A SIMPLE GEOCHEMICAL MODEL FOR BASALT AND RHYOLITE GENESIS ALONG THE YELLOWSTONE HOTSPOT TRACK

Conflicting explanations have been advanced for the petrogenesis of basalts and low-δ¹⁸O rhyolites of the central Snake River Plain. We show that, when the Yellowstone hotspot track including the Columbia River and Steens basalts is treated as a single province derived ultimately from a common mantle source, the isotope geochemistry (Nd, Pb, Sr, and O) of these lavas and tuffs can be explained to a first order by simple binary mixing of asthenosphere-derived basalt and granitic crust of the Idaho batholith.

Gravity, seismic, and petrologic evidence suggests that a thickness of 7-16 km of basaltic material has been added to the crustal column beneath the province since the middle Miocene. There is a consensus that early Steens and Imnaha basalts represent the least lithospherically contaminated mantle-derived magma erupted during the mid-Miocene main phase of the CRBG. Given the spatial and temporal continuity of the entire ‘Yellowstone Hotspot’ province, we adopt these primitive basalt compositions as representative of the mantle component involved in SRP magmatism.

The crustal component chosen in our modeling consists of Idaho Batholith granitoids, representing the shallow roots of caldera systems active during the Eocene, with pervasively meteoric-hydrothermally altered low δ¹⁸O zones that extend into the Cretaceous granite country rock. These granitoids outcrop both to the north and south of the CSRP region, and some are truncated by the Snake River plain. Zones of low-δ¹⁸O alteration in caldera hydrothermal systems have been shown to affect volumes of rock as thick as 2 km, hence we make the reasonable assumption that a large mass of low-δ¹⁸O rock was available for anatexis beneath the present area of the central SRP.

In summary, mixing models using 10-40% Steens/Imnaha basalt and 60-90% Idaho Batholith granite produce isotopic values for Nd, Sr, Pb, and O consistent with CSRP rhyolites.