Geometry and construction history of a complex intrusive center in the shallow crust, Mount Pennell, Henry Mountains, Utah

Abstract

Large igneous intrusions grow from incremental emplacement of relatively small batches of magma. The Henry Mountains have been the subject of investigation because multiple, well-exposed intrusive centers are present on the Colorado Plateau. Mount Pennell is an asymmetric intrusive center in the Henry Mountains of southern Utah. Recent field work on the eastern flank of Mount Pennell reveals evidence of construction of a primary laccolith and subsidiary intrusions from multiple, discernable pulses of magma. Igneous rock textures and compositions at Mount Pennell are highly diverse, providing methods of differentiating between magma batches, not obvious at the other Henry Mountains intrusions. Nine igneous rock units with distinct composition and texture were identified, each belonging to one of three communities that were emplaced during three phases (or pulses) of construction. The geometry and relative timing determinations describe a construction history with a primary phase of mushroom-shaped, laccolithic emplacement, and two secondary phases of emplacement where intrusions crosscut the early igneous rock and emplaced as shallow sills, or underplated the early laccolith and expanded laterally.