THREE-DIMENSIONAL MAGMATIC FABRIC OF A SHEETED LACCOLITH, HENRY MOUNTAINS, UTAH

The Henry Mountains are one of several Tertiary igneous centers intruded into flat-lying Colorado Plateau sediments. Excellent exposure and lack of overprinting by regional tectonic strain make the intrusions in the Henry Mountains ideal for studying pluton emplacement. Our work concentrated on the Maiden Creek laccolith, a thin (less than 100 m thick, where exposed), plagioclase-hornblende porphyry. The intrusion is located on the eastern flank of Mt Hillers, one of the five intrusive centers in the Henry Mountains. Contacts between the intrusion and the surrounding sediments are well exposed on steep cliffs and in several canyons.

The fabric of this intrusion was determined using field measurements of aligned elongate minerals and laboratory determination of anisotropy of magnetic susceptibility (AMS). The agreement between field and AMS measurements was corroborated with image analysis and X-ray computed tomography. Petrographic work indicates a lack of solid-state deformation, allowing the fabric data to be interpreted in terms of magmatic flow direction. The data indicate consistent directions of movement, even along a series of vertical cross-sections.

The field and magnetic data indicate that the thin Maiden Creek laccolith is composed of multiple sheets of porphyry emplaced in rapid succession, rather than a single injection pulse. Field evidence further suggests that each sheet solidified before the next sheet was emplaced. From these data, we infer that magmas intruded near their solidus temperature and rapidly solidified after emplacement. The constant orientation of the fabric throughout the intrusion suggests that each pulse of magma originated from a single source area.