Miller, D.M., 1995, Petrogenesis of adjacent calc-alkaline and tholeiitic volcanoes on Umnak Island, Aleutian Islands, Alaska: New York City, New York, Columbia University, Ph.D. dissertation, 455 p., illust., maps.
A comprehensive new petrographic, chemical, and isotopic data set representing samples from four field expeditions has been used to constrain the petrogenesis of Umnak's three major volcanoes. Okmok, on northeastern Umnak, and Vsevidof, the currently-active volcano on southwestern Umnak, exhibit tholeiitic differentiation trends, while the older Recheshnoi on southwestern Umnak exhibits a calc-alkaline trend. Lavas from the fault-bounded Idak plateau on the northeast flank of Okmok are also distinct from those of Okmok proper, with samples from Idak exhibiting a large range in Ce/Yb. On both ends of the island, there is a general evolution from highly variable compositions that range to high Ce/Yb, toward more constant compositions with lower Ce/Yb. Major and trace-element data show that Okmok and Recheshnoi parental magmas have distinct compositions, most likely resulting from different extents of melting in the mantle wedge. Modeling based on the rare-earth elements suggests approximately 7% melting for the calc-alkaline parent and 20% for the tholeiitic parent. Larger enrichments of fluid-mobile elements such as boron and cesium in the Okmok source suggest that variability in the volume of fluid flux from the subducted lithosphere may be responsible for the different extents of melting. Isotope and incompatible element ratios show that the magma sources for Okmok and Vsevidof are more homogeneous than for Recheshnoi, and suggest that both a fluid phase derived from the subducted crust in the absence of melting and a partial melt of subducted sediments are required to explain the overall enrichment of the mantle source of the Umnak lavas. The tholeiitic Okmok differentiation sequence can be produced by low-pressure crystallization of anhydrous minerals. However, the high alumina contents of aphyric Okmok basalts require suppression of plagioclase crystallization that likely results from partial hydration at pressures of up to approximately 2 kilobars. The calc-alkaline Recheshnoi lavas represent a complex history that likely includes multiple generations of primary melt, magma mixing, high-pressure in situ crystallization that includes amphibole as a major crystallizing phase, and lower-pressure crystallization of an anhydrous mineral assemblage.
Theses and Dissertations