Koolau shield basalt as xenoliths entrained during rejuvenated-stage eruptions: perspectives on magma mixing

• Published in: Bulletin of volcanology Vol. 66; no. 2; pp. 182 - 199
• Main Authors: Weinstein, J. P., Fodor, R. V., Bauer, G. R.
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer 01.02.2004; Springer Nature B.V
• Subjects: Basalt; Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Lava; Magma; Mineralogy; Salt lakes; Trace elements; Volcanic cones; Volcanoes; Volcanology; Kilauea; Polynesia; Hawaii; Hawaii County Hawaii; Oahu; Hawaii Island; Mauna Loa; depletion; chain silicates; chabazite; Magma replenishment; mixing; olivine; Zeolites; Oceania; rare earths; mineral composition; enrichment; xenoliths; trace-element analyses; lava; basalts; East Pacific Ocean Islands; cones; okenite; sheet silicates; silicates; framework silicates; Shield volcano; Basalt mineral compositions; Koolau; electron probe data; zeolitization; Magma mixing; nesosilicates; processes; volcanic rocks; eruptions; Hawaiian basalt; igneous rocks; magmas; chemical analysis; pyroclastics; zeolite; tuff; shield volcanoes; apophyllite; tholeiitic basalt
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-003-0302-1

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 'end member', 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 determine how these seemingly random samplings of the Koolau profile compare to established Koolau geochemistry. Analyses reveal that 24 are shield tholeiitic basalt--the focus of this study--and 4 are rejuvenated-stage basaltic rocks. The tholeiitic xenoliths represent largely upper Koolau shield lavas, as these samples (8.3 to 5.8 wt% MgO) have, with one exception, overall major- and trace-element compositions that overlap those of Koolau subaerial shield lavas. Secondary processes, however, created some distinctions--namely, enrichments/depletions in K, Ba, Sr, SiO^sub 2^, 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--Fo^sub 87.8-72^, orthopyroxene Mg#s 85-72, and An^sub 74-60^. Several xenolithic basalts have both normally and reversely zoned orthopyroxene and plagioclase with a variety of core compositions (e.g., orthopyroxene-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 proportions; however, extreme MgO lavas (~13 and <5 wt%) are not observed as either subaerial or xenolithic basalt, but are indicated by phenocryst cores of Fo^sub 87.8^ and orthopyroxene-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.[PUBLICATION ABSTRACT]