THE LYLE SPRING MAGMATIC SYSTEM OF THE YELLOWSTONE VOLCANIC FIELD

Cyclic eruption patterns within the Yellowstone Volcanic Field have long been recognized based on geochemical and geochronological information about caldera-forming eruptions and the small-volume effusive eruptions that precede and follow caldera formation. However the petrogenetic relationships between caldera-forming and effusive eruptions are relatively under-studied. Here we present geochemical, isotopic, and new high-precision ⁴⁰Ar/³⁹Ar ages for four small-volume eruptions located on the western rim of the Henry’s Fork caldera produced by eruption of the 1.3 Ma Mesa Falls Tuff. These four units are mineralogically and geochemically similar, however the Bishop Mountain Flow and the Tuff of Lyle Spring have ages of ~ 1.45 Ma along with Pb-isotopic ratios that are distinct from the Mesa Falls Tuff, younger effusive eruptions, and rhyolites associated with the older Huckleberry Ridge Tuff system. The Pb-isotopic signatures and older eruption ages led us to propose a magmatic system unrelated to that of the caldera-forming system. Analysis of geochemical, rock magnetic, and detailed ⁴⁰Ar/³⁹Ar dating provides insights into the dynamics of this earlier system. Antecrystic inheritance is prevalent within the matrix of the Tuff of Lyle Spring, yet absent within individual pumice, with inherited grains forming statistically resolvable age populations separated by several thousand years. One of these populations is consistent with our new age for the Bishop Mountain Flow, suggesting that the Tuff of Lyle Spring disaggregated solids associated with earlier periods of magmatism. We use the distribution of antecrystic populations as a proxy for the pace of crustal accumulation in bimodal volcanic fields and propose a system that generates chemically, isotopically, and temporally distinct magmas in a low-flux state that is periodically punctuated by larger caldera-forming eruptions.