ALTERATION AND THE MAGMATIC HISTORY OF MT. RAINIER, WASHINGTON

The age and spatial distribution of alteration at Mt. Rainier reflect both the detailed magmatic history of the volcano and the longer-term geologic history of the region. Prior to the Osceola collapse event of 5.6 ka, alteration formed an ENE - WSW belt across the upper edifice, closely associated with similarly striking dikes and fractures. Recently emplaced radial dikes served as radiators, driving hydrothermal circulation in their vicinities, and perhaps providing magmatic sulfur. The largest, most abundant, and farthest reaching dikes intruded during two stages of greatly heightened magmatic activity 500 - 400 ka and 280 - 190 ka (K-Ar and ⁴⁰Ar/³⁹Ar) and much alteration on the WSW and ENE flanks probably formed during these times. The younger high-output stage waned gradually, with smaller dikes and alteration affecting rocks as young as 130 ka preserved on the upper E flank. The Osceola collapse removed altered rock from the volcano's summit, core, and upper ENE flank, and shed altered debris to the NE (Osceola lahar) and S (Paradise lahar). Geologic relations require that altered debris shed down the S flank from the former summit region was erupted and altered at < 40 ka, younger than the 280 - 190 ka high-output stage. Exploratory ⁴⁰Ar/³⁹Ar measurements of jarosite and alunite from the Paradise and Osceola lahars provide limiting alteration ages < 100 - 130 ka, supporting inferences of a young and probably active hydrothermal system limited to the upper edifice vent and conduit system at the time of the Osceola collapse. This active system was flanked to the ENE and WSW by older, dike-produced fossil alteration from the high-output stages. The consistent ENE - WSW to NE - SW strikes of dikes over the history of the volcano matches the strikes of abundant pre-Mt. Rainier joints in the Tertiary basement, that formed normal to the axes of large, NNW - SSE trending Tertiary folds. Pleistocene Mt. Rainier magmas exploited these preexisting weaknesses at shallow levels, leading to diking and alteration localized to the WSW and ENE sectors of the volcano. The N and S flanks of the volcano have not been cut by radial dikes, are largely unaltered, and are structurally relatively coherent.