Abstract
The Edgecumbe volcanic field is a Holocene volcanic province located on Kruzof Island, SE Alaska. Exposed within the 260 km2 field are basalt, andesitic basalt, andesite, dacite and rhyodacite. The rhyodacites were erupted after the basalts and before the andesites. The volcanics, which are Al-rich (14–18 wt%) and lack an iron enrichment trend, range from tholeiites (47 wt% SiO2) through rhyodacites (72%), but a compositional gap of approximately 9 wt% separates the dacites and rhyodacites. Initial 87Sr/86Sr ranges from 0.70297 in the basalts to 0.70440 in a pyroxene andesite. δ 18O increases across the suite: 5.8‰ to 7.9‰. Plagioclase (An32–86) is the dominant phenocryst in all but one lava. Olivine (Fo58–86) occurs in the basic lavas (<53 wt% SiO2), but is replaced by orthopyroxene (En43–73) and clinopyroxene (En31Wo41-En48Wo40) in the more siliceous volcanics. In the basalts and rhyodacites, plagioclase is weakly zoned, but extreme zoning (<30 mole% An) is characteristic of phenocrysts in the intermediate lavas. Fractionation of the observed phenocryst assemblages could not have produced the more silicous volcanics. Instead they were generated by partial melting of intrusive basement (87Sr/86Sr=0.70487; δ 18O: 8.7–9.3) by basaltic magma and subsequent assimilation. Mass balance calculations show the rhyodacites are almost pure partial melt (<5% basaltic component) whereas the intermediate lavas contain between 30 and 60% partial melt.
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Myers, J.D., Marsh, B.D. Geology and petrogenesis of the edgecumbe volcanic field, SE Alaska: The interaction of basalt and sialic crust. Contr. Mineral. and Petrol. 77, 272–287 (1981). https://doi.org/10.1007/BF00373543
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DOI: https://doi.org/10.1007/BF00373543