Geochemistry, petrogenesis, and tectonic implications of central High Cascade mafic platform lavas

Petrographic and geochemical data allow the distinction of normal basalts (53 wt.% SiO (sub 2) ) from two types of basaltic andesites with associated high-silica basaltic andesites (53-62 wt.% SiO (sub 2) ) in the central High Cascades. All units yield fractionated REE and incompatible element-enriched patterns characteristic of continental tholeiitic magmas. The Mt Washington type basaltic andesites are distinguished from the North Sister ones by relatively higher total REE and other LIL element abundances. Based on trace-element signatures, separate magma sources are postulated for normal basalt and basaltic andesites with associated high-silica ones and models of their magma formation are proposed. Eruptions of near-primary basaltic andesite were catalysed by fluids which had been derived from a subducted slab. Basaltic andesites accompanied by locally evolved calc-alkaline silicic units, were interspersed with voluminous eruptions of basalt due to the juxtaposition of an extensional regime upon the older Cascade province.