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A tonalite (∼66 wt % SiO2; 2·4 wt % K2O) xenolith from the Mauna Kea summit region provides information on the origin of silicic liquids in mafic magma systems. This leucocratic rock has ∼40 vol. % quartz interstitial to and enclosing Ca-andesine, plus phlogopite (∼11 vol. %), clinopyroxene (mg-number ∼74), orthopyroxene, and Fe–Ti oxides. Also, it contains lithic fragments (≤5 cm) of gabbro (MgO 7·2 wt %; An60–50; clinopyroxene mg-number ∼78–75; phlogopite). The tonalite has Sr–Nd–Pb isotopic ratios of 87Sr/86Sr 0·703610, 144Nd/143Nd 0·512976, 206Pb/204Pb 18·58, 207Pb/204Pb 15·49, and 208Pb/204Pb 38·15, which agree with the isotopic composition of Mauna Kea post-shield Hamakua Volcanics (tholeiitic and alkalic basalts). A positive Eu anomaly and poikilitic texture indicate a cumulate origin. Leucocratic diorite (∼50–53 wt % SiO2; 1–2·4 wt % K2O) xenoliths coexist with the tonalite. These have intergranular andesine–oligoclase (∼68–85 vol. % of An50–15), evolved clinopyroxene (mg-number 76–69), and biotite (± orthopyroxene, zircon). The tonalite represents SiO2-rich liquids that accumulated, perhaps in a small reservoir, after segregation from a hydrous basalt-magma solidification front; gabbro inclusions within the tonalite are probably remnants of that source. Hydrous conditions (implied by abundant phlogopite) were necessary for this silicic-liquid segregation; namely, relatively high H2O facilitated crystallization of SiO2-deficient phases (oxide; phlogopite) to leave SiO2-enriched residual liquids, and H2O vapor pressure helped segregate silicic liquids by gas-pressured filter pressing. The diorites are coarse-grained equivalents of hawaiite and mugearite produced by clinopyroxene-dominant crystallization of Hamakua alkalic basalt. On the basis of normative Ol–Di–Ne, diorite xenoliths originated under intermediate and high pressures, apparently reflecting the transition from low- to high-P crystallization regimes that are identified for Mauna Kea post-shield magmas (where the high-P regime is approximately at the crust–mantle boundary). Leucocratic xenoliths, particularly tonalite, provide additional insights into Hamakua post-shield magma evolution by manifesting differentiation that is more extensive and complex, approaching granitic compositions, than that represented by Hamakua lavas, and by showing that quartz can crystallize appreciably during Hawaiian magmatism. Quartz-bearing rock on Hawaii has global relevance in terms of providing better understanding of the circumstances for oceanic basalt–rhyolite magmatism.