Plutons record time-integrated intrusive processes. Using field geology and igneous petrology tools (whole-rock and mineral compositions, and thermodynamic modeling), we decipher the processes that primitive arc magmas experienced in the upper crust. The Emigrant Gap mafic complex, the most primitive intrusive complex of the Sierra Nevada, reveals relatively low-water mafic magmas that experienced open-system processes including infiltration and subsequent reaction of melts transiting through crystal rich magma “mush” and hybridization between the mush and a new batch of felsic melt in an upper-crustal volcanic reservoir.