VOLCANIC STRATIGRAPHY AND STRUCTURES ASSOCIATED WITH THE ERUPTION OF THE EARLY MIOCENE PINTO PEAK LACCOLITH, IRON AXIS MAGMATIC PROVINCE, SOUTHWEST UTAH

The Pinto Peak laccolith is located in the northern Pine Valley Mountains and belongs to a northeast-trending belt of Miocene (22 to 20 Ma) plutons, of mostly quartz monzonite porphyry, known as the Iron Axis. Most volcanic activity during this magmatic episode is represented by a sequence of ash-flow tuffs and lava flows mapped by previous workers as belonging to the Rencher Formation and originating from a laccolithic complex in the Bull Valley Mountains to the west. Detailed mapping of the stratigraphy and structures of the Pine Valley Mountains shows the Pinto Peak laccolith also underwent concurrent volcanism (Hacker, 1998). These volcanic rocks are here informally named the “rocks of Paradise” and separated from the Rencher Formation. Field evidence also shows the previously defined Paradise intrusion (Cook, 1957) to be a thick lava flow derived from the Pinto Peak laccolith.

The ~ 400 m thick composite section of the rocks of Paradise includes from bottom to top: 1) a poorly to moderately welded, crystal-rich (25-35%), dacitic ash-flow tuff, 90 m thick (thinning to 8 m 15 km to the SW), consisting of at least five flow units locally separated by thin (2 m) of pyroclastic fall and surge deposits containing bomb sags and slump features; 2) a resistant crystal rich (30-45%) dacitic, prorphyritc lava flow unit consisting of a 120 m thick lower gray glassy flow, locally flow brecciated, and a 180 m thick upper light to dark red devitrified flow; and 3) a poorly resistant, 8 m thick, cross-bedded eolian sandstone. The ash-flow tuff and lava flow units can be found in a vent area (300 m diameter) on the east side of Pinto Peak. The vent phase consists of ~ 10 m of tuff and tuff breccia intruded by vertical dikes of vertically flow banded porphyritic glass that grades down into intrusive quartz monzonitic porphyry of the Pinto Peak laccolith.

The Pinto Peak laccolith (~ 3 km² exposed) intruded from the north into the sedimentary Claron Formation at a depth of ~ 500 m. On the south flank of the laccolith, the ash-flow tuff unit is underlain by a 2.5 km long, 100 m thick mass of brecciated pre-intusive volcanic and sedimentary rocks of the roof sequence. This mass represents a gravity slide initially sloughed from the uplifted flank of the laccolith just prior to the eruption of ash flows suggesting sliding may have initiated volcanism of the laccolith.