PETROGENESIS, AGE, AND HYDROTHERMAL ALTERATION OF THE MIOCENE TATOOSH INTRUSIVE SUITE, MOUNT RAINIER NATIONAL PARK

The Miocene Tatoosh Intrusive Suite (TTIS) in Mount Rainier National Park, south of Mount Rainier, is composed of four granodioritic plutons associated with the ancestral Cascades arc. The intrusive suite is reversely zoned in that the oldest and youngest plutons are most silicic and least silicic, respectively. Each pluton has diagnostic textural, mineralogic, and geochemical features and lacks internal contacts. At the time of emplacement and solidification, each of these magma bodies was largely homogeneous in bulk composition and petrographic features. Crystal-rich magma that formed the plutons accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth (≤1-2 km). Internal geochemical variation within each pluton is inconsistent with two-component magma mixing. Rather, their geochemical and petrographic variation (including variably developed cumulate textures) is most consistent with differential residual melt loss, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma.

New ion-microprobe U–Pb zircon data indicate crystallization ~19.2 Ma (Stevens pluton), ~18.5 Ma (Pyramid and Reflection plutons), and ~17.5 Ma (Nisqually pluton). The Stevens pluton includes rare, statistically distinct ~20.1 Ma zircon antecrysts. Source(s) for the intrusive suite were active for at least ~2.6 m.y. and major crystallization episodes were separated by intervals of 0.7–1.0 m.y. during which each newly emplaced pluton solidified and was cut by sparse hydrothermal veins.

Although most TTIS rocks are affected by weak hydrothermal alteration, significant mineralization is absent. Alteration is in accord with late H₂O-rich volatile saturation, low magmatic sulfur contents and (or) relatively reducing conditions, preferential partitioning of base metals into magma rather than hydrothermal fluid, and lack of closely-spaced fractures. These features precluded development of strong hydrothermal systems associated with porphyry copper deposits like those associated with other Miocene plutons in the ancestral Cascades arc.

The TTIS was assembled by episodic intrusion at shallow depth and may be representative of mid-sized plutonic complexes beneath arc volcanoes that produce moderate-volume silicic ignimbrites and dome fields.