CONSTRAINTS ON EMPLACEMENT HISTORY OF SUBVOLCANIC MAGMA SYSTEMS FROM THERMAL MODELING, HENRY MOUNTAINS, UTAH

Volcanism on the surface of Earth is driven by subsurface magma systems, which grow and change through repeated injections of magma pulses, usually as sub-horizontal sheets fed by dikes. This research uses thermal modeling to provide constraints on the growth history of these subvolcanic magma systems. We use Oligocene hypabyssal igneous intrusions now exposed in the Henry Mountains of southeastern Utah as a model for modern subvolcanic magma systems. Previous work demonstrates that the igneous intrusions of the Henry Mountains have a laccolith geometry and were constructed through injection of multiple magma pulses. The detailed 3-D geometry of the laccoliths is well known, but the number, volume, and timing of the pulses are poorly constrained. Additionally, published geochronologic and host rock thermochronology data provide constraints respectively on the total duration of magmatism and the thermal history of sedimentary host rock adjacent to the intrusions. We use thermal modeling of these laccolith systems to better understand their igneous emplacement history. The modeling allows exploration of a range of factors related to igneous emplacement history that influence thermal history, including magma pulse sizes, number of pulses, timing between pulses, stacking order of magma sheets (e.g., over-accretion versus under-accretion), etc. The goal of the project is to explore the possible combinations of these factors that produce thermal histories compatible with existing data such as intrusion geometry, geo- and thermochronology data, observed igneous textures, etc. Preliminary results provide a time of ~37,000 years for cooling of an instantaneously emplaced single pulse of magma into the subsurface. This is a much faster cooling time than what published U/Pb zircon geochronology suggests and supports the notion that these intrusions were formed through multiple injections of magma. Further results will provide more insight into how shallow magma systems grow and evolve.