Xenoliths from the southern flank of Mauna Kea volcano form two broad categories. (1) Ultramafic: porphyroclastic dunite, wehrlite, and olivine clinopyroxenite (Fo89.4–83.6, clinopyroxene mg-number 90.3–86.3, spinel mg-number 57–42, spinel cr-number 7–52, no palgioclase); and granular wehrlite and olivine clinopyroxenite (Fo83–76) with plagioclase (An84–69) ± orthopyroxene, and Cr-magnetite. (2) Gabbroic: granular gabbro, gabbronorite, and troctolite composed of olivine + clinopyroxene frameworks (Fo82–74, mg-number 85–79) enclosing plagioclase (∼An79–69) ±orthopyroxene, and Fe–Ti oxides; and plagioclase (4 wt %). These Mauna Kea xenoliths are plutonic complements to postshield lavas (Hamakua volcanics), and they identify that stage of volcano development with 15–5 wt % MgO magmas that underwent processes intrinsic to mafic-layered intrusions; e.g. in situ and gravity-settled crystallization, extensive differentiation, varieties of layering, mobilizations of late-stage, evolved liquids, compaction and connective disturbances in reservoirs. [ABSTRACT FROM AUTHOR]