GEOMETRY AND CONSTRUCTION HISTORY OF THE COPPER RIDGE LACCOLITH, MOUNT ELLEN, HENRY MOUNTAINS, UTAH

The Copper Ridge laccolith is a tongue-shaped intrusion located on the southeastern margin of Mount Ellen in the Henry Mountains, Utah. The intrusion is about 8 km wide, 4 km long and, at its thickest point, 335 m thick, with an estimated magma volume of about 3 km³. To reconstruct the emplacement history of Copper Ridge both field and laboratory methods were employed. New fieldwork and laboratory results suggest the intrusion was built incrementally through the injection of at least two separate magma batches. Dioritic magma intruded the upper 2-3 km of crust into subhorizontal Cretaceous strata. Rapid cooling of the magma led to the formation of plagioclase-hornblende porphyry and preservation of magmatic fabrics. The 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 exposed locally, and numerous natural cross sections through the interior exist. These characteristics make the Copper Ridge laccolith an ideal location to study emplacement of magma in the shallow crust. Studying such intrusions may provide information about volcanism, economic mineral deposits, geothermal energy, and crustal growth.

Evidence for multiple sheet emplacement can be observed in the field and in laboratory data. The upper sheet is separated from the lower sheet by well-preserved, variably metamorphosed Cretaceous Tununk shale. Consistent textural differences, which may indicate separate magma batches, are apparent in the phenocryst size of the two sheets: the upper sheet has relatively larger phenocrysts, up to 10 mm, and the lower sheet has smaller phenocrysts, up to 4 mm. Preliminary laboratory data also support multiple sheet emplacement: anisotropy of magnetic susceptibility data show that the rocks from the upper sheet have significantly higher susceptibility values than those from the lower sheet.

Field measurements of magmatic fabric suggest lateral emplacement toward the southeast from a conduit located towards the center of Mount Ellen. Preliminary laboratory data show that, within each sheet, the anisotropy of magnetic susceptibility fabric orientations are broadly consistent, and that the fabric orientations within the upper sheet differ from those in the lower sheet.