DEVELOPMENT OF THE MINERAL MOUNTAINS SHEET COMPLEX, MINERAL MOUNTAINS BATHOLITH, SOUTHWESTERN UTAH

Field, structural, and petrographic studies suggest that the west-central portion of the 18 Ma Mineral Mountains batholith in southwestern Utah was emplaced as a sheet complex that developed via incremental injection of compositionally diverse magmas. The sheet complex extends laterally for 14 km parallel to the north-south trending margin of the pluton and occupies an ~2 km-thick section between Precambrian basement rocks to the west and homogeneous granite to the east. Its northern exposures consist of Miocene-age screens of gabbro, diorite, and coarse- and fine-grained granite that vary in thickness from <1 to >800 m. Contacts dip to the east 45-70º and range from gradational to sharp, with the sharpest contacts located at mafic/felsic and fine-/coarse-grained boundaries. Gradational contacts often display evidence for physical mixing of granite and hornblende-rich gabbro. Its southern exposures consist of 100-400 m-thick swaths of foliated hornblende granodiorite and coarse- and fine-grained granite (all of Miocene age) that separate thinner screens of Precambrian gneiss and schist. Foliation in both the Precambrian metamorphic rocks and hornblende granodiorite dips 45-70º to the east.

The uniformity of foliation and sheet orientation across the field area, along with the presence of distinct sheet boundaries, indicate that: 1) the intrusive sheets were likely emplaced either as parallel dikes or as horizontal layers that were subsequently tilted, 2) a direct relationship may exist between sheet orientations and gneiss/granodiorite foliations (i.e., foliation in the Precambrian rocks is pre-Miocene and served as planes of weakness along which sheets intruded, or the foliation is Miocene and reflects synintrusive strain), and 3) at least the western portion of the pluton was emplaced via the intrusion of small magma pulses, implying that the active fractionating/mixing magma chamber was likely not as large as the current pluton. Ongoing paleomagnetic and geochronological studies will test whether the western margin of the pluton is tilted and discern the time period over which the sheet complex grew.