Volume 38: January - December 1997

Issue 4: April 1997

Abstract

Ultra high pressure-temperature melting experiments on an SiO2 -rich lamproite from Smoky Butte, Montana: derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle

  • Ultra high pressure-temperature melting experiments on an SiO2 -rich lamproite from Smoky Butte, Montana: derivation of siliceous lamproite magmas from enriched sources deep in the continental mantle

    • AD. Edgar and RH. Mitchell2 1Department of Earth Sciences University of Western Ontario, London, Ontario, Canada N6A 5B7 and 2Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 531Corresponding author

    ABSTRACT

    Liquidus to near-solidus phase relations between 0.8 ad 12.0 GPa and 900 and 1200oC were determined for an SiO2-rich lamproite from Smoky Butte, Montana. At 7-10 GPa, assemblages between liquidus and solidus are clinopyroxene+garnet, garnet+clinopyroxene+coesite, garnet+clinopyroxene+coesite+rutile+K-Ti silicate, and garnet+clinopyroxene+K-Ti silicate+phlogopite+coesite+rutile. At 12 GPa, comparable assemblages are garnet, garnet+clinopyroxene+coesite, and garnet+clinopyroxene+K silicate+coesite+rutile. At 6-12 GPa, K-Ti silicates and K-Ba phosphate compounds occur between 1200 and 1400oC. At 12 GPa, K silicate replaces the K-Ti silicate compound present at lower pressures. At >6 GPa, clinopyroxene and garnet incorporate K in their structures, particularly at lower temperatures. Near-liquidus phase relations of the Smoky Butte lamproite are compared with those of previously investigated lamproite of a similar composition. It is considered unlikely that near-liquidus garnetite mineral assemblages at 7-12 GPa could reflect the source of these magmas. A more likely source for these SiO2-rich lamproitic magmas is an assemblage comparable with the near-solidus assemblage of garnet+clinopyroxene+phlogopite and minor K-, Ti-, Ba-enriched compounds. On partial melting such an assemblage would yield a composition comparable with that of the SiO2-rich magma from Smoky Butte. SiO2-rich lamproite magmas may be primary melts from an enriched source from deep in the lithospheric mantle.

    Keywords: lamproite; Smoky Butte; melting experiments

    Pages: 457 - 477

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