A Tale of Two Plutons: Petrographic and Mineralogic Constraints on the Petrogenesis of the Red Lake and Eagle Peak Plutons, Central Sierra Nevada, California

Abstract

The Red Lake and Eagle Peak plutons are similar in age (K-Ar: 87-89 m.y.) and range in composition from granodiorite to granite. The Red Lake pluton is equigranular, has a locally greisened marginal zone, and shows only minor mineralogical and chemical zoning, whereas the Eagle Peak pluton is mineralogically, compositionally, and texturally zoned, with an equigranular margin and a porphyritic core. Modal and normative trends within each pluton are consistent with separation of plagioclase and mafic minerals leading to a residual liquid enriched in quartz and alkali feldspar. The formation of the prominent alkali feldspar megacrysts in the Eagle Peak core was promoted by plagioclase separation, $$H_{2}O$$ undersaturation, and a slower cooling rate relative to the Red Lake magma. Early loss of volatiles and a resulting increase in magma (melt + crystals) viscosity inhibited crystal segregation within the Red Lake pluton and prevented the development of zoning. Late stage aplites associated with both plutons formed from residual liquids created by crystal fractionation. Biotite and amphibole equilibria suggest internally buffered oxygen fugac-ity through most of the crystallization of the plutons. Comparison of mineralogic and whole rock compositional data with experimental studies on natural and synthetic sytems suggests a minimum intrusive temperature of 750°C and a load pressure of $$1.0 \pm 0.5 kb$$. Late stage hydrothermal alteration within each pluton reequilibrated feldspar compositions at subsolidus conditions. The presence of homogeneous cores in the plagioclase, early titanite crystallization in the Red Lake magma, and irregular hornblende compositions suggest refractory material was present when the magmas were intruded. Inferred source regions for the two plutons are amphibolite for the Red Lake magma and a more biotite-rich amphibolite for the Eagle Peak magma.