EVIDENCE OF PULSED CONSTRUCTION OF A MID-TERTIARY LACCOLITH BY DISTINGUISHING COMPONENT MAGMAS AT MOUNT PENNELL, HENRY MOUNTAINS, UTAH

Mount Pennell, of the Henry Mountains in Utah, is a mushroom-shaped laccolith, intruded into sub-horizontal strata of the Colorado Plateau. Roof strata are domed upward nearly concentrically from the intrusion center. The laccolith has a central core that is overwhelmingly igneous in composition, a peripheral zone of inclined sills between strata, and satellite intrusions that are furthest from the core. Recent field work on the eastern flank of Mount Pennell reveals evidence of pulsed construction of the primary laccolith structure from multiple stacked sheet intrusions, including smaller-scale laccolith, tongue, and flue structures.

Our recent work demonstrates significant evidence for multi-stage disruption of early intrusions and strata by later magmatism. In many locations, exposed edges of intrusive sheets have fingered geometry extending into both sedimentary and igneous host rock. There are multi-scale appearances of cross-cutting dike development, deformation of host rock, and sill emplacement.

There is consistent erratic intrusion placement of certain late-stage syenite magmas with distinct coarse texture. Hydrothermal epidotization of magmas near the intrusion center appears inconsistently within a defined radius, and the degree of metamorphism varies locally and with igneous host rock composition. Also, toward the intrusion center there is increased incorporation of xenoliths, and more exposed breccia.

Construction histories of significant intrusions on the eastern flank of Mount Pennell are constrained by expanding on cross-cutting relationships of compositionally distinct magmas. Geometry of individual intrusions of different rock types suggests that intrusion form may be linked to rheology. Magma categorization is based on petrologic composition or texture seen in thin sections or hand samples, magnetic susceptibility, and major and trace element geochemistry. Previous work recognized seven distinct magma types in the central igneous body. Our work indicates that these magma types extend beyond the central body and additional magma types exist.