Emplacement and Assembly of Shallow Intrusions, Henry Mountains, Southern Utah

Exceptional exposures of igneous intrusions of the Henry Mountains of southern Utah allow for the detailed study of three-dimensional pluton geometries as well as igneous emplacement and assembly processes. Examination of the geometry, fabric, and wallrock structures of relatively small intrusions (<3 km map-view diameter) suggests that several plutons in the region (Maiden Creek sill, Trachyte Mesa laccolith, Black Mesa bysmalith) represent snapshots of a continuum of pluton geometry recording volume increase through emplacement of successive magma pulses. Intrusions begin as thin sills and inflate into laccoliths through incremental injection of magma sheets. Space is made through wallrock strain and upward rotation. Further sheet emplacement leads to the formation of a subvertical fault at the margin of the inflating intrusion. This fault accommodates piston-like uplift of the intrusion roof and results in the formation of an intrusion geometry referred to as a bysmalith. Margins of laterally propagating igneous sheets are commonly comprised of numerous finger-like magma lobes that coalesce into contiguous sheets with further lateral growth. As intrusion volume grows through incremental assembly, pluton geometry becomes increasingly simple (fewer marginal lobes, etc.) and evidence for pulsed assembly becomes progressively more cryptic. Thermal modeling suggests these relatively small intrusions were assembled on time scales of weeks to years. These intrusions grew as 'satellites' on the margin of the larger Mt Hillers intrusive complex. Satellite intrusions are not evenly distributed around the main intrusive centers. For example, numerous satellite intrusions exist on the NW half of the Mt Hillers intrusive center, but the SE half is devoid of them. No clear explanation exists for this spatial variation in satellite intrusion development. The main intrusive center is a large laccolith comprised of numerous sills and dikes above a voluminous central pluton. The central pluton includes an enigmatic 'shattered zone' of complexly intermingled sedimentary host rock and magmas with a wide range of textures. Geochronology suggests all of Mt Hillers, including the satellite intrusions, was assembled within no more than ~1 m.y. from several geochemically distinct magma pulses. Evidence from the satellite intrusions demonstrates that each of these pulses is itself emplaced on shorter time scales as a series of pulses.