ISOTOPIC AND CHEMICAL EVIDENCE CONCERNING THE GENESIS AND CONTAMINATION OF BASALTIC AND RHYOLITIC MAGMA BENEATH THE YELLOWSTONE PLATEAU VOLCANIC FIELD

Since 2.2 Ma, the Yellowstone Plateau volcanic field has produced approximately 6000 km3 of rhyolite tuffs and lavas in > 60 separate eruptions, as well as approximately 100 km3 of tholeiitic basalt from > 50 vents peripheral to the silicic focus. Intermediate eruptive products are absent. Large calderas collapsed at 2.0, 1.3, and 0.6 Ma on eruption of ash-flow sheets representing at least 2500, 280, and 1000 km3 of zoned magma. Early postcollapse rhyolites show large shifts in Nd, Sr, Pb, and O isotopic compositions caused by assimilation of roof rocks and hydrothermal brines during collapse and resurgence. Younger intracaldera rhyolite lavas record partial isotopic recovery toward precaldera ratios. Thirteen extracaldera rhyolites show none of these effects and have sources independent of the subcaldera magma system. Contributions from the Archaean crust have extreme values and wide ranges of Nd-, Sr-, and Pb-isotope ratios, but Yellowstone rhyolites have moderate values and limited ranges. This requires their deep-crustal sources to have been pervasively hybridized (and the Archaean components diluted) by distributed intrusion of Cenozoic basalt, most of which was probably contemporaneous with the Pliocene and Quaternary volcanism. In hybrid sources yielding magmas parental to the subcaldera rhyolites, half or more of the Nd and Sr may have been contributed by such young basalt. Parents for the extracaldera rhyolites, generated beyond the leading edge of the northeast-propagating focus of basaltic intrusion and deep-crustal mobilization, contained smaller fractions of mantle-derived components. Most Yellowstone basalts had undergone cryptic clinopyroxene fractionation in the lower crust or crust-mantle transition zone and, having also ascended through or adjacent to crustal zones of silicic-magma generation, most underwent some crustal contamination. A high fraction of the Pb in most basalts is of crustal derivation. Anomalously low seismic velocities to a depth of approximately 250 km and a high flux of He-3 at Yellowstone suggest sublithospheric magma contributions. Elevated baseline Nd- and Sr-isotope ratios suggest additional contributions from old lithospheric mantle, but this is hard to quantify because of the crustal overprint. Foundering of crustally contaminated main-stage cumulates into the low-viscosity upper mantle beneath the principal focus of basaltic intrusion may influence the isotopic compositions of low-K tholeiites and Snake River olivine tholeiites subsequently generated along the Snake River Plain axis in the wake of the main migrating melting anomaly.