GEOCHEMICAL AND PETROGRAPHIC ATTRIBUTES OF LAVAS ERUPTED AT SMALL VOLCANIC CENTERS IN NORTHERN OREGON; IMPLICATIONS FOR MAGMA FORMATION WITHIN AN INTRA-VOLCANIC ARC GRABEN

Quaternary volcanism in the northern Oregon Cascade Range is dominated by eruption of calc-alkaline andesite to rhyodacite lava flows and associated pyroclastic deposits at long-lived, subduction-driven composite volcanoes such as Mt. Hood. This study investigates small and more mafic Quaternary volcanoes in the Mt. Hood vicinity, including Clear Lake Butte (CLB), Pinhead Buttes (PHB), and Olallie Butte (OB). These volcanoes are located within an intra-volcanic arc graben, which developed approximately 7-8 Ma to the south and propagated northward to the Mt. Hood region (Smith et al, 1987). This study compares geochemical and petrographic characteristics of Quaternary lavas erupted at small volcanoes within the northern graben to those of primary and near-primary lavas mainly erupted within southern graben regions. The research objective is to identify geochemical and petrographic similarities and differences among intra-arc graben lavas in order to better understand conditions of magma formation at small volcanoes in the central to northern Oregon Cascade Range. Lavas at CLB, PHB, and OB are low to medium-K basalts and basaltic andesites (SiO2 = 45.9-57.3 wt%; K2O = 0.28-0.73 wt%), similar in composition to intra-graben primary and near-primary basalts interpreted to have formed via decompression-induced melting (Conrey et al., 1997). However, CLB, PHB, and OB lavas are characterized by notably lower ranges in MgO (4.0-9.0 wt%) and CaO (5.8-9.2 wt%), and higher ranges in Al2O3 (16.6-18.6 wt%) and Rb (21-44 ppm). CLB, PHB, and OB lavas exhibit LREE enrichment, but lack significant HFSE depletions. CLB, PHB, and OB basalts and basaltic andesites are petrographically similar to intra-graben primary and near-primary basalts containing abundant pyroxene and plagioclase phenocrysts, but are dissimilar in that olivine is not ubiquitously present. These data suggest that although CLB, PHB, and OB lavas share certain geochemical and petrographic characteristics with intra-graben primary and near-primary magmas attributed to decompression melting, subduction-related processes cannot be ruled out for magma formation at small northern graben volcanoes.