PETROLOGY AND GEOCHEMISTRY OF OKMOK AND WRANGELL VOLCANOES, ALASKA

Okmok is a tholeiitic volcano in the central Aleutians. Two primary liquid compositions from Okmok are described, a picrite (14% MgO) and a magnesian basalt (11% MgO). The 1330(DEGREES)C calculated liquidus temperature for the picrite requires vigorous induced counterflow beneath the Aleutians and revision of estimates of the thermal structure of arcs. The inability of either crystal fractionation or melting of a homogeneous source to produce the observed variations in primary liquid compositions implies varying metasomatic enrichment of a depleted source. Based on the shapes of the REE patterns and isotopic compositions, the evolved rocks from the volcano are most closely related to the picrite. Magmas evolve to high-Ti ferrobasalts (2.3% TiO(,2), 13% FeO*), almost identical to some oceanic basalt suites, emphasizing that the low Ti contents of arc rocks is due to high D(,Ti) during fractionation. Okmok magmas have LILE enrichment and LILE/REE ratios typical of arc magmas.;As an example of volcanism in the Wrangell Mountains, Mt. Wrangell is described in detail. Volcanism is dominated by eruption, over short time intervals, of large batches of magma which are not directly related to previous batches. This eruption style is characterized by thick piles (up to 700 m) of extremely homogeneous flows and individual porphyritic acid andesite flows up to 50 km long.;In contrast to Okmok, primary magmas at Wrangell preclude production from a homogeneous mantle and require models involving variable degrees of source metasomatism. Pb, Sr and Nd isotopic compositions at Wrangell are similar to previously reported Aleutian values, and specifically preclude involvement of the continental crust in Wrangell magmagenesis.;The Wrangell volcanoes are a group of exceptionally large Pleistocene-Recent volcanoes (individual volcanoes up to 890 km('3)) in south-central Alaska which formed in response to subduction of a small sliver of oceanic crust ahead of an accreting microplate. The Wrangell eruption rate is the highest yet reported for non-rifting convergent margins. The high eruption rate is provisionally ascribed to compression during microplate accretion. This compression may force the rise of what otherwise would become deepseated intrusive bodies.