RAPID EMPLACEMENTS OF MIOCENE IRON AXIS LACCOLITHS, SW UTAH

Studies of large and broadly homogenous plutons indicate that they can accumulate incrementally over several million years (e.g., Glazner et al., 2004). However, a growing number of recent studies indicate rapid emplacement in shorter time frames on the order of hundreds to thousands of years (e.g., Habert et al., 2006; Clemens and Mawer, 1992). We present evidence from the laccoliths of the Iron Axis that support rapid emplacement of large single batch plutons into the shallow crust. The Iron Axis consists of a NE-trending belt of calc-alkaline plutons whose trend is partly controlled by older NE-striking, SE-verging Sevier-age thrust faults. Intrusions were forcibly emplaced within 3.0 to 0.2 km of the surface and formed structural laccoliths of high relief, with some roofs failing and producing large gravity slides and eruption of ash-flow tuffs and lava flows. Laccoliths appear homogeneous in composition and lack evidence of chemical or pertrographic zoning or visible presence of internal contacts. Paleomagnetic and AMS fabric studies on several laccoliths support field evidence of the lack of multiple intrusions. Magmatism in the Iron Axis occurred in three distinct pulses distinguished by field relations, petrographic and geochemical characteristics, and geochronologic data. The first pulse included the main cluster of quartz monzonite laccoliths associated with older Sevier thrust faults. Concordant 40Ar/39Ar dates from seven plutons and three ash-flow tuff products show emplacement between 22.0 and 21.76 Ma (<0.09 Ma duration). These laccoliths formed immediately following the eruption of the compositionally identical Harmony Hills Tuff (22.03 Ma) from an unidentified caldera. The second pulse (after 1.3 Ma hiatus) included the quartz monzonite Pine Valley laccolith (>300 km2) at 20.46 Ma and the gabbro-diorite Iron Peak laccolith at 20.2 Ma. Nine concordant 40Ar/39Ar dates from the Pine Valley intrusion and its lava flows indicate rapid emplacement (<0.03 Ma duration). The third pulse included the granitic Mineral Mountain laccolith at 12.1±1.9 Ma and the associated Ox Valley Tuff with compositions closer to Caliente caldera complex magmas. The affinity of these laccoliths to caldera-forming eruptions probably reflects large power inputs from sources below, thus resulting in rapid emplacement of the shallow plutons.