MAFIC INCLUSIONS IN MOUNT HOOD LAVAS: LINKS TO PARENTAL MAGMAS OF A LONG-LIVED ANDESITIC VOLCANO

Mount Hood is a 500 ka composite volcano located in the Cascade Range of north central Oregon. The volcano is dominantly calc-alkaline andesite with SiO₂ ranging 57-63 wt %.  Several flank vents on Mt Hood have erupted basaltic andesite (53-57 wt % SiO₂) during the same time span. The andesitic lava flows contain inclusions ranging from ~2 cm to ~15 cm in diameter that are more mafic in composition than host lavas. The basaltic andesites flank lavas do not contain inclusions.  Inclusions and host lavas of samples collected (ages 200 ka to 200 yr BP) have 53-59 wt% SiO₂ and 59-62 wt% SiO₂ respectively.

The host lavas and inclusions are porphyritic and contain 15-20% and <10% phenocrysts respectively. In both, two principal phenocryst assemblages can be distinguished based on the cpx/amp ratio.  If the ratio is >1, the phenocryst assemblage is 65% plag; 25% opx; 10% cpx; <1% amp; whereas if the ratio is <1 then it is 75% plag; 12% opx; 9% amp; 1-2% cpx.  In addition to differences in phenocryst contents, groundmass textures differ between the inclusions and host lavas. The groundmass of the host lavas contain >20% glass whereas the inclusions contain <20% glass and are coarser grained.  The groundmass has the same crystal phases as the phenocrysts and there is a close correspondence between inclusions and host lavas as to whether mineral assemblage is amp or cpx rich.  In contrast, the majority of basaltic andesite flank lavas range from aphyric to porphyritic (~10% phenocrysts) with phenocryst phases of plag, opx, cpx, ± amp and ± olivine, and the groundmass of most samples contain >20% glass.

Bulk chemical data show a wider variation in incompatible trace elements in the inclusions than in the host lavas.  For example, inclusion compositions (in ppm) are: Nb (~5-15); Sr (~500-1600); Rb (~7-22); Zr (~100-220); whereas host compositions (in ppm) are: Nb (~8-12); Sr (~500-800); Rb (~16-26); Zr (~150-200).  The range in inclusion compositions are within the range seen in nearby primitive basalt compositions (Conrey, et al., 1997).  These preliminary results suggest that the wide range in some elements in the inclusions are the signatures of variably depleted to enriched parental magmas to Mount Hood andesite.  The mineralogy and geochemical data of host lavas and inclusions imply that magma mixing is dominating compositional variations.