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 differences in texture, geochemistry, etc. may be subtle or non-existent. However, differences in injection crystallization histories may be recognizable through analysis of the late-crystallizing groundmass in porphyritic subvolcanic igneous rocks. In igneous bodies built from component magma sheets, early injections cool rapidly relative to later injections, resulting in distinct groundmass crystal size distribution (CSD) in the youngest, slowly cooled magma sheets compared to older, faster cooled sheets. Previous work demonstrates the ~28 Ma Copper Ridge laccolith (Henry Mountains, Utah) was constructed at a depth of ~2 km from at least two texturally distinct igneous sheets stacked atop one another, and suggests these two sheets may themselves include multiple injections of magma. For this work, we test the hypothesis that the relative timing between intrusive sheets in a laccolith can be constrained using CSD analysis of groundmass texture.

To test this, we collected a suite of porphyritic diorite samples from a natural cross-section through the entire 400-m-thick Copper Ridge 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 was conducted on EBSD mineral phase maps of quartz, anorthite, and orthoclase in the groundmass (crystals <100 microns). Preliminary results suggest the upper sheet was intruded after the lower sheet, and the lower sheet is built of three or more component pulses of magma. Ongoing work includes adding data from more samples and synthesizing results from three analyzed mineral phases. We discuss the implications for both the construction history of this shallow magma system and other shallow magma systems.