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Title: Contributions to the Mineral Chemistry of Hawaiian Rocks. V. Composition and Origin of Ultramafic Nodules and Megacrysts in a Rhyodacite from Oahu, Hawaiian Islands 
Author: Fodor, R.V.; Keil, Klaus; Bauer, G.R.
Date: 1977-04
Publisher: University of Hawaii Press
Citation: Fodor RV, Keil K, Bauer GR. 1977. Contributions to the mineral chemistry of Hawaiian rocks. V. Composition and origin of ultramafic nodules and megacrysts in a rhyodacite from Oahu, Hawaiian Islands. Pac Sci 31(2): 211-222.
Abstract: Dunite nodules (Fo85) and megacrysts of olivine (Fo83 - 84 ) and
clinopyroxene (FS13 Wo43 En44) are present as rare inclusions in the rhyodacite
( ~ 66wt%SiO 2 ) of Kauaopuu Ridge, Oahu, Hawaii. The rhyodacite is interbedded
with caldera-filling tholeiitic lavas of the Waianae volcano. Results:
(I) Dunite nodules (< 1 cm) consist of xenomorphic-granular olivine and minor
chrome spinel and clinopyroxene (Fs6 Wo47 En47); olivine (~Fo85) reacted
with the rhyodacite magma to form amphibole and an Fe-enriched margin
(~Fo80). (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 of the 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.
ISSN: 0030-8870

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