Okmok and Recheshnoi are adjacent volcanoes on the island of Umnak in the Aleutian Arc. Ninety-five new chemical analyses of lavas from the two volcanoes show that Okmok exhibits a classical tholeiitic and Recheshnoi a calc-alkaline differentiation trend. Both volcanoes have erupted lavas that range in composition from basalt to rhyolite. This allows investigation of differences in both the differentiation systematics and the parental magma compositions. In contrast to the predictions of many recent models for calc-alkaline and tholeiitic volcanism, the major and trace element data show that the parental magmas for the two volcanoes have different compositions. These different parental compositions might themselves be produced by in situ differentiation or other complex fractionation processes from a very magnesian parental magma (16% MgO) with Okmok being derived by low-pressure fractionation and Recheshnoi by in situ fractionation at higher pressures. An alternative and simpler explanation is that the inferred high-MgO Okmok parent and the Recheshnoi parent are derived by different extents of melting in the mantle wedge. Modelling based on the rare earth elements suggests approximately 7% melting for the calc-alkaline parent and 20% for the tholeiitic parent. For these two volcanoes therefore, there may be a correlation between extent of melting and the tendency to follow a calc-alkaline or tholeiitic differentiation trend. Larger enrichments of highly fluid-mobile elements such as boron and cesium in the tholeiitic Okmok source suggest that variability in the volume of fluid flux from the slab may be responsible for the different extents of melting. If the partial melting model is applied generally to the Aleutian Arc, it provides an alternative explanation for the volcanic regularities of the arc described by Kay et al. (1982). Smaller extents of melting lead to less melt, fractionation at higher pressures, often including hydrous phases, a preponderance of plutonic rather than volcanic rocks, and smaller calc-alkaline volcanoes; Large tholeiitic volcanoes are often associated with fracture zones on the subducting Pacific plate (Kay et al., 1982; Marsh, 1982). Since fracture zones are more extensively altered than average oceanic crust and might also serve as sediment traps, they could serve as sources of a larger volatile flux from the slab, leading to greater extents of melting and large, tholeiitic volcanoes. If this explanation is correct, then the origin of the volcanic segmentation of the arc may be found within the subducted slab. This contrasts with the alternative model of control by the stress regime of the overlying plate (Kay et al., 1982; Singer and Myers, 1990; Kay and Kay, 1991). Inferences from Okmok and Recheshnoi may also apply to global variations in convergent margin chemistry. In general, arcs built on thick crust tend to be more calc-alkaline in character. On the basis of the negative correlation between convergent margin crustal thickness and inferred extent of melting (Plank and Langmuir, 1988), lower extents of melting may contribute to a tendency towards calc-alkaline differentiation on a global basis.
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