Abstract: In the Wrangell Volcanic Belt (WVB) a northwesterly increase in volume of calc-alkaline versus transitional (sodic alkaline/calc-alkaline) magmatism is accompanied by a migration in the locus of magmatic activity. The space-time-composition relationships reflect oblique convergence between the North American and Pacific plates over the last 17.3 million years. Compositional- temporal trends are particularly well preserved in the four stages of volcanic stratigraphy in the St. Clare Creek field (17.3-6.5 Ma). Initially, alkaline olivine basalts, hawaiites and mugearites were erupted from small, isolated shield volcanoes in the axis of a continental molasse basin. The alkaline lavas were followed by an early stratovolcano stage of transitional trachybasalts and high-Fe basaltic trachyandesites, succeeded by basaltic trachyandesites, trachyandesites, trachytes, rhyolites and rare basaltic andesites. Widespread basaltic fissure eruptions dominated the third volcanic stage. The late stratovolcano stage consisted of renewed eruption of intermediate and felsic transitional lavas. A systematic temporal-chemical relationship between early alkaline and younger transitional and calc-alkaline lavas in the St. Clare Group is illustrated by a decrease in FeO/MgO, Na+K/Si, NB/Zr/Y, and an increase in Rb/Zr with increasing stratigraphic levels. Primitive basalts are non-primary and show variable degrees of fractionation between large ion lithophile (LILE) and high field strength element abundances. A model is proposed in which the alkaline shield volcano and early stratovolcano stage magmas formed by progressive melting of a rising mantle diapir in response to local extension along the Duke River fault. Early Fe-rich magmas may have undergone clinopyroxene fractionation at high pressures, but most magmas appear to have differentiated in the near surface environment via fractional crystallization and local magma mixing. With the onset of Yakutat subduction, progressively larger amounts of slab-derived, LILE-enriched fluids metasomatised overlying peridotite, which in turn melted to form primitive, late-stratovolcano stage magmas.

Authors: Skulski, T.

Citation: Skulski, T., 1988. The Origin and Setting of Anomalous Arc Magmatism in the Wrangell Volcanic Belt, Southwest Yukon. In: Yukon Geology Volume 2, J.G. Abbott (ed.), Exploration & Geological Services Division, Indian & Northern Affairs Canada, p. 88-98.

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