This paper tentatively relates the comparative petrology and chemistry of three volcanic suites on Umnak and Bogoslof islands in the eastern part of the Aleutian Island arc to tectonic position with respect to the axis of the arc.

Umnak is a large geologically complex island on the Aleutian Ridge; Bogoslof, 22 miles north of Umnak, is the top of a Recent, largely submarine volcano that rises about 5000 feet from the floor of the Bering Sea basin. Southwestern Umnak is underlain by a pre-Quaternary basement of low-grade metamorphic rocks and post-Oligocene mediosilicic plutonic rocks, which are overlain by two Quaternary andesitic stratovolcanoes. Northeastern Umnak is underlain by a late Tertiary(?) and Quaternary basaltic shield volcano with a large central caldera. The lavas exposed at Bogoslof in 1947 consisted largely of historically dated volcanic domes ranging in composition from hornblende andesite to hornblende basalt.

Fractional crystallization was probably the dominant process in the formation of quantitatively minor andesite and rhyolite masses associated with the basaltic shield volcano of northeastern Umnak. An extensive blanket of welded andesitic agglomerate and subordinate rhyodacite ash associated with the caldera-forming eruption of the basaltic shield volcano probably owes its composition to a combination of processes, including fractional crystallization, mixing with remelted basalt wall rock, and possibly assimilation of sialic rock.

Most volcanic rocks of southwestern Umnak are similar in composition to the underlying plutonic rocks now exposed. The relatively high chromium and nickel in the southwestern Umnak quartz diorite and in its extrusive equivalent, the hypersthene-bearing labradorite andesite, indicate that these rocks probably did not originate from fractional crystallization of less siliceous rock. The presence of hypersthene in the andesites of southwestern Umnak is probably related to their high aluminum content ultimately derived by assimilation of aluminous sedimentary rocks. A similar origin is postulated for the hypersthene in the High Cascade lavas.

The slightly alkalic lavas of Bogoslof, typified by hornblende, became progressively less siliceous from 1796 to 1927, but the magmatic diversification in the underlying chamber may have been well advanced prior to 1796.

The petrochemical differences in the three volcanic suites of Umnak and Bogoslof are probably related to tectonic position with respect to the Aleutian Ridge. The hypersthenic aluminum-rich lavas of southwestern Umnak were probably generated at relatively shallow depth from pre-existing plutonic rocks of similar composition. The position of the andesitic volcanoes on a high-standing, possibly upwarped erosion surface is in accord with Benioff's proposal that the maximum heat would be generated in the crust at the zone of maximum bending across a volcanic arc. The dominantly tensional features associated with the basaltic shield volcano on northeastern Umnak are in accord with its position on the north flank or inner side of the arc, where a zone of tension may prevail. The slightly alkalic rocks of Bogoslof Island are derived possibly from a tension fracture normal to the arc in the Bering Sea basin.

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