dolomite and calcite filling vesicles and veins in the spilites yield ages of 201 ± 15 Ma and 202 ± 47 Ma, respectively, consistent with the stratigraphic emplacement interval of these lavas during the Upper Triassic. Notably, the U-Pb system in dolomite has preserved hydrothermal conditions related to magmatic emplacement, without resetting during two subsequent geological events at temperatures approaching 300°C, emphasizing its reliability as a chronometer in this context. The obtained ages overlap with the temporal framework of the Central Atlantic Magmatic Province (CAMP) and geochemical signatures of the spilites correspond to medium/high-Ti transitional to alkaline basalts, comparable to continental basalts such as those of the CAMP. This suggests a shared tectono-magmatic context with mantle-derived magmatism. Furthermore, the spatial proximity of the studied spilites to lower crustal CAMP-related magmatism in the Internal Alps supports a potential genetic relationship, with magma ascent likely facilitated by inherited tectonic structures during the Upper Triassic extension and the opening of the Alpine Tethys. Hydrothermal alteration, marked by spilitization and carbonate precipitation, occurred under low- to moderate-temperature conditions (70–360°C), possibly driven by marine or continental-derived fluids. By providing the first absolute geochronological constraints on spilites in the External Western Alps, this study expands the recognized extent of CAMP. It underscores the utility of carbonates as reliable archives for unraveling hydrothermal and magmatic histories.