PETROLOGY AND GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE BASEMENT BENEATH MOUNT HOOD, CASCADE RANGE, OR

The Cascade Range is dominated by stratovolcanoes that erupt intermediate magma compositions. The general chemistry and mineralogy of these volcanics is generally well described, but significant knowledge gaps remain concerning the processes that produce the erupted magmas and whether these vary through the lifetime of individual volcanoes. Here we report results from petrographic and geochemical study of samples that predate the current Mount Hood edifice, ranging in age from ~500-1200 ka, relative to the ~450 ka age of the current Mount Hood edifice. These older rocks are chemically and mineralogically similar to Mount Hood lavas, and are considered to represent the eroded remains of earlier stratovolcanoes. However, it remains unknown if the same set of petrologic processes that drive compositional variations and eruptions at Mount Hood also occurred in the earlier eruptions.

Geochemical data indicates these older volcanics are similar to modern Mount Hood, and were also produced by quasi-binary mixing of mafic magma recharge within a more evolved magma reservoir. Petrographic studies show spongy dissolution zones and high anorthite rims are common on plagioclase phenocrysts from both older and modern Mount Hood samples, and together with Mg, Fe and Ti variations are consistent with mafic recharge being a common process. There are also variations – in particular crystals with high resorbed anorthite rich cores – interpreted to reflect plagioclase derived from a mafic source. Our interpretation is that magma recharge and magma mixing plays a key part in the eruptions and formation of andesitic complexes, such as Mount Hood, but that in detail petrological evolution may vary over time.