Microstructures and composition of melt inclusions in a crustal anatectic environment, represented by metapelitic enclaves within El Hoyazo dacites, SE Spain

Abstract

This article reports microstructures and compositions of melt inclusions in anatectic metapelites found as enclaves within El Hoyazo dacites, in the Neogene Volcanic Province of southeastern Spain. The enclaves represent fragments of continental crust partially melted at ∼ 800–850 °C and 5–7 kbar, and brought to surface rapidly within the host volcanics while they still were in a molten state. Rapid cooling produced the solidification of silicate melt to glass in the rock matrix and in inclusions within minerals. Melt inclusions (MI) are present within nearly all mineral phases in the enclaves, and their associated microstructures indicate a primary or pseudo-secondary origin. The entrapment mechanisms of MI within plagioclase were associated with: (1) the presence of micron-sized solids (mostly graphite) on the surface of growing crystals; (2) partial resorption of crystals, generation of embayments, and later growth in the presence of melt; and (3) entrapment of melt during crystal growth within crystallographic-controlled planar structures, e.g. on crystal faces. Melt inclusions in garnet are commonly associated with regularly oriented planar discontinuities filled with glass, interpreted either as spaces left between adjacent mineral growth spirals or as crystallographic-controlled cracks generated in-between stages of mineral growth. Melt was trapped or percolated along these microstructures and, later on, necking down phenomena individualized inclusions with negative crystal shape. Melt inclusions in biotite are abundant, show negative crystal shape, and parallel cleavage planes. Melt inclusions in cordierite, alkali feldspar and ilmenite are isolated, sparse and sometimes rather large. The composition of glass from all MI is leucogranitic and peraluminous, with small differences among glasses in each mineral host. There is some variation in glass composition within each textural location as well. Compositional heterogeneity can be partially explained by a combination of processes such as generation of boundary layers during mineral growth, crystallization of daughter phases, and hydrogen loss via diffusion through the host. Mean glass compositions in the several textural locations, however, can be tentatively interpreted as reflecting the evolution of melt chemistry during the prograde low-to-medium pressure anatexis of quartz-poor metapelites.