Abstract
Three contemporaneous yet mineralogically and chemically distinct lava series which range in composition from olivine basalt through basaltic andesite through two-pyroxene andesite, have been analyzed for their strontium isotopic composition. The Mammoth-Modoc lavas produce a mantle isochron which suggests derivation from a source approximately 243 m.y. old. The Medicine Lake shield-forming extrusives produce a minimum mantle isochron; the age of this source is likely greater than 760 m.y. old. Encompassing the entire time span of eruption for the two basalt-andesite series are a number of low potassium high alumina olivine tholeiite lavas which, when analyzed as a coherent group, produce a zero age. Lavas related to these three magma series have been extruded during the last 105 years and cover a limited geographic area. The isotope data suggest a model with substantial heterogeneity in the subcontinental lithosphere and asthenosphere, at least beneath the Medicine Lake Highland portion of the southern Cascade petrographic province.
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Mertzman, S.A. Strontium isotope geochemistry of a low potassium olivine tholeiite and two basalt-pyroxene andesite magma series from the Medicine Lake Highland, California. Contr. Mineral. and Petrol. 70, 81–88 (1979). https://doi.org/10.1007/BF00371874
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DOI: https://doi.org/10.1007/BF00371874