PETROLOGY OF ANDESITE BODIES IN THE PUGET LOWLAND: OLIGOCENE MAGMATISM WEST OF THE MAIN CASCADE ARC

Scattered throughout the Puget Lowlands are small andesite bodies that represent Oligocene-Miocene magmatic activity about 5 miles west of the main Cascade arc and 15 miles west of Mt. Rainier. This study focuses on two such bodies that are exposed as small hills about 2 kilometers apart rising from mudflow and glacial deposits of the Puyallup River valley near Enumclaw, WA. Previous workers have interpreted these bodies either as hypabyssal intrusions (stocks or sills) or as the remnant cores of Fife’s Peak lava flows (Frizzell, 1984). The two main goals of the present study are: (1) to determine the mechanism of emplacement of these bodies, and (2) to compare the geochemistry of these rocks with analyses of other Cascade lavas. Both of the areas chosen for study contain quarries that provide excellent sections through the rock bodies. The exposed thicknesses of these andesite bodies are 340 feet at Enumclaw Quarry and 365 feet at Veazy Quarry, but neither the top nor the base of either body is visible. Both bodies exhibit columnar jointing from bottom to top of the exposed sections, and the columns have an average diameter of about 1 meter. The columns at Veazy Quarry are near vertical, whereas those at Enumclaw Quarry dip ~65 degrees to the west. Petrographic analysis of samples from Veazy Quarry reveals that the rock is a porphyritic 2- pyroxene andesite with phenocrysts of plagioclase (An 56 – An 54), orthopyroxene, and clinopyroxene. Brown glass is present in the intersticies. Results to date reveal no petrographic differences between samples from different stratigraphic positions in the body. Ongoing work, including petrography and XRF analysis, will reveal whether there are petrologic differences between these two bodies and how they compare with Fife’s Peak lavas.