Petrological insights on the last 1000 years of explosive activity at La Soufrière volcano, St. Vincent (Lesser Antilles)

Abstract

The petrological features of scoria samples and enclosed holocrystalline xenoliths from the last ~1000 years of activity of the La Soufrière volcano of St. Vincent (Lesser Antilles) are described in detail here. The products of the investigated prehistoric eruptions (pre-1440, 1440, 1580 CE) are relatively homogeneous basaltic andesites (with a paragenesis consisting of plagioclase, clinopyroxene, orthopyroxene and opaques) commonly hosting gabbroic xenoliths. Scoria samples from the historical eruptions (1718–1812 and 1902–03 CE) are mostly similar to the former in terms of whole-rock composition, mineral chemistry and types of xenoliths, but include also a mafic-rich scoria type (plagioclase + clinopyroxene + olivine) in the products of the 1902–03 eruption that is less evolved in composition and encloses ol-gabbro and ultramafic xenoliths. Both scoria types were likely affected by crystal contamination and accumulation processes, as indicated by their moderately to strongly porphyritic textures and large compositional variation of the crystal populations. Recycling of xenocryst, protocryst and/or antecryst phases, however, did not significantly modify the original magma composition, as suggested by the overall consistency of whole-rock differentiation trends with petrography, mineral chemistry, xenolith types and glass compositions (within both inclusions and the groundmass) and experimental petrology observations, crystal fractionation therefore still appearing as the main evolutionary process. Most of the gabbro and ol-gabbro xenoliths have mineral compositions in line with that of the coexisting phenocrysts and textural features consistent with an intrusive origin, i.e., representing fully-crystallised melt aliquots. Conversely, ultramafic, troctolitic and noritic xenoliths (plus some ol-gabbro) are likely representing cumulitic assemblages, i.e., mineral phases removed from the magma by crystal fractionation. The typical association of An-rich plagioclase and relatively Fo-poor olivine of the troctolite (and ol-gabbro) cumulates is ascribed to crystallisation from a hydrous (H2O ~4.5–6 wt%) high-Al2O3 basalt/basaltic andesite magma crystallising at shallow depths (~0.2 GPa, i.e., at ~7 km). On the other hand, the ultramafic cumulitic xenoliths are thought to derive from a less hydrous basalt magma stalling at deeper levels (~0.4 GPa, ~13 km). The tapping of such magma could mark a major rejuvenation event, which is the process that likely triggered the 1902–03 eruption.