Extensive hybridization of mafic and silicic magmas at the confluence of the Cascade Arc and High Lava Plains: The Newberry Volcano of Central Oregon

Abstract

Newberry volcano lies at the intersection of the north-south trending Cascade Arc with the margin of the High Lava Plains trend. Newberry is unusual as a Cascade volcano in that it is dominantly basaltic and geochemically bimodal between basalt and silicic rocks. The youngest calc-alkaline mafic to intermediate lavas of Newberry Volcano have major and trace element characteristics consistent with up to 35–40 wt% mixing and hybridization of an evolved tholeiitic basalt with silicic magmas similar to that present as domes and flows within the caldera of the volcano. This unusual extensive hybridization is the combined result of voluminous influx of basaltic magma into the crust, fractionation within subvolcanic chambers, anatexsis of crustal rocks to produce silicic magmas, superheating of these silicic magmas, and complex interaction of mafic and silicic magma within the substructure of the volcano and especially within a dike swarm underlying the northwest rift of the volcano.The most primitive basaltic rocks of the volcano include both tholeiitic and calc-alkaline varieties derived from melting of variously subduction-influenced lithospheric mantle at depth. The evolved tholeiitic end member of mixing within the Northwest Rift was produced by fractionation of a primitive tholeiitic parent. This evolved tholeiite mixed substantially with at least two silicic magmas to generate the calc-alkaline intermediate rocks of the Northwest Rift of the volcano.