Magma mixing in a main stage of formation of Montagne Pelée: the Saint Vincent-type scoria flow sequence (Martinique, F.W.I.)

Abstract

Contrasting with the first stage of activity at Montagne Pelée, which built an andesitic volcano chiefly formed of brecciated lava flows and pyroclastic breccias, the second main stage (> 40,000 to 20,000 years B.P.) is characterized by the prevalence of scoria flows of more mafic composition. Many cross-sections of the deposits of stage 2 show an upward magmatic evolution from early acid andesites (plinian air-fall and pumice flow deposits) to late basaltic andesites (scoria flow deposits). Heterogeneous “banded” rocks, complex mineralogical associations and mineralogical disequilibrium features in the whole series are explained by mechanical mixing of contrasting magmas. Reversed zoning characterized plagioclase phenocrysts of both the acid and basic terms and also pyroxene phenocrysts of the mafic terms. Grossly uniform composition of residual glass in most macroscopically homogeneous rocks implies that chemical exchanges between liquids and local hybridization occurred. Plutonic rocks with cumulate textures are found in upper units of the sequence. Their phenocrysts (anorthite, salitic clinopyroxene, pargasitic hornblende and Ti-magnetite) are in equilibrium with a subalkaline basaltic interstitial liquid. The same phases are commonly present as xenocrysts in the basaltic andesites. It is inferred that basaltic andesites of stage 2 were firstly derived from a subalkaline basalt by crystal fractionation. Mixing of the basaltic andesites (52% SiO2) with acid andesites (62% SiO2) occurred in late stage of evolution of the series, producing heterogeneous rocks of intermediate composition. Successive eruption of acid up to mafic magmas, dominant amount of mafic rocks in the sequence and short duration of the magma mixing event shown by narrow rims of the reversely zoned phenocrysts, suggest that mixing was produced by intrusion of mafic magma into a differentiated magma body. We propose that hydrostatic overpressure induced by a high rate of intrusion triggered the eruption. Thus, magma mixing would be due to a large volume of mafic magma passing through the resident magma. Rapid admixture of the two batches increased gas exsolution, resulting in a highly explosive eruption. Magma mixing can thus explain the explosive dynamics of the basaltic andesites eruptions, which surprisingly contrast with the relatively effusive style of the preceding andesitic stage.