DETAILED LACCOLITH EMPLACEMENT HISTORY DEMONSTRATES MECHANICAL INTERPLAY BETWEEN HYPABYSSAL MAGMA AND HOST ROCK: MOUNT ELLEN, HENRY MOUNTAINS, UTAH

Theoretical models of sheet intrusion (e.g. sills, dikes, laccoliths) emplacement in the upper crust all assume these igneous bodies grow through propagation of a single contiguous pressurized magma sheet. However, abundant evidence from a wide range of sources (field examples, geophysical studies of ancient intrusions, geodetic studies of active upper crustal systems, and analog models) demonstrate many intrusive sheets grow through a process of lobe amalgamation. These contiguous sheet intrusions have superficially simple morphologies (e.g. sill, laccolith, etc.) but were constructed from successively emplaced lobate component intrusions.

The Mount Ellen laccolithic intrusive center in the Henry Mountains of Utah provides an opportunity to study in detail hypabyssal magma emplacement. Exceptional exposure of this Oligocene (~28 Ma) diorite porphyry intrusion and surrounding host rock allows for detailed reconstruction of the 3-d morphology of the laccolith. Regional subhorizontal stratigraphy is deflected upward by the laccolith (maximum of ~2000 m) over a region with a diameter that ranges from 15 to 20 km. The laccolith dome is elongated in map view in a NW-SE direction, but numerous relatively small (<3 km diameter) bulges project radially outward from the main dome. The total igneous rock volume responsible for the doming is ~100 km³. Available U/Pb zircon geochronology data suggest the entire Mount Ellen intrusive center was constructed over ~1 m.y.

The first-order laccolith dome was assembled from no fewer than 20 second-order component lobate intrusions, many of which are separated by host rock screens. Each of the second-order intrusions has an igneous volume of 0.5 to 3.0 km³. Field evidence demonstrates that many of these second-order intrusions were themselves emplaced from multiple third-order component magma pulses. Second-order component intrusions can be separated into three groups based on their morphology. Spatial patterns of these morphologies allow us to infer a history of igneous assembly for the entire laccolithic intrusive center – early component intrusions influenced the geometry of later intrusions. This assembly history suggests that the mechanical relationship between magma and host rock evolved continuously throughout hypabyssal emplacement.