The Relevance of Crystal Transfer to Magma Mixing: a Case Study in Composite Dykes from the Central Pyrenees

Two composite dykes containing abundant mafic enclaves within a felsic host crop out in the Maladeta Plutonic Complex, Pyrenees, Spain. Field, petrographic and geochemical criteria reveal mixing between gabbroic and aplitic magmas, giving rise to a variety of hybrid compositions. The rocks contain spongy plagioclase, quartz ocelli and amphibole-biotite clots that are interpreted as early crystals destabilized by reaction with the hybrid melts. Spongy plagioclase and quartz ocelli were mechanically transferred from the felsic to the mafic magma, whereas amphibole-biotite clots are former pyroxene crystals from the mafic magma. Multivariate statistics (principal component analysis) have been used to examine variations in the mineral trace element compositions, which are best explained by the crystal transfer process. This study shows that crystal transfer represents a mixing mechanism that can overcome some of the physical limitations of interaction between rheologically contrasting magmas and explain deviations of the hybrid compositions from the theoretical mixed chemical composition. In particular, hybrid whole-rock compositions show non-linear correlations in inter-element variation diagrams for elements that are enriched or depleted in preferentially transferred crystals. This effect has been quantified by extending magma mixing modelling to include crystal transfer. The composite dykes studied could be regarded as scale models of the behaviour of larger-scale magmatic systems, so this investigation has important implications for interpreting the petrogenesis of igneous suites by magma mixing.