DEVELOPMENT OF A VERTICALLY-STRATIFIED DACITE-RHYOLITE MAGMATIC SYSTEM WITHIN THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

Some ash-flow tuffs that preserve dacite-rhyolite compositional transitions are interpreted as the erupted contents of stratified reservoirs in which rhyolitic magmas overlie the dacitic “mushes” from which they segregated. The youngest member of the Mount Whitney Intrusive Suite (MWIS) may preserve a plutonic record of such a compositionally-stratified system.

The MWIS was emplaced between 90 and 83 Ma near the eastern edge of the Sierra Nevada batholith and its youngest member, the Whitney pluton, intrudes both older members and wall rocks warmed by them. The pluton grades from marginal granodiorite at low elevations to granite in its upper central part, and mass balance calculations indicate the granite can derived from the granodiorite by modest fractionation of hb + pl + mt. Alkali-feldspar megacrysts comprise 6-11% of the body and are coarser towards its center. Evidence of repeated resorption and growth of Ba-rich zones in the megacrysts indicates open-system behavior during which influxes of hotter magma are inferred to have selectively dissolved small alkali-feldspar crystals and led to coarsening. The inward coarsening of megacrysts suggests that the longest history of recharge occurred near the center of the Whitney pluton where it would have promoted differentiation by episodically raising temperature and lowering crystallinity.

Field relations, chemistry, and megacryst textures suggest that relatively slow cooling at the center of the suite held the lower dacitic part of the Whitney pluton above its rigid percolation threshold long enough (10⁴-10⁵ years) for melt to segregate and form the overlying granite. A coupled thermal-deformational model of the MWIS agrees with similar models in predicting that intrusion at an average rate greater than 10^-3 m³/m²/yr would have been required to sustain a differentiable reservoir at the center of the Whitney pluton.