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

Abstract

Details of magma migration paths in the shallow crust are poorly understood, in part because there are few locations with well-preserved contacts of igneous intrusions and surrounding host rock. The exposures in the Henry Mountains of southern Utah are a notable exception. This study focuses on rocks exposed on the eastern flank of the Mt. Hillers (one of the Henry Mountains) intrusive center where sedimentary strata have been deformed and upturned to make room for the central intrusive body. Previous work suggests that the Mt. Hillers intrusive center was constructed from multiple magma pulses. The primary objective of this project is to study the construction history of the Mt. Hillers intrusive center. A detailed geologic map of a key area on the eastern flank of Mt. Hillers shows that a network of sills and dikes extends outward from a central intrusive body into surrounding sedimentary strata. Some of these sills and dikes feed small-volume satellite intrusions in subhorizontal host rock away from the central body. Field fabric orientation, phenocryst shape preferred orientation, thin section petrography, and geochemical data provide information on the magmas that moved through conduits to small satellite intrusions. Field textures, crystal size distribution, petrographic characteristics, and trace element geochemistry show evidence of multiple magma pulses. These results allow the presentation of a hypothesized construction history for the Mt. Hillers intrusive center. In this model, the early magma pulses intruded as sills in flat lying sedimentary country rock; subsequent intrusions rotated and uplifted both the sedimentary strata and overlying prior intrusions. The satellite intrusions represent the latest magma phase and were fed through a network of conduits from the large central intrusion.