Post-caldera volcanism at the heise volcanic field: Implications for petrogenetic models

Abstract

The Heise volcanic field is the second youngest caldera complex of the Yellowstone-Snake River Plain province (USA) and represents a polycyclic caldera system with rhyolitic volcanism extending over more than 2 Myr. The products of the Heise volcanic field include four regionally extensive ignimbrites, including the Blacktail Creek and Kilgore tuffs, which both have volumes estimated at >1000 km3, separated by sequences of smaller volume tuffs, lavas and sedimentary deposits. Rhyolites from the Heise volcanic field are both normal-δ18O and low-δ18O, making it a key locality for investigating rhyolite petrogenesis. However, the occurrence of abundant young basaltic lava has limited our ability to fully characterise this volcanic centre, particularly in terms of post-caldera volcanism. Here we describe rhyolitic samples from both a >700m thick section of drillcore within the Snake River Plain and the exposed outflow stratigraphy on the margins of the plain. Based on a combination of bulk-rock and mineral geochemical, isotopic, and geochronological evidence, we conclude that the rhyolites from the drillcore are not exposed at the surface, nor are the surficial rhyolites found in the drillcore. High-precision isotope dilution thermal ionisation mass spectrometry U-Pb geochronology dates the rhyolite at the base of the drillcore to 4·024860±0011 Ma, ~0·4 Myr younger than the youngest caldera-forming ignimbrite at Heise, the 4·48Ma Kilgore Tuff, whereas U-Pb secondary ionisation mass spectrometry dates the uppermost portion of rhyolite in the drillcore to 3·8660±19 Ma. The combined geochemistry and stratigraphic relations suggest that the drillcore penetrates the intracaldera stratigraphy. The intracaldera rhyolites are compositionally and mineralogically similar to the outflow stratigraphy with high-temperature magmas (>800°C) persisting for the full >3 Myr history of the Heise centre. The δ18O values of pyroxene, sanidine, and quartz from the unaltered drillcore samples are consistent with high-temperature equilibrium and return magma δ18O values that are low (4·1-6·0 based on Δ18O melt-sanidine of 0·6) but somewhat higher than the value for the preceding Kilgore Tuffmagma of 3·3. Buried deep within the drillcore are also hydrothermally altered rhyolites with bulk δ18O ranging from-3·5 to + 1·0 (SMOW) with complex X-ray diffraction spectra revealing the presence of epidote, quartz and chlorite. These altered samples are, however, not markedly different in bulk major or trace elemental geochemistry from the unaltered Heise rhyolites. Rhyolite-MELTS models using these hydrothermally altered samples as potential assimilants can reproduce the compositions, mineralogy, and crystallinity of the low-δ18O Kilgore Tuffwith 40-50% assimilation while also satisfying the mass balance constrained on the basis of δ18O. These results support a cannibalisation model for Heise volcanism while highlighting that the lowest δ18O rhyolites may require large amounts of extremely 18O-depleted hydrothermally alteredmaterial available for assimilation.