MAGMA SHEETS AND OTHER CONTRIBUTIONS TO THE QUATERNARY PLUTON UNDER CRATER LAKE CALDERA, OREGON

The timing, composition, and aspects of the geometry of contributions to a >25 km² shallow pluton can be inferred from magmas and lithic blocks ejected by Mount Mazama volcano. The ~7.7-ka caldera-forming climactic eruption vented ~50 km³ of chiefly homogeneous crystal-poor rhyodacite magma generated mainly by crystallization differentiation of basaltic to andesitic sills repeatedly ponded between cumulate mush and overlying derivative silicic magma. Rapid partial crystallization of sills yielded rhyodacitic melt that escaped upward, perhaps by gas-driven filter pressing, into the convecting silicic magma chamber. This vigorous activity, concurrent with regional eruptions of primitive basalt, followed ~400 kyr of intermediate volcanism atop an older silicic lava field. In contrast, granodiorite lithics are samples of crystal-laden intermediate magmas that froze during times of declining thermal input. SHRIMP U–Th model ages for zircons from granodiorites concentrate near 50–70, ~110, and ~200 ka and correspond to periods of dacitic volcanism dated by K–Ar. Multi-modal age spectra for individual blocks are interpreted as due to recycling of antecrysts from earlier intrusions and late zircon crystallization in high-crystallinity stalled magmas. Variation in final crystallization ages of blocks from around the caldera suggests a composite pluton underlies Mount Mazama. Although granodiorite blocks in the climactic ejecta are 0–50% fused, showing that the climactic magma chamber was melting its walls, climactic rhyodacite magma was far from zircon saturation and dissolved virtually all zircon from any assimilated rock. Postcaldera volcanism produced 98% andesite and 2% derivative rhyodacite, as though the climactic eruption exhausted, or possibly pressure quenched, the crystal-poor magma at that time. Were it exposed, the composite pluton likely would consist of irregular gabbro to granodiorite intrusions, probably with discontinuous or faint contacts, intruded by a stack of horizontal magma sheets formed of a cumulate pile laced with sills, the upper sheets commingled with minor granite, and all affected by foundering of the roof block. Zircons would record a >300 kyr history of freeze and thaw cycles in a modest-sized pluton.