FLOW UNITS OF THE LITTLEFIELD RHYOLITE, EASTERN OREGON, CONSTRAINING AGE AND STORAGE SITES OF GRANDE RONDE BASALT MAGMAS

The widespread Littlefield Rhyolite is one of the large, hotspot-related rhyolites that erupted in eastern Oregon and which had a previous volume estimate of ~100 km³ covering ~850 km². This may represent half the erupted volume and actual distribution. The focus of this study has been to investigate the stratigraphy and petrology of the Littlefield Rhyolite (LFR) and whether field and geochemical relationships exist to help constrain the timing and storage sites of Grande Ronde Basalt (GRB) magmas. Although indistinguishable in the field, our data reveal that the LFR consists of two geochemically distinct rhyolite flow packages that are designated here as lower and upper Littlefield Rhyolite, according to stratigraphic relationships in Malheur River Gorge. Rarely viewed in sequence, these rhyolites are distinguished by Zr, Ba, Nb, TiO₂ and FeO contents and ⁴⁰Ar/³⁹Ar ages (16.12±0.04 vs 16.05±0.04).

Rhyolites known either as ‘rhyolite of Cottonwood Mountain’, or ‘rhyolite of Bully Creek Canyon’, and which are exposed around Cottonwood Mountain, NW of Vale, have identical compositions to samples of lower LFR. Additionally, single crystal ⁴⁰Ar/³⁹Ar ages of two samples (16.12±0.07, 16.20±0.08) are statistically indistinguishable. Together, this provides unequivocal evidence that rhyolites of the lower Littlefield Rhyolite erupted over a minimal distance of 40 km from vents observed within Malheur Gorge in the south to Cottonwood Mtn. in the north.

Among units sandwiched between the lower and upper LFR are several lava flows and a one-meter thick agglutinated spatter deposit of local Grande Ronde Basalt (GRB) units. The spatter deposit thickens to 10s of meters over a distance of 800 m where the deposit is strongly welded. We now recognize this as a venting site of local Grande Ronde Basalt. Ages of LFR constrain eruption of local Grande Ronde Basalt to an age span of ~100k years between 16.05 – 16.12 Ma.

One local variant of late-stage GRB is icelanditic (~61 wt. % SiO₂) and is found at a number of places including a location near the southern extent of the upper LFR. Geochemical modeling strongly suggests that icelandite lavas resulted from mixing of GRB and upper LFR magmas, thereby tying a GRB magma storage site to within the greater Malheur River Gorge area and indicating contemporaneity of rhyolitic and GRB magma reservoirs.