REE-ISOTOPE STUDY OF MAFIC TO FELSIC DIFFERENTIATION, STOKES MOUNTAIN REGION, WESTERN SIERRA NEVADA BATHOLITH, CA

The Early Cretaceous Stokes Mountain plutonic suite includes layered troctolite cumulates, various hornblende and enstatite-rich gabbros, tonalites, granodiorites and lesser granites (SiO₂ = 41-79 wt. %). This lithologically diverse suite was emplaced west of the 0.706 line during a low-flux period immediately proceeding the mid-to Late Cretaceous magmatic flare-up within the Sierra Nevada arc. Oxygen-strontium-neodymium isotopic data demonstrate that the suite is dominated by a depleted mantle-derived magma that differentiated, in part, by assimilation of continental source(s) coupled with fractional crystallization. Field observations and new rare earth element data are consistent with fractional crystallization producing magmas having SiO₂ contents up to 62 wt. %. In contrast, samples having approximately 62 to 68 wt. % SiO₂ are interpreted as resulting from magma mixing between the products of assimilation-fractional crystallization and variably LREE-enriched crustal melts. In addition, comparison of trace element data from three neighboring ring-dike complexes supports the presence of small-scale heterogeneities in the mantle source region and the existence of adjacent, approximately coeval, plutonic systems that were geochemically distinct.