THE GARDINER BASALTS: PETROLOGICAL AND GEOCHEMICAL COMPARISON WITH THE HEPBURN MESA AND YELLOWSTONE NATIONAL PARK BASALTS

North of the Yellowstone National Park (YNP) boundary lie two distinct series of basaltic lava flows. The ~0.6 Ma (total gas date; preliminary) Gardiner basalts lie just outside the YNP boundary. The 2.2 Ma (⁴⁰Ar/³⁹Ar) Hepburn Mesa basalt flows are 30 miles to the north near Emigrant, MT. The Hepburn Mesa basalt flows represent some of the first magmatic activity in the Yellowstone Valley associated with the hotspot migration. The multiple Gardiner basalt flows are contemporaneous with the last pulse of magmatism in YNP. These basalts are variably phenocrystic with a mineral assemblage of olivine + plagioclase ± pyroxene and accessories of sphene ± magnetite. They are approximately tholeiitic in major element composition. Geochemical characterization of these basalts reveal slightly enriched LREE (~50X chondrites for Hepburn Mesa; 30 to 60X chondrites for Gardiner), small positive Eu-anomalies and flat HREE (~10X chondrites). Ti-Zr-Y ternary plots show that these basalts all lie within the intraplate boundary field. Rb/Nb ratios range from 0.36 to 0.85, overlapping the field for all YNP basalts.