PETROLOGY OF THE NORTHWEST RIFT ZONE, NEWBERRY VOLCANO, OREGON

The Northwest Rift Zone (NWRZ) constitutes the youngest mafic volcanism (~7ka) at Newberry Volcano. Over 100 vents and fissures erupted across ~35 km from near Bend, OR, to the high flanks south of Newberry’s summit caldera. The compositionally zoned eruption (50.7 to 58.7 wt% SiO₂) took place mostly from small spatter vents but includes 2 large cinder cones on the lower NW side of the volcano. The lavas and tephras from all vents are poorly porphyritic (10-20 vol%, vesicle free) and contain a typical mineral assemblage comprised of plagioclase (average 72%) + olivine (average 20%) + clinopyroxene (average 8%) ± spinel (<1%). New mineral data show that plagioclase compositions (rims and cores, combined) range from An30–92 with a main mode at An66. Normal and reverse zoning are observed, and larger phenocrysts sometimes exhibit minor sieve and dissolution textures. Clinopyroxene show normal and reverse zoning with compositions ranging from Mg#63–82. Olivine crystals range from Fo70–85 and dendritic textures at the apices provide evidence for rapid growth. The most An-rich compositions may reflect crystals disaggregated from sparse clots of olivine + clinopyroxene + plagioclase found in some lavas, whereas the lower An values may be related to rare partially melted granitic inclusions. The whole rock compositions indicate that eruptions in the NWRZ specifically tapped magmas that experienced varying degrees of evolution, whereas the overlapping mineral chemistry data suggests some similarities in the overall magma evolution within the shallow, crustal plumbing system. Interestingly, the wide range in Th/La vs Ba/Nb (0.05-0.4 vs 22.6-37.9) suggests multiple mantle sources. This indicates that the deeper plumbing systems for vents along the NWRZ are likely not connected completely along the rift.