Contributions to the Mineral Chemistry of Hawaiian Rocks. V. Composition and Origin of Ultramafic Nodules and Megacrysts in a Rhyodacite from Oahu, Hawaiian Islands

Dunite nodules (Foss) and megacrysts of olivine (Fos3 s4 ) and clinopyroxene (FS13W043En44) are present as rare inclusions in the rhyodacite (~ 66wt %SiO z ) of Kauaopuu Ridge, Oahu, Hawaii. The rhyodacite is interbedded with caldera-filling tholeiitic lavas of the Waianae volcano. Results: (I) Dunite nodules (::;; I cm) consist of xenomorphic-granular olivine and minor chrome spinel and clinopyroxene (Fs6 W047 En47 ); olivine (~Fos s) reacted with the rhyodacite magma to form amphibole and an Fe-enriched margin (~Foso)' (2) Olivine megacrysts (2-6 mm) contain chrome spinel and melt inclusions; they are resorbed and rarely are slightly enriched in Fe at the margins. (3) Clinopyroxene megacrysts (0.5-1.2 cm) contain ilmenite, ferropseudobrookite, and melt inclusions; they are slightly resorbed and enriched in MgO at unresorbed margins. Conclusions: (1) Olivine from the dunite and olivine megacrysts compositionally resemble olivine in typical dunite inclusions in alkalic olivine basalts of Hawaii, as well as olivine phenocrysts in basalts of Hawaii. Clinopyroxene in the dunite resembles that in typical dunite inclusions of Hawaii, whereas clinopyroxene megacrysts are like phenocrysts in basalts of Hawaii. (2) The reaction relationship between the dunite and the rhyodacite magma suggests that the nodules are accidental. A positive gravity anomaly over Waianae volcano indicates a dense, perhaps olivine-rich zone beneath the volcano-a possible source for the nodules. (3) The resorption ofthe megacrysts and their compositions indicate that they are probably remnant phenocrysts of basaltic magma from which the rhyodacite formed by igneous differentiation. (4) Rhyodacite magma was derived from a basaltic parent and it later incorporated dunite fragments, probably during ascent. I This paper was first presented as a talk at the Cordilleran Section meeting, Geological Society of America, 29-31 March 1974, at Las Vegas, Nevada. The work was supported in part by United States National Aeronautics and Space Administration grant NGL 32-004-064 and by a University of New Mexico-NASA Ames Research Center cooperative working agreement (K. Keil, principal investigator). Manuscript accepted 23 September 1976. 2 University of New Mexico, Department of Geology and Institute of Meteoritics, Albuquerque, New Mexico 87131. Current address: North Carolina State University, Department of Geosciences, Raleigh, North Carolina 27607. 3 University of New Mexico, Department of Geology and Institute of Meteoritics, Albuquerque, New Mexico 87131. 4City and County of Honolulu, Board of Water Supply, 630 South Beretania Street, Honolulu, Hawaii 96813. 211 MAFIC AND ULTRAMAFIC inclusions in the volcanic rocks of Hawaii have been studied by White (1966), Jackson (1968), Kuno (1969), Jackson and Wright (1970), and Beeson and Jackson (1970). Most inclusions occur in basaltic rocks (mainly of the alkalic and nephelinic suites) and some are in more highly differentiated rocks such as hawaiite and trachyte (Jackson 1968). Only rarely have inclusions been found in rocks of the tholeiitic suite (Powers 1955: 90, White 1966) and none have been previously observed in rhyodacite, a rock interpreted as the most highly differentiated member of the tholeiitic suite (Macdonald and Katsura 1962, 1964; Bauer et al. 1973). Here, rare nodules of dunite and mega212 PACIFIC SCIENCE, Volume 31, April 1977 FIGURE I. Texture of a dunite nodule in rhyodacite of Kauaopuu Ridge, Oahu, Hawaiian Islands. Note triplepoint junctures between some olivine grains, indicating recrystallization. A fine-grained discontinuous rim of amphibole is present between the margin of the nodule and the glass of the rhyodacite host (black area, bottom). Crossed nicols. Scale bar equals 0.5 mm. crysts of olivine and clinopyroxene from near the base of the rhyodacite lava flow at Kauaopuu Ridge, Oahu, Hawaiian Islands, were studied by microscopic and microprobe techniques (Keil, Fodor, and Bunch 1972; Bauer et al. 1973). Such inclusions in a host rock of '" 66 wt %S102 pose some interesting questions-for example, whether the rhyodacite was at one time in contact with mantle material (accidental inclusions) or whether the inclusions are of high-pressure cognate origin, thus providing an estimation of minimum depth of formation for the rhyodacite.