EMPLACEMENT AND GROWTH OF COMPLEX INTRUSIONS IN THE SHALLOW CRUST, HENRY MOUNTAINS, UTAH

Details of subvolcanic processes like magma migration paths in the shallow crust are poorly understood, mainly because there are few locations that have well preserved contacts between igneous intrusions and surrounding host rock. Spectacular exposures in the Henry Mountains of southern Utah are a notable exception. We studied a portion of the Mt. Hillers (one of the Henry Mountains) intrusive center where sedimentary strata have been deformed and upturned to make room for a large central intrusive body. A detailed geologic map of the field area shows a network of sills and dikes extending outward from the central intrusive body between and through the sedimentary strata. Some of these sills and dikes feed small-volume satellite intrusions in the more flat lying undeformed host rock. Field fabric orientation, phenocryst shape preferred orientation, thin section petrography, and XRF geochemistry data are employed to constrain the conduits through which magma has migrated from the central intrusive body to the small satellite intrusions. Evidenced that more than one pulse of magma exists in the intrusive center comes from a variety of data sets, including; distinctive field textures, crystal size distribution, petrographic characteristics, and trace element geochemistry. Additional data will help constrain the actual number of pulses. By integrating our data with previous works, we have constructed a hypothetical construction history for the Mt. Hillers intrusive center. The first pulse of magma intruded as sills in flat lying sedimentary country rock; subsequent intrusions rotated and uplifted the sedimentary rock and prior intrusions. The satellite intrusions represent the latest intrusive phase and grew from small sills and dikes.