ALONG-ARC GEOCHEMICAL VARIATIONS OF CASCADES MAFIC MAGMAS: A ROLE FOR SUBDUCTED SEDIMENT IN THE SOUTHERN ARC?

Mafic magmas erupted in the Cascade arc should be relatively water-poor, due to the young, hot slab being subducted along Cascadia. However, researchers have shown that some mafic magmas erupted in the arc are water-rich, including some of the highest melt H₂O contents yet measured (Mt. Shasta). The mafic magmas also exhibit a wide range of compositions erupted in close proximity: calc-alkaline basalts (CAB), low-K tholeiites (LKT) and OIB. We have compiled published geochemical data on mafic magmas (MgO>7 wt%; both CAB and LKT) erupted throughout the Cascade arc to investigate along-arc trends in major and trace elements and isotope ratios (Sr-Nd-O), in order to illuminate variations in the origins of these magmas and the contributions of a subduction component/s along the arc. These data, combined with new analyses of mafic magmas (from the Lassen area of the southern arc and from the north Cascades) and from sediments overlying the Gorda plate, point to an increased subducted sediment component in mafic magmas erupted in the southern Cascades.

Our dataset illustrates that tracers of fluid addition (Pb/Ce, Ba/La, Ba/Nb, Sr/La) in the mafic magmas show a large range, but also the highest values, at the southern end of the arc. Interestingly, the concentrations of these ratios overlap for CAB and LKT magmas suggesting that the mantle source for both magma types received some subduction-related component. The ratio Th/La, an indicator of subducted sediment involvement, is highest in southern Cascades magmas. ⁸⁷Sr/⁸⁶Sr values increase from north to south, whereas ¹⁴⁴Nd/¹⁴³Nd decrease toward the south and δ¹⁸O data are variable but highest in the southern arc (up to +5.8‰ in Lassen area olivine and +6.2‰ in Mt. Shasta). Together, these data can be explained by an increased contribution of subducted sediment to the mantle in the southern portion of the arc. To test this theory, we are analyzing the geochemistry of sediment from the Gorda plate, offshore of the southern arc. Preliminary results show these sediments have δ¹⁸O values of +13-18‰ that could easily explain the elevated values in some southern arc magmas. With further research, the contribution of subducted sediment to the southern Cascades magmas can be better understood.