Petrology of Quaternary volcanics of the Broken Top and Diamond Peak areas, central and south-central Oregon High Cascades: Evidence for varied magmatic processes from two contrasting volcanic centers

Quaternary volcanism in the Broken Top (BT) area of the central Oregon High Cascades ranges from basalt to rhyodacite. Most non-BT lavas were generated through low pressure crystal fractionation (CF) in small, short-lived shallow (ca. 2.5-3 km deep) chambers. Lavas erupted from BT are dominantly basaltic andesite (53.5-58 wt.% SiO;Andesitic volcanism (since 0.5 Ma) dominated the Diamond Peak Wilderness (DPW) area of the south-central Oregon High Cascades. Early stage basaltic andesites (0.5-0.1 Ma) erupted at Mt. Yoran (55.5-56.9 wt.% SiO;Significant differences between magmatic processes operating at BT and DPW are related to local structure and tectonics. Extension and the related graben structure in the BT area promoted development of shallow magma chambers where low pressure processes operated. Milder extension in the DPW area allowed significant underplating of basalt, leading to lower crustal melting and mixing. However, extension was significant enough that resulting magmas reached the surface without any significant low pressure overprint.