EMPLACEMENT AND GROWTH OF A SHALLOW-LEVEL MEGA-LACCOLITH: THE PINE VALLEY INTRUSION, SOUTHWEST UTAH

The early Miocene Pine Valley intrusion (~20.5 Ma) is the largest and most enigmatic igneous mass within the NE-trending Iron Axis group of laccolithic intrusions. The exposed main part of this monzonitic porphyry body is 30 km long, 11 km wide, and as much as 900 m thick. Geologic mapping indicates that this gigantic igneous mass capping the Pine Valley Mountains is a laccolith that originally occupied an area of ~800 km², as delineated by erosional outliers, subsurface extensions, and sedimentary debris flow deposit patterns. The concordant floor of the intrusion and peripheral deformation of the wall rocks demonstrates forceful emplacement of the intrusion as a voluminous single magma batch beneath an extremely thin cover (≤200m). Wall rocks are preserved as remnants of highly-attenuated and overturned peripheral flexures, and in gravity slide masses (as much as 10 km²) that detached from the oversteepened roof. The peripheral structures and gravity slides are partially covered by lava flows of the Pine Valley Latite that erupted from the intrusion. Field data suggest that the magma inflated the laccolith from the bottom up by injecting new magma to the bottom of the intrusion as sills and lifting the magma mass above it. An emplacement model for this gigantic laccolith shows continuous development from (1) initial rapid sill emplacements to their full lateral extent (~ 45 by 20 km) within the upper Claron Fm., (2) vertical inflation by incremental sill injections at its base, (3) gravity sliding from the upturned roof and extrusion of lava of about 40 km³ over the flanking structures, and (4) continued vertical inflation to about 1 km. The Pine Valley “mega-laccolith”, at ~350-400 km³, greatly exceeds the typical size of recorded laccoliths. Its large size and shallow depth of emplacement indicate very rapid development. ⁴⁰Ar/³⁹Ar ages indicate a total duration of emplacement of ~100 kyr and a magma supply rate of ~0.11 m³/s.