Magmatic to solid-state evolution of a shallow emplaced agpaitic tinguaite (the Suc de Sara dyke, Velay volcanic province, France): implications for peralkaline melt segregation and extraction in ascending magmas

Abstract

In the last decades the mush model has been generalized to the complete trans-crustal magmatic system in which differentiation would be driven by segregation and extraction of trapped melts from crystal-rich mushes. Melt extraction processes involved are porous flow and strain localization, the latter being regarded as the main process acting during transfer through dykes and necks along which high differential stresses are acting on. We combine structural measurements together with petrological analyses and textural observations to constrain the model of emplacement and finally emphasize how shear deformation and strain localization structures promoted the residual melt segregation that occurred in a shallow silica-undersaturated peralkaline intrusion (Suc de Sara, Velay volcanic province, French Massif Central). In this study, we demonstrate that segregation and subsequent extraction of the CO2-rich residual melt occurred during magma ascent and final emplacement of the Suc de Sara tinguaite. Contrasting features of shear deformation between the margins that exhibited different permeabilities highlight that melt segregation started by compaction as a loose packing of emerging microlites and continued with melt filling of an anastomosed C/C′ band network developing in the crystal-rich mush subjected to high shear strain. Subsequent melt extraction throughout the country rock was controlled by the permeability of the hanging wall. Along the western hanging wall of the intrusion, extraction of the residual melt was prevented by the 15 cm thick chilled margin. In contrast, segregated melt circulated through the highly porous and permeable eastern margin, causing the fenitization of the country rock.