Contributions to the Mineral Chemistry of Hawaiian Rocks. V. Composition and Origin of Ultramafic Nodules and Megacrysts in a Rhyodacite from Oahu, Hawaiian Islands
| dc.contributor.author | Fodor, R.V. | |
| dc.contributor.author | Keil, Klaus | |
| dc.contributor.author | Bauer, G.R. | |
| dc.date.accessioned | 2008-04-11T02:34:42Z | |
| dc.date.available | 2008-04-11T02:34:42Z | |
| dc.date.issued | 1977-04 | |
| dc.description.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. | |
| dc.identifier.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. | |
| dc.identifier.issn | 0030-8870 | |
| dc.identifier.uri | http://hdl.handle.net/10125/1193 | |
| dc.language.iso | en-US | |
| dc.publisher | University of Hawaii Press | |
| dc.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 | |
| dc.type | Article | |
| dc.type.dcmi | Text |