Interim Geologic Map of the West Part of - Utah Geological Survey

(Anderson, 1965; Spurney (1984); magnetite veins are present throughout theintrusion and are as much as 10 feet (3 m) in width, but most are less than oneinch (2.5 cm) wide (Spurney, 1984); base of laccolith is well exposed in the northcanyon wall of Little Creek, which has incised through the laccolith to revealnumerous feeder dikes; originally referred to as the Iron Point laccolith byAnderson (1965) and Anderson and Rowley (1975), as the namesake peak wasthen known, but the peak was renamed and is now referred to as Iron Peak;intruded at the stratigraphic level of Brian Head Formation and is preserved in agraben at the west edge of the Markagunt Plateau, about 5 miles (8 km) northeastof Paragonah; roof rocks are not preserved; yielded K-Ar whole-rock age of 19.7± 0.5 Ma (Fleck and others, 1975); exposed thickness is as much as about 800 feet(240 m).Forms the easternmost laccolith of the Iron Axis, a northeast-trending beltof early Miocene calc-alkaline laccoliths and concordant stocks that rose at about22 to 20 Ma above the roof of an inferred large batholith (Blank and Mackin,1967; Cook and Hardman, 1967; Rowley, 1998; Rowley and others, 1998); IronPeak is the second youngest and most mafic of the Iron Axis intrusions; most ofthe central quartz monzonite plutons appear to be partly controlled by northeaststriking,southeast-verging Sevier-age thrust faults and were emplaced at shallowdepths, mostly within about 1.2 miles (2 km) of the surface (Mackin and others,1976; Van Kooten, 1988; Hacker and others, 2002, 2007; Rowley and others,2006), but the Iron Peak laccolith exhibits no such structural control; like theother laccoliths in the belt, the Iron Peak laccolith probably formed rapidlyfollowing a two-stage emplacement process – injection of a sill immediatelyfollowed by inflation – at shallow crustal depth of less than 4000 feet (1.2 km)based on stratigraphic reconstructions (Spurney, 1984; see also Hacker and others,2002, 2007; Willis, 2002); rapid inflation of the laccoliths commonly led to partialunroofing by gravity sliding, immediately followed by volcanic eruptions(Mackin, 1960; Blank and Mackin, 1967; Hacker and others, 1996, 2002, 2007;Hacker, 1998; Willis, 2002), although it is unclear if the Iron Peak laccolithexperienced a similar history; Spurney (1984) interpreted exposures immediatelyeast of the Iron Peak laccolith as a peripheral breccia complex and describedvolcanic rocks of similar composition to the south in the adjacent Red CreekReservoir quadrangle that suggest that the intrusion erupted and produced lavaflows or block and ash flow breccias; Maldonado and others (in preparation),however, interpreted the eastern exposures as older Bear Valley breccia; ongoingmapping in the Red Creek Reservoir and Cottonwood Mountain quadrangles mayfurther elucidate the emplacement history of the Iron Peak laccolith.Emplacement of the Iron Peak laccolith was suggested as one possiblesource of the Markagunt megabreccia (Sable and Maldonado, 1997a), butAnderson (1993, 2001) suggested that the intrusion was too small to haveproduced such a large gravity slide; however, because the laccolith is onlyexposed in a graben, we do not know its original extent, particularly how far westit may hhave once reached; we mapped megabreccia deposits (here lumped withthe Markagunt megabreccia for lack of suitable criteria for differentiation) on the37