SNAPSHOTS OF PROGRESSIVE INTRUSIVE CENTER GROWTH IN THE SHALLOW CRUST, HENRY MOUNTAINS, UTAH

The intrusions of the Henry Mountains of southern Utah provide an exceptional setting for the study of igneous emplacement processes in the shallow crust. The five separate intrusive centers intruded the stratigraphy of the Colorado Plateau at 2 to 4 km depth. The intrusions are mid-Tertiary in age and therefore postdate the minor Laramide orogenic activity that affected this part of the Colorado Plateau. Consequently, displacement of wall rock resulting from emplacement of magma is well constrained and fabric within the intrusions primarily reflects finite strain produced by magmatic flow during emplacement.

Each of the five separate intrusive centers in the Henry Mountains preserves a different stage of the evolution of an igneous system constructed in the shallow crust. All five intrusive centers are comprised of a central composite laccolith and associated smaller intrusive bodies. Exposures of the system with the smallest magma volume, Mount Holmes, are primarily dikes and small tongue-shaped laccoliths; the main central igneous body is not exposed. Mount Hillers represents an intermediate stage of intrusive center development. Component intrusions include an early network of sills and dikes, relatively voluminous central igneous body, and a late group of satellite intrusions developed around the periphery of the intrusive center. Mount Ellen records an advanced stage of development, with a relatively voluminous central igneous body and dozens of satellite intrusions.

Collectively, the five intrusive centers provide a series of snapshots of the progressive growth of an igneous system in the shallow crust. Using data field, geochemical, geochronogical, and paleomagnetic data from these intrusive centers, we present a generalized model for progressive construction of a shallow igneous complex.