FIELD, PETROLOGIC AND GEOCHEMICAL EVIDENCE FOR UPPER-CRUSTAL ULTRAMAFIC CUMULATE FRACTIONATION TO GENERATE INTERMEDIATE PLUTONS AT EMIGRANT GAP, SIERRA NEVADA, CALIFORNIA

The Jurassic Emigrant Gap intrusive complex of the Sierra Nevada batholith (N. California) comprises ultramafic to granodioritic rocks that represent the crystallization products of near-primitive low to medium-K basalts or basaltic andesites (Mg# as 100*Mg/[Mg + Fe_T] < ~69) at upper-crustal depths. Cumulates include dunite, wehrlite, pyroxenite and gabbros (olivine-gabbronorite, gabbro, amphibole gabbro). Fine-grained gabbronorites and gabbrodiorites have melt-like compositions. Intermediate rocks include two-pyroxene diorite and extensive granodiorite; they have sharp contacts with mafic-ultramafic rocks, brecciating the latter. Dunites and wehrlites through gabbronorites have gradational mutual contacts, crosscut each other as dikes (which possibly represent replacement reactions where ultramafic).

We report whole-rock geochemistry of 60 samples, mineral major and trace element chemistry, petrography, and U-Pb zircon geochronology in the context of field observations. Rock compositions range from 40–41% SiO₂/Mg# 83–88/ASI=0.1–0.4 in dunite to 61–64% SiO₂/Mg# 46–55/ASI=0.7–0.9 in diorite-granodiorite. Trace element compositions calculated for melt in equilibrium with clinopyroxenes in cumulates match whole-rock analyses of gabbronorite through granodiorite samples, suggesting that they represent cumulate-melt complements. Crystallization ages for three intermediate-felsic samples (concordant U-Pb zircon weighted mean ages by LA-ICPMS) are ~4–9 Ma younger than published ages from bulk zircon separates. A 2-pyroxene diorite yields 160.4 ±1.8 Ma, overlapping in age with a diorite associated with the granodiorite (158.5 ±1.5 Ma). Last, a coarse amphibole gabbro (with two whole rock analyses, 42–46% SiO₂/Mg# 52–69/ASI=0.7–0.75) that brecciates pyroxenite also overlaps in age with the granodiorite with an age of 156.9 ±3.0 Ma. We show how shallow crystallization-differentiation starting from primitive magma can generate granodioritic compositions, with intrusive context.