Robert R. Coats
1952
<p><span>Samples of 17 </span>volcanic<span> </span>rocks<span> of </span>Tertiary<span> and </span>Quaternary<span> age from </span>Adak<span> and </span>Kanaga<span> </span>islands<span> have been chemically analyzed and studied microscopically. Spectrograms have been made of 10 of them. The </span>rocks<span> from </span>Adak<span> represent one center of possibly older </span>Tertiary<span> age and two centers of younger </span>Tertiary<span> or </span>Quaternary<span> age. The </span>rocks<span> from </span>Kanaga<span> </span>Island<span> represent both a shield volcano of possibly </span>Tertiary<span> age, partly destroyed by the formation of a caldera, and a young cone of </span>Quaternary<span> age that has grown within the caldera. All the </span>rocks<span> are basalt or andesite. Modally, all are characterized by relatively large crystals of plagioclase more calcic than andesine, and by one or more of the following ferromagnesian minerals: olivine, hypersthene, augite, and hornblende. Apatite and iron ores are common, and late silica minerals and orthoclase occur interstitially </span>in<span> the groundmasses of some </span>rocks<span>. As analyses of no more than four samples are available for each center, the small differences between sets of analyses representing different centers are of doubtful significance. Consequently, the analyses representing all the centers have been plotted on each of the several diagrams used. The several types of variation diagrams show that the province is a calc-alkaline one. The alkali-lime index is </span>in<span> the neighborhood of 63. This very high value is comparable with that for Katmai and is only slightly less than the maximum for the Japanese </span>volcanic<span> </span>rock<span> series. The quantities of minor constituents present are not exceptional for the </span>rock<span> types analyzed; the </span>rocks<span> from </span>Adak<span> are apparently more strontium-rich than those from </span>Kanaga<span>. The chemical analyses of the more basic </span>rock<span> types, as compared with the average analysis of plateau basalt, suggest that the </span>Aleutian<span> parental magma could have been derived from a plateau basalt magma by the addition of plagioclase and the subtraction of pyroxene, iron ore, and some quartz. The distribution of the minor elements can be explained more easily by postulating that, at least on </span>Kanaga<span> </span>Island<span>, some sediments have been assimilated. The derivation of the analyzed </span>rocks<span> from the </span>Aleutian<span> parental magma is most easily explained by the hypothesis that the plagioclase remained </span>in<span> suspension while the ferromagnesian minerals were settling out. </span></p>
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10.1130/0016-7606(1952)63[485:MDITAQ]2.0.CO;2
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Geological Society of America
Magmatic differentiation in tertiary and quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska
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