A SNAPSHOT OF THE VOLCANIC ACTIVITY DURING THE EARLY EVOLUTION OF THE LIGURIAN-PIEDMONT OCEAN: INSIGHTS FROM THE PILLOW LAVA SEQUENCE OF THE BALAGNE NAPPE (ALPINE CORSICA)

Abstract

In Alpine Corsica, the Upper Units consist of weakly metamorphosed Middle to Late Jurassic ophiolites and their Late Jurassic to Late Cretaceous sedimentary covers. The Navaccia Unit in the Balagne area is one of the best preserved within the Upper Units. It is characterized by a sequence of pillow lava basalts, which is at least 700-800 m thick, which occurs at the top of an oceanic basement, consisting of serpentinized peridotites and  gabbros, topped by ophiolitic breccias. This sequence of pillow lavas features dolerite sills as well levels of radiolarites and continental-derived silty sandstones and siltstones.

Geochemically, the Navaccia pillow lavas are mainly tholeiitic basalts with minor basaltic andesites, displaying an overall N-MORB affinity but clear enrichment in incompatible elements such as Ti, Nb, Th, Ta, Hf, and LREE relative to other Corsican ophiolites. Their chondrite-normalized REE patterns, with HREE depletion (Dy_N/Yb_N = 1.29-1.59), indicate a mantle source influenced by residual garnet. Accordingly, based on Dy/Yb vs. Ce/Yb co-variation, the Navaccia basalts can be classified as garnet-influenced basalts like those formed at the ocean-continent transition zone. They are characterized by hump-shaped LREE patterns (La/Nd)_N = 0.64-0.93) indicating source heterogeneity and/or source enrichment. Petrogenetic modelling (Ce, Dy, Yb, Th, and Nb) suggests primary magmas derived from 8-12% partial melting of an enriched spinel-facies mantle, combined with 20-30% melting of a garnet-pyroxenite component. The exceptional thickness and preservation of the sequence enable a chemostratigraphic reconstruction of OCTZ volcanism, revealing uniform incompatible-element contents and Mg# (63-47) throughout the pile. This implies construction through repeated small eruptions tapping magmas with comparable partial-melting degrees and fractional-crystallization histories, likely governed by stable magma-chamber processes. Overall, its geochemistry and the occurrence of continental-derived sediments indicates that the sequence of pillow lavas originated in the initial stage of spreading of the Ligurian-Piedmont oceanic basin, close to the European continental margin.