HYBRIDIZATION OF MAFIC AND SILICIC MELTS IN POST-CALDERA LAVA OF NEWBERRY VOLCANO, CENTRAL OREGON, USA

Newberry volcano is a large Quaternary basaltic shield volcano located behind the Cascade Arc near Bend, Oregon. Although largely composed of mafic lava, a summit caldera on the volcano formed during eruption of a zoned ignimbrite about 75,000 ybp, after which silicic magmatism continued in plinian eruptions that produced tephra falls, pumice cones and obsidian lava, most recently about 1,300 ybp. The youngest mafic lavas formed about 7,000 ybp during activity along the volcano’s northwest rift zone. Previous workers have documented the intimate relations between mafic and silicic magmas in the caldera wall of the volcano and within the 1,300-year-old Big Obsidian Flow and as partly fused rhyolitic or granitoid inclusions within several of the youngest mafic lavas.

Major elemental oxides and trace element systematics document extensive mixing of silicic magma within the post-caldera mafic to intermediate lavas of the volcano. Harker variation diagrams for these units (basalt to andesite) are markedly linear. Incompatible element ratio plots showing dramatic decreases of K/Rb and Ba/Rb versus Rb suggest as much as 40 wt.% mixing of silicic melt represented by analyses of the Big Obsidian Flow within northwest rift andesite. These elemental ratios should not be extensively changed during normal crystal fractionation of phenocysts (pc, px, FeTi oxides). Similar extensive mixing is also suggested by Sr and Ni plots as wells as rare earth and multi-element variation (“spider”) plots. The well-documented disparity between magmatic temperatures and viscosities of mafic and silicic melts argue against such extensive mixing. The Big Obsidian Flow, however, is nearly aphyric, suggesting that is was superheated and this may have facilitated hybridization of silicic with basaltic melt.

Newberry volcano is located at the intersection of the High Lava Plains trend and the Cascade arc. Abundant injection of High Lava Plains mafic magma into the crust may have led to crustal melting and generation of silicic magma, providing the opportunity for mixing of the two magma end members as the volcanic substructure heated up over time.