EVIDENCE OF LATE STAGE PHREATOMAGMATIC ERUPTION OF THE STRAWBERRY VOLCANICS

The mid-Miocene Strawberry Volcanics of northeastern Oregon erupted through several volcanic vent locations between ~16.2 to 12.5 Ma. Here we investigate the style, eruptive conditions and geochemical variations of a single mafic pyroclastic unit near one of the proposed vent sources. Field mapping indicates the pyroclastic flow reached ~69.5 m in height before transitioning to lava flows. This deposit shows a pattern of magmatic pulses from which larger bombs up to 90 cm size are deposited within a matrix of palgonite lapilli and ash. 12 episodic eruptions took place to form this pyroclastic deposit. Observed bombs sags and palagonite weathering suggest the eruption took place within the presence of water. Geochemical analysis of the pyroclastic deposit indicate that these erupted as tholeiitic basaltic andesites with ~55 wt%. SiO₂. The lavas capping the pyroclastic flow transition to calc-alkaline andesites with ~60 wt% SiO₂. Incompatible trace elements become progressively enriched from the bottom to the top of the pyroclastic deposit (e.g. Zr 165 – 213 ppm; La 27 – 35 ppm; Yb 2.1 – 2.9 ppm) possibly due to fractional crystallization.