Episodic Assembly of Igneous Intrusions in the Shallow Crust at two Spatial and Temporal Scales, Henry Mountains, Utah

We describe episodic assembly of igneous intrusions in the shallow crust at two spatial and temporal scales, focusing on examples from the Henry Mountains (Utah, U.S.A.) intruded into flat-lying sandstones and shales. The lack of regional deformation during Oligocene assembly of these intrusions makes the Henry Mountains an excellent location to study intrusion processes without tectonic modification. At the scale of individual small (< 1 km3) intrusions we focus on three examples with exceptionally well exposed 3-D geometries. In order of increasing volume these include: (1) a lobate sill, (2) a laccolith, and (3) a bysmalith (a cylindrical, fault-bounded laccolith). Field evidence demonstrates that each intrusion was assembled from multiple horizontal sheet-like magma pulses. We suggest these intrusion geometries reflect the time evolution of an idealized upper crustal pluton as total magma volume increases. Thermal constraints for these intrusions require that assembly of each occurred in less than 100 years. Geochemical data indicate that (1) each intrusion is geochemically homogeneous, and (2) the sill and laccolith were fed by a magma batch distinct from that of the bysmalith. Episodic assembly is also apparent at the scale of a relatively large (~ 50 km3) laccolith-like intrusive center (Mt Hillers) that formed through amalgamation of dozens of component intrusions, including the three discussed above. Paleomagnetism demonstrates that early sills were uplifted and tilted by later underlying intrusions, and structural analysis allows us to establish the relative timing of other component intrusions. Geochemical data indicate that at least five separate magma batches contributed to assembly of the intrusive center. The duration of assembly is not yet well established, but is probably less than 1 M.y. Our data demonstrate that even small geometrically simple intrusions can have complex assembly histories, and that similar intrusive processes operate at both local and regional scales. Similar complex structural features and assembly histories have been inferred for larger syntectonic intrusions elsewhere, but unequivocal evidence of this complexity is rare. The interplay between deformation and magmatism commonly ensures that, as intrusions grow in size and duration of assembly, many of the features recording the details of such complex histories are modified or destroyed. Studying these processes in relatively simple locations like the Henry Mountains is therefore essential to unraveling assembly histories elsewhere.