CONSTRAINTS ON SUBVOLCANIC MAGMA PLUMBING SYSTEM EVOLUTION FROM CRYSTAL SIZE DISTRIBUTION ANALYSIS OF IGNEOUS GROUNDMASS, HENRY MOUNTAINS, UTAH

Shallow magma systems drive surface volcanism and are commonly built through multiple injections of magma. Recognizing these separate magma injections can be difficult because of the typically subtle differences in their igneous rock characteristics. Because of differences in injection cooling histories, analysis of the late-crystallizing groundmass in porphyritic subvolcanic igneous rocks may provide evidence of the order in which adjacent magma bodies were injected. Earlier magma injections cool rapidly relative to later injections, resulting in smaller crystals in the groundmass of the youngest magma sheets and larger crystals in the groundmass of the oldest magma sheets. For this work, we test the hypothesis that the relative timing between intrusive sheets in a laccolith can be constrained by using crystal size distribution (CSD) analysis of groundmass texture in samples of porphyritic andesite from a well-exposed cross section through the 400-m-thick Copper Ridge laccolith in Utah’s Henry Mountains. The ~28 Ma laccolith was intruded at a depth of about 2 km as part of the larger Mount Ellen intrusive center, which was constructed from dozens of component intrusions similar to the Copper Ridge laccolith.

Previous work demonstrates the Copper Ridge laccolith was constructed from at least two texturally distinct igneous sheets stacked atop one another, and that these two sheets may themselves include multiple injections of magma. We collected a suite of samples from a natural cross-section through the entire intrusion including samples at well-exposed upper and lower contacts of the laccolith with sedimentary host rock, at contacts with an intercalated layer of host rock within the laccolith, and within the upper and lower igneous sheets themselves. CSD analysis of the samples is being performed on SEM images of thin sections. Mineral phases in the groundmass are identified using EDS. We will present in-progress results of this work and discuss the implications for both the construction history of this shallow magma system and other shallow magma systems.