Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California

• Published in: Journal of petrology 1988-10, Vol.29 (5), p.965-995
• Main Authors: BEARD, JAMES S., DAY, HOWARD W.
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/29.5.965

Gabbroic plutons are part of the intrusive substructure of the Smartville Complex, a late Jurassic, rifted, ensimatic arc located in the northern Sierra Nevada of California. The plutons range from unzoned, equant bodies of olivine gabbro less than 1 km in diameter to elongate intrusions up to 25 km in length that are reversly zoned from olivine gabbro cores to quartz diorite rims. The felsic rocks dip inward beneath the mafic core, indicating that this zoning reversal continues to depth. The zoned plutons have relatively shallow keels. We interpret the reversed zoning as an emplacement feature, analogous to the compositional zoning in a zoned tephra sheet. It formed as a result of tapping an already zoned, deeper level magma chamber. Whether the original zoning of the magma was concentric or stratiform cannot be readily deduced. During emplacement, considerable amounts of cumulate rocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutons indicate that they contain two suites of rocks: a cumulate suite represented by olivine gabbro and olivine clinopyroxenite and a differentiated suite of non-cumulate olivine gabbros, gabbronorites, and diorites that lie along a compositional continuum and approximate liquid compositions. Plagioclase and olivine compositions in the Smartville Complex cumulate suite are identical to those in modern arc cumulates and are characteristic of the arc cumulate suite. The differentiated rocks form a compositionally continuous series that is geochemically very similar to a differentiated suite of arc tholeiitic basalts and andesites. Fractionation modeling indicates that removal of mineral phases found in the cumulate gabbros from the mafic members of the differentiated suite can produce the lithologic variation seen in the zoned plutons. Plutons such as those in the Smartville Complex indicate that there is a genetic link between cumulate rocks and a basalt-andesite fractionation trend in arcs, supporting the hypothesis that arc andesites form by crystal fractionation. The gabbroic plutons and related Alaska-type ultramafic complexes contain ultramafic cumulates that can rectify the discrepancy between the cumulate mode predicted by fractionation models and the observed mode of gabbroic cumulates in arcs.