Geometry and Construction History of the Copper Ridge Laccolith, Mount Ellen, Henry Mountains, Utah

Abstract

The Copper Ridge laccolith is an asymmetric tongue-shaped intrusion located on the southeastern margin of Mount Ellen in the Henry Mountains, Utah. The late-Oligocene to early-Miocene igneous rocks have no syn- or post-emplacement tectonic overprint. In addition, exposure of the laccolith is exceptional: well-preserved sedimentary strata cap the intrusion, the lower contact is locally exposed, and numerous natural cross sections can be studied. These characteristics make the Copper Ridge laccolith an ideal location to study emplacement of magma in the shallow crust. Field mapping shows the intrusion is about 3.5 km wide, 2.0 km long and, at its thickest point, 425 m thick. The estimated magma volume is about 2.9 km³. Field work, crystal size distribution, and geochemistry, suggest that the intrusion was built incrementally through the injection of two separate magma batches, resulting in an upper and a lower sheet. Field work shows that the upper sheet is separated from the lower sheet by well-preserved, variably metamorphosed Cretaceous Tununk shale. Data from anisotropy of magnetic susceptibility analysis suggest a subhorizontal sill fed the intrusion from the west-northwest, and magma flowed to the southeast in a fanning pattern. X-ray diffraction data show that the preserved Tununk experienced low-grade contact metamorphism. Multiple data sets suggest a construction model in which the upper sheet intruded first, lifting and deforming the sedimentary strata. The lower sheet intruded second and off center relative to the upper sheet. The asymmetric lifting caused the upper sheet to fracture adjacent to the southern margin of the lower sheet. The fractures likely facilitated weathering and erosion, causing the upper sheet to appear segmented in current exposure.