Deep Mafic Roots to Arc Volcanoes: Mafic Recharge and Differentiation of Basaltic Andesite at North Sister Volcano, Oregon Cascades

The deep crustal magmatic history of arc volcanoes is obscured by diversity in mantle inputs, modest isotopic contrast between magma and wall-rock, and overprinting processes in the middle and upper crust. To identify and quantify processes in the deep arc crust, we investigated the evolution of the mafic composite North Sister Volcano, the oldest and most mafic of the Three Sisters Volcanic Field of the central Oregon Cascade arc. Here, intra-arc extension limits the degree of magma interaction with the mid- to upper crust and the range in primitive magmas delivered from the mantle is known. North Sister Volcano has produced low-K basaltic andesitic magmas (0·5^0· 8w t % K2O) for � 400 kyr during four central-vent eruptive stages and along the late, 11km long Matthieu Lakes Fissure. Although restricted in bulk composition (53^55 wt %S iO 2), North Sister basaltic andesites from different stages cluster into elemental and isotopic groups. Over time, North Sister basaltic andesites generally have decreasing compatible elements, such as Ni (from 112 to 40 ppm), and increasing Al2O3 and TiO2. Concurrently, incompatible elements remain the same or decrease (e.g. from 302 to 247 ppm Ba). Isotopic variations at North Sister are small, but systematically progress toward more mantle-like ratios with time; 87 Sr/ 86 Sr decreases (from 0·70369 to 0·70356), and 144 Nd/ 143 Nd increases (from 0·51285 to 0·51292). We present a multi-stage petrological model for the evolution of North Sister magmas to account for: (1) the generation of low-K basaltic andesite; (2) geochemical variations within the eruptive stages; (3) evolution of the magma system over time to more mantle-like compositions. The earliest and most isotopically ‘crust-like’ (highest 87 Sr/ 86 Sr and lowest 143 Nd/ 144 Nd) North Sister magma is consistent with two-component mixing of regionally typical mantle-derived, low-K tholeiites with partial melts of the crust. Crustal melts must be high in SiO2 and Al2O3, and most probably result from low-degree melting of plagioclase^clinopyroxene amphibole-bearing gabbro at high pressure. Variations in highly compatible elements within compositional groups (e.g. 60 ppm Ni within a single group) reflect fractionation of plagioclase, olivine, and clinopyroxene and recharge by more primitive basaltic andesite that overprint longer-term variations between groups. To understand the evolution of the North Sister basaltic andesite magmas through time, we use an energy-constrained model that balances assimilation of refractory gabbroic wall-rocks and abundant recharge by mantle-derived low-K tholeiites. These complementary processes allow Sr and Nd isotopic ratios to become more like those of the regional basalts while maintaining high Ni concentrations. Low-K basaltic andesites like those of North Sister Volcano are found along the Oregon Cascade arc and they imply that low-K tholeiitic magmas interact with a refractory mafic underplate along its length. Dominantly basaltic andesite volcanoes are common in arcs and provide insight into the extensive, albeit compositionally cryptic mafic underplating and intraplating that affects arc crust.