Emplacement and evolution of a fault-controlled rhyolite series: the Obsidian Creek and Roaring Mountain members of Yellowstone. Insights from major and trace element and Pb isotope geochemistry.

Lesser known than the broadly coeval intra-caldera Central Plateau Member (CPM) rhyolites, the porphyritic Obsidian Creek and largely aphyric Roaring Mountain members of Yellowstone comprise small extra-caldera rhyolite flows and domes dated from ~526 to ~80 ka. Most lavas lie in a graben adjacent to the northern caldera rim, the Norris-Mammoth corridor. New major and trace element data reveal that regardless of their member assignment, most samples define a common evolution trend exhibiting pronounced enrichments in LILE and HFSE with decreasing MgO, Sr, Ba and LREE. This trend differs from and intersects the CPM trend, exhibiting larger incompatible element variations and smaller compatible element variations. Overall, this trend is consistent with fractional crystallization of two feldspars, quartz, pyroxenes, Fe-Ti oxides, apatite, and chevkinite. Two lava flows, each erupted >20 km SW and NE from the corridor also lie on this trend. However, eruption ages do not correlate with the degree of evolution, and within-flow compositional variations, sometimes observed at outcrop scale, are substantial and may span much of the trend. Within-flow crystallinity variations are equally significant, challenging the existing classification into two members. New in situ glass Pb isotope data complement existing whole-rock data and reveal that whereas lavas within the corridor broadly resemble one another with Pb radiogenic signatures of 207Pb/206Pb~0.9020.907 and 208Pb/206Pb~2.212.23, the two lavas from outside the corridor have distinct 207Pb/206Pb~0.9230.929 and 208Pb/206Pb of 2.262.31. Together, these elements argue instead for a common evolution process of distinct magma batches which erupted sporadically over ~400 ka. At the northern end of the corridor and farthest from the caldera, the porphyritic Gardner River rhyolite is a clear geochemical outlier, plotting at the primitive end of the CPM trend. This lava, mingled with basaltic andesite, has 207Pb/206Pb~0.893 and 208Pb/206Pb~2.17 which closely resemble the CPM lavas. Its eruption age of ~300 ka and geochemical affinity allow to speculate that it might be a precursor of the earliest CPM lavas, erupted ~ 260250 ka. Here, regional faulting might have aided lateral magma transport with minimal crustal interaction.