A tale of two plutons: petrographic and mineralogic constraints on the petrogenesis of the Red Lake and Eagle Peak plutons, central Sierra Nevada, California

The two 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 presence of homogeneous cores in the plagioclase, early titanite crystallization in the Red Lake magma and irregular hornblende compositions suggest that 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.