A complex magmatic system beneath the Devès volcanic field,Massif Central,France: evidence from clinopyroxene megacrysts

Clinopyroxene megacrysts and mineral aggregates with clinopyroxene occur in the volcanic deposits at Mont Briançon and Marais de Limagne, which are located in the northern part of the Devès volcanic field (Massif Central, France). The clinopyroxenes can be subdivided into five groups based upon their major and trace element chemistry. Types 1a, 1b and 1c have mg# ～0.80 and are relatively Al-rich and low in Na and Fe³⁺. Subdivision into three groups is based on differing trace element signatures. Type 2 clinopyroxenes have mg# = 0.63–0.65 and higher Na and Fe³⁺ (Fe³⁺/ΣFe > 0.4) contents and may contain apatite inclusions. A type 3 megacryst is Fe-rich (mg# = ～0.52) and has the highest Na and Fe³⁺ contents, as well as containing titanite and apatite inclusions. High Fe³⁺ contents in all clinopyroxenes investigated emphasises the need to consider Fe³⁺/Fe²⁺ when assessing the petrologic origin of such megacrysts. The large range in mg# means that the clinopyroxenes could not all have crystallised from the same melt; in fact comparison with the basanitic host lavas from the two localities reveal that nearly all of the megacrysts are xenocrystic in the strict sense. The clinopyroxenes are mostly genetically related, having crystallised from related melts within the magmatic system that had undergone various degrees of differentiation. Similarities in clinopyroxene chemistry indicate that both volcanic centres are linked to the same magmatic system at depth. Assessing the depth of crystallisation reveals that types 1a and 1b formed in the lithospheric mantle, near the asthenosphere–lithosphere boundary, whereas types 1c, 2 and 3 formed in crustal magma chambers or conduits. Eruption was induced by a pulse of Mg-rich magma from the asthenosphere that entered the existing magmatic system, entraining clinopyroxene as megacrysts at several stages of ascent, before erupting at the surface. The style of eruption at Mont Briançon (cinder cone) and Marais de Limagne (maar) is different and most likely reflects local differences in near-surface hydrology. The essentially identical variety in megacrysts at the two localities suggests that eruption must have been nearly contemporaneous.