Geochemical signals of mafic-felsic mixing: Case study of enclave swarms in the Bernasconi Hills pluton, California

Mafic enclaves are ubiquitous in granitic plutons and in some cases represent evidence of mingling between mafic and felsic magmas. To better understand whether such mingling, as seen on the outcrop (meter) scale, plays an important role in the generation of intermediate magmas at the crustal (>km) scale, we characterized the geochemical signatures of mixing through a fine-scale geochemical study of a mafic enclave swarm (Bernasconi Hills in the northern Peninsular Ranges Batholith, California, USA). The enclaves are basaltic to andesitic in composition and occur as angular to rounded clasts and as highly attenuated schlieren within a host pluton with silica contents ranging from dacitic to rhyolitic in composition. Centimeter- to meter-scale sampling showed clear textural, mineralogical, and compositional evidence for incipient mixing between the mafic enclaves and felsic host. The enclaves and host magmas define linear element-element arrays between the basaltic enclaves and dacitic end member of the host magma (65–70 wt% SiO₂). In particular, crystal fractionation generated strongly arcuate P₂O₅-SiO₂ differentiation trends, so the linear arrays here are interpreted to reflect local-scale mixing. Plagioclase phenocrysts within the host pluton are homogeneous, showing no evidence for the reverse zonation typical of mafic recharge or reheating. This indicates that the enclaves were entrained as cold, solidified xenoliths rather than as hot, basaltic liquids, and that entrainment likely occurred near the end of the magma’s thermal life, providing too little time for the host magma and its phenocrysts to have chemically registered the entrainment of mafic enclaves. As such, mixing appears to have been largely mechanical, driven by flow within the magma body. However, despite textural and bulk-rock geochemical evidence for local mixing, variation diagrams, such as K₂O and Zr versus SiO₂, clearly indicate that the most silicic components of the pluton (>70 wt% SiO₂) did not participate in mixing. These highly silicic components are last-stage residual liquids, which were locally and internally expelled after the pluton had crystallized to the point at which the magma body locked up, terminating mechanical mixing. Finally, our case study of local-scale mixing may shed some light on crustal-scale geochemical systematics of arc crust. While P₂O₅-SiO₂ systematics at Bernasconi Hills show linear mixing arrays between basaltic and dacitic end members, arc volcanic rocks tend to show arcuate patterns, suggesting that crystal fractionation explains most of the spectrum of arc lavas, including intermediate lavas, with mafic-felsic mixing playing a small role. Arc plutonic rocks, however, show both arcuate and linear arrays, suggesting that mixing may become more important during pluton formation and evolution.