COMPARING PLAGIOCLASE TEXTURES AND COMPOSITIONS IN LAVAS AND MAGMATIC INCLUSIONS FROM MOUNT HOOD, OREGON

The textures and compositions of plagioclase and other minerals in volcanic rocks from the Cascade Arc and other subduction zone settings provide important constraints on magmatic processes. Here we compare the textures and compositions of plagioclase in lava and different types of magmatic inclusions from a ~120 ka andesite lava flow from Mount Hood, Oregon. Mount Hood lavas are typically crystal rich, with plagioclase the dominant modal phase. Lavas also contain a variety of magmatic inclusions, including abundant finer-grained inclusions, and less common coarser grained plutonic textured inclusions. Plagioclase and other minerals provides a potential way to test hypotheses about the relationship between inclusions and lava samples that carry them.

Two types of plagioclase occur in Mount Hood andesite lavas: (i) large (typically >> 1 mm), equant and complexly zoned crystals, and (ii) crystals that are smaller (< 500 µm), simply zoned, and more elongate. The larger plagioclase have compositions consistent with growth in a more silicic magma reservoir, whereas the smaller and more elongate crystals have higher Mg and Fe consistent with a more mafic source. The juxtaposition of these two plagioclase populations occurs via mafic recharge into a silicic stored magma or crystal-rich mush to produce the erupted mixed andesites. From this we hypothesis that the plagioclase in the quenched mafic inclusion are predominantly from a mafic source and this is consistent with observed textures. We further hypothesize that plagioclase in plutonic textured inclusions derive from the silicic mush disrupted by mafic recharge. Further chemical and textural analyses will test these ideas.