GEOMETRY AND CONSTRUCTION HISTORY OF AN ASYMMETRIC LACCOLITHIC INTRUSIVE CENTER: MOUNT HILLERS, HENRY MOUNTAINS, UTAH

The shallow crustal intrusions of the Henry Mountains in southern Utah inspired G.K. Gilbert to recognize that magma can deform its hot rock and to coin the term ‘laccolite’ (later ‘laccolith’). The idealized laccolith is an axisymmetric mushroom-shaped intrusion with a sub-horizontal base and a convex-up roof that concordantly domes overlying strata. However, recent detailed geologic mapping on the northern portion of Mount Hillers, one of the Henry Mountains, demonstrates the central igneous body does not concordantly dome sedimentary strata around the entire mountain. On the southern flank of Mount Hillers, steeply dipping Permian sedimentary strata are in contact with the main intrusive body. On the northern side of the mountain, shallowly to moderately dipping Cretaceous-Jurassic strata are in contact with the main igneous body. The main igneous body of the Mount Hillers intrusive center is in fact a discordant laccolithic structure that has experienced multiple stages of magma emplacement and host rock deformation.

The new data presented here are concentrated on the northern side of Mount Hillers. However, our analysis incorporates the previously well-studied southern portion of the mountain to provide a comprehensive geometric configuration of the intrusive center. After establishing an accurate geometry of the entire intrusive center in the field, various laboratory datasets allow us to better understand how the asymmetric relationship between the main igneous body and host rock progressively developed. Cross-cutting relationships between igneous bodies with clear variations in igneous texture allow us to identify the timing of different component intrusions. Anisotropy of magnetic susceptibility data allow us to constrain paleo-magma flow patterns preserved as fabrics in the plagioclase hornblende porphyry igneous rocks. Geochemical data and thin section analysis allow us to distinguish between different magma families by evaluating mineralogical and textural variations among igneous samples.

Multiple cross sections illustrate the detailed architecture of the Mount Hillers intrusive center mapped at a 1:10,000 scale. A detailed study of this intrusive center reveals crucial stages of magma emplacement and host rock deformation that were not recognized during previous mapping.