HIGH-RESOLUTION MAPPING ILLUSTRATES LATE-STAGE MAGMA MIXING PROCESSES

Many studies of granitic plutons have shown that they commonly contain mafic enclaves. This has led to the interpretation that mafic and felsic magmas mix to produce magmas of intermediate composition and that mixing may be an important process in the formation of continental crust. Despite these observations, the exact process by which mixing occurs remains speculative. Here we present a field-based study of mafic-felsic mixing in the Bernasconi Hills pluton (BHP) in order to provide a geological basis for petrographic and geochemical observations.

We mapped and sampled mafic enclaves and granitic host rocks across two lateral transects (~60 m and 150 m, respectively) of the BHP at a scale of 5 m. Our mapping indicates that enclaves are most abundant in swarms structurally above larger mafic intrusions and that schlieren typically occur adjacent to swarms. Enclaves are mineralogically and texturally zoned between a coarse hornblende and quartz-rich interior and a fine-grained biotite-rich rind. In many instances the rind appears to erode off the enclave to form schlieren. Interestingly, we observe systematic enrichments and depletions in hornblende and biotite abundance, respectively in granitic rocks adjacent to enclave swarms and schlieren.

The fact that enclave orientations are representative of late-stage magmatic stress fields (Paterson et al., 2004) suggests that schlieren and biotite-rich rinds are also products of late-stage magmatic processes. By extension, this implies that late-stage processes also produce the petrographic zonation observed in enclaves as well as the observed mineralogical variations in adjacent granitic rocks. These textural features are defined by hydrous minerals and therefore demonstrate that H₂O is an important component in this process. Based upon these observations we propose that mafic-felsic mixing can occur during the late stages of plutonism by infiltration and reaction of aqueous magmatic or metasomatic fluids with enclaves and that these reactions result in the mechanical mixing of mafic and felsic components to produce the field relations noted here.