Kahoolawe Island is a ∼ 1.4 to 1 Ma shield volcano composed of shield, caldera-fill, and postshield tholeiitic lavas, and postshield alkalic basalt and hawaiite lavas. One postshield vent erupted alkalic lavas (K2O 1.1–1.6 wt.%) with resorbed olivines of wide compositional range (cores, Fo86–71), resorbed Na-plagioclase (An57–30; largely andesine) and clinopyroxene (evolved; Mg#s 76–71), and groundmass orthopyroxene (Mg# ≤62). They also contain tholeiitic gabbro xenoliths, which, as a suite, have a continuum of mineral compositions — clinopyroxene Mg#s 83–74, orthopyroxene Mg#s 83–76, and plagioclase An69–35 (e.g., includes andesine gabbro). Lava compositions do not fall on expected ‘Hawaiian’ fractionation trends due to MgO ‘enrichment’ (e.g., CaO ∼ 7 wt.% @ MgO ∼6 wt.%). This assortment of mineral and rock components within alkalic lavas with apparent Mg enrichment is owed to a complex history that began with protracted mixing among primitive and differentiated tholeiitic magmas, probably near the end of shield building. These hybrid magmas crystallized a compositional variety of olivines that were resorbed during reservoir replenishments, and also crystallizedin situ to form orthopyroxene-bearing gabbro on reservoir walls. When magma production rates declined during the shield to postshield transition of tholeiitic to alkalic magmatism, the tholeiitic hybrids in reservoirs fractionated to yield highly evolved phases such as andesine and clinopyroxene with Mg# < 75. When postshield hawaiite magmas subsequently entered reservoirs, alkalic-tholeiitic hybridization occurred; the resulting `complex' mixture of hawaiite+tholeiitic hybrids resorbed andesine and clinopyroxene crystals and, upon eruption, entrained xenoliths of gabbro. Mass balancing suggests that the alkalic-tholeiitic hybridization involved ∼44% hawaiite mixed with a nearly equal amount of tholeiitic hybrid (MgO ∼9.5 wt.%) plus olivine and andesine. This type of complex hybridization is a logical process for magmatism associated with tholeiitic to alkalic transitions and waning magma production, and this Kahoolawe example is the first to document such mixing in Hawaiian reservoirs.