Granitoid compositional zoning by side-wall boundary layer differentiation: Evidence from the palisade crest intrusive suite, central Sierra Nevada, California

W. N. Sawka*, B. W. Chappell, R. W. Kistler

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    64 Citations (Scopus)

    Abstract

    Compositionally zoned plutons are an important feature of the Sierra Nevada batholith, California. Two such plutons have been examined to determine the mechanism by which crystals separate from a magma. The Tinemaha pluton shows continuous compositional variation from 58 to 67% SiO2, whereas the McMurry Meadows pluton is bimodal, with an outer margin of mafic granodiorite (59-60% SiO2) and an inner core of granite (66-69% SiO2). Extreme differentiates also occur as small isolated masses within the suite and may contain up to 76% SiO2. Both plutons are uniform in strontium isotopic composition but are different from each other, with initial 87Sr/86Sr values of 0·70719 and 0·70651 respectively. The Tinemaha pluton is both horizontally and vertically ({reversed tilde} 1000 m) zoned, with fractionation occurring both inward from the contacts and upward. The vertical trends in relative mineral proportions are not consistent with crystal settling. Both the vertical and horizontal variations in the chemical composition of 50 elements, in mineralogy, and in accessory mineral light rare-earth element zoning, are all directly relatable to side-wall crystallization which created a less-dense melt that buoyantly moved upward along the wall towards the top of the magma chamber. The different rates for diffusive heat exchange and compositional diffusion within the magma initiated the double-diffusive gradient in the magma chamber. Compositional variations in the side-wall crystal accumulation zone occur as boundary layer melts evolve, reflecting changes in the bulk convecting magma. The compositional gap in the McMurry Meadows pluton is the result of a similar but more efficient side-wall fractionation process, related to a higher proportion of melt to crystals in the initial magma and a slower rate of side-wall solidification as a result of the thermal blanket created by the enclosing Tinemaha pluton.

    Original languageEnglish
    Pages (from-to)519-553
    Number of pages35
    JournalJournal of Petrology
    Volume31
    Issue number3
    DOIs
    Publication statusPublished - Jun 1990

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