MULTIPLE GENERATIONS OF VOLATILE-ASSISTED BRECCIATION ACCOMMODATE EMPLACEMENT OF THE BUCKHORN RIDGE SILL, HENRY MOUNTAINS, UTAH

Igneous intrusions in Utah's Henry Mountains were emplaced at a depth of 2 to 3 km into flat-lying sediments of the stable Colorado Plateau during the mid-Tertiary. The intrusions are therefore ideal for studying shallow igneous emplacement processes in the absence of tectonic overprinting. Mt Holmes, one of five intrusive centers in the Henry Mountains, is composed of numerous sills, dikes, and other igneous bodies that together form a complex laccolith. The largest exposed component intrusion on Mt Holmes is the Buckhorn Ridge sill, which has a minimum thickness of ~80 m near its emergence from the main body and thins gradually until its distal termination ~1300 m away.

Magnetic fabric analysis and field measurements of phenocryst fabric demonstrate that foliation in the intrusion is typically sub-parallel to the sill's contacts and lineation has a radial pattern. Fabrics throughout the intrusion are, however, poorly developed, even near contacts with wallrock. This contrasts with observations of more distal intrusions in the region, where strong fabrics developed near wallrock contacts.

A meter-thick breccia layer at the base of the intrusion demonstrates that much of the emplacement-related strain was accommodated by brecciation and granular flow of volatile-rich wallrock. The number of breccia generations decreases away from the main Mt Holmes body, but the thickness of the brecciated layer remains effectively constant, indicating that the same already deformed rock was reworked during emplacement. This reworking was aided by volatile phases that migrated preferentially along mechanical heterogeneities, resulting in local weakening. Several distinct structural features developed as a result of volatile-assisted strain localization, including duplexes composed of decimeter-scale lozenges of igneous rock surrounded by centimeter-thick breccia layers, and gneiss-like interlayering of igneous and brecciated rock.

The proximity of the sill to its parent igneous body may explain this unique emplacement history. Magma driving pressure was probably higher than for more distally located intrusions, allowing rapid volatile-assisted brittle deformation to dominate during emplacement.