T203 The Life and Times of Arc Volcanoes from Bottom to Top I

Wednesday, 13 October 2021
8:00 AM - 12:00 PM
A106

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

Paper number: T203-10
Program: Technical Sessions
Time: 10:10 AM – 10:25 AM

Authors

Abstract

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 + FeT] < ~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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-370122

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