THE EIGHT MILE DACITE, PINE VALLEY MOUNTAINS, SOUTHWEST UTAH: EMPLACEMENT OF LAVA DOMES ALONG A FISSURE SYSTEM

The Eight Mile Dacite (of Pleistocene? age) is a light-bluish-gray to brown porphyritic lava with 40% phenocrysts of plagioclase (75%, as long as 1 cm), hornblende (23%), and Ti-Fe oxides (2%) set in a glassy groundmass containing abundant microlites. The average major oxide concentrations (wt. %) of three samples are: SiO2=67.15, TiO2=0.47, Al2O3=14.7, Fe2O3=3.2, MgO=1.02, CaO=3.05, Na2O=3.42, K2O=3.15, MnO=0.53, LOI=2.07, and confirms the composition as dacite. Flow structures are well preserved in the form of platy partings that locally exhibit strong folding, some being recumbent. The lava is exposed in four isolated outcrops aligned along a 15 to 20 degree northeast trend over a total distance of 5 km. The outcrops are more or less oval in plan, ranging from ~0.01 km2 to ~1.65 km2 with the long axes aligned along the same northeast trend. Earlier studies considered the outcrops to be erosional remnants of a single lava flow that once filled a paleovalley descending southwest from Eight Mile Spring. However, current field analyses of the flow structures suggest the outcrops are individual lava domes. Flow foliations dip radially outward from, or inward toward, an area in each dome that represents a vent location. Over the vent area on the smallest dome, the foliations are vertical and the platy parting surfaces show vertical lineation grooves that indicate vertical extrusion, as well as the viscous nature of lava. The dome at the highest elevation, near Eight Mile Spring, shows extensive flow brecciation zones, with clasts up to boulder size, around its southern periphery (its main flow direction) that are not present in the other domes located at lower elevations. The northeast-southwest alignment of the domes suggests they formed along a fissure similar to those of nearby basaltic cinder cones of the same age that are also aligned along NE-SW trends. Thus the Eight Mile Dacite and basalts represent bimodal magmatism that erupted lavas along fissures of the same orientation produced by Basin and Range crustal extension. Basin and Range tectonism produced NE-SW and NW-SE trending fault systems in this part of the Pine Valley Mountains with volcanism occurring predominantly along the NE-SW fault segments.