Fractionation, recharge, and hybridization in a shallow, bimodal magma reservoir

The 15.7 Ma Aztec Wash pluton, emplaced at a depth of 4--7 km, preserves a record of bimodal (mafic-felsic) magma fractionation, recharge, and hybridization. This study examines a mafic lobe at structurally deep levels in the pluton. Within the lobe, mafic and felsic magmas form gently dipping sequences (10--20 m thick) composed of mafic sheets (0.5--4 m thick) with felsic interlayers (2 cm--2 m thick). The mafic sheets and felsic interlayers show chemical and textural evidence for increasing chemical and mechanical hybridization with structural height. Previous work has shown that both open and closed system processes operated within the active magma chamber. The new isotopic data in this study place important constraints upon these processes, and show that generation of the observed lithologies involved single- or multi-stage events of mafic magma fractionation, mafic recharge, and hybridization of mafic and felsic magmas and their cumulates.