KOOLAU SHIELD BASALT AS XENOLITHS ENTRAINED DURING REJUVENATED-STAGE ERUPTIONS: PERSPECTIVES ON MAGMA MIXING

Rejuvenated-stage tuff cones (Honolulu Volcanics) on Koolau volcano, Oahu, Hawaii, contain xenoliths of Koolau shield basalt. Because Koolau subaerial shield lavas represent a Hawaiian geochemical 'endmember', and submarine shield lavas have compositions with some affinities to Mauna Loa and Kilauea, we analyzed 28 xenolithic basalts from Salt Lake and Koko Head cones to establish how these seemingly random samplings of the Koolau profile compare to established Koolau geochemistry. Analyses reveal that 24 are shield tholeiitic basalt and four are rejuvenated-stage basaltic rocks. Highlights include that the rejuvenated-stage tuffaceous eruptions entrained largely upper Koolau shield lavas, as the tholeiitic basalt samples (8.3 to 5.8 wt.% MgO) have, with one exception, overall compositions that overlap those of Koolau subaerial lavas. There are some distinctions, however, from secondary effects -- namely, enrichments/depletions in K, Ba, Sr, SiO2, and FeO, and, due to zeolitization (chabazite with attending okenite and apophyllite), elevated CaO. One xenolithic basalt with 8.2 wt.% MgO has higher Ti, Zr, Nb, and Sc, and lower Zr/Nb than subaerial lavas, and appears to represent relatively early, deeper shield -- thereby reinforcing that the Koolau shield source varied temporally. Olivine, orthopyroxene, and plagioclase are the phenocrysts (clinopyroxene is rare), and their core compositions range widely across the suite -- Fo87.8-77, opx Mg#s 85-72, and An74-60. Several xenolithic basalts have both normally and reversely zoned orthopyroxene and plagioclase grains with a variety of core compositions (e.g., opx-core Mg#s 82, 77, and 72, all in one sample). These compositions and zonations record evidence for wide compositional ranges of replenishment (MgO ~13-8 wt.%) and reservoir (MgO ~7 to <5 wt.%) magmas mixing in varying volumetric proportions, from preponderances to minor relative amounts of each type; however, extreme MgO lavas (~13 and <5 wt.%) are not observed as either subaerial or xenolithic basalt, but are indicated by phenocryst cores of Fo87.8 and opx-Mg# 72. The Koolau magma-mixing history resembles that of Kilauea, and is unlike the 'steady-state' mixing known for Mauna Loa. Finally, these basalt samples show that any xenolithic occurrence of Koolau lava is subject to the zeolitization prevalent in the tuff-cone hosts.