RECOGNITION OF A LARGE VOLCANIC LAVA DOME WITHIN THE EARLY MIOCENE PINE VALLEY LATITE, PINE VALLEY MOUNTAINS, SOUTHWEST UTAH

The Pine Valley Latite (~20.5 Ma) is a voluminous (77+ km3) series of porphyritic lava flows that erupted from several vent areas in the northern Pine Valley Mountains. The latite contains 45-60% phenocrysts (65-75% plagioclase, 5-10% sanidine, 5-10% biotite, 6-15% pyroxene, 1-2% Fe-Ti oxides, and trace hornblende) in a glassy to stony groundmass. The uniformity of mineralogy and texture within the lava led Cook (1957) to originally map the flows as a single unit. More recently the latite mass has been divided into two flow members based on differences in flow structures and color (Hacker, 1998). The slightly older Rencher Peak flow to the north apparently formed a lava dome type flow above a central vent area (at Rencher Peak) and is partially covered by the Timber Mountain flow to the south that emanated from the comagmatic Pine Valley laccolith following its deroofing by gravity sliding. Current field mapping (1:24,000) in conjunction with analysis of 1:16,000 stereoscopic aerial photographs, LANDSAT 7 imagery, and 3D perspective views of analyzed data sets (from 1:24,000 USGS DEM), confirms and adds to the lava dome interpretation of the Rencher Peak flow member. The unit forms a single thick flow over 11.3 km long (E-W) and 6.1 km wide (N-S), with a preserved thickness of over 450 m. The flow consists of light- to dark-gray and purple porphyry above a 3 to 50 m thick basal dark-gray to black vitrophyre that grades laterally into a massive flow breccia. The lava emanated radially from the Rencher Peak area, which is the present highest preserved surface elevation of the flow. Flow foliations dip radially outward from this area. Farther away they dip moderately to steeply back toward the peak. This pattern of flow can best be observed on aerial and satellite images where concentric surface ridges (or ogives) wrap around the vent area in a nearly circular pattern. Differential weathering of dipping foliation and parting planes that are mostly concave in the upflow direction form the ridges. In addition, these surface ridges outline the presence of a large lava lobe (or coulee type flow) extending to the SW from the lava dome area with a preserved length of over 6 km. The Rencher Peak flow does not appear to be directly related to the main Pine Valley laccolith, but it is possible that the flow discharged from a smaller unknown and buried laccolith.