THE CHEMISTRY AND MINERALOGY OF MAFIC DIKES IN THE LOOKOUT MOUNTAIN - WEST POINT AREA, SKAMANIA COUNTY, WASHINGTON: FEEDERS FOR THE BASALTIC ANDESITE OF ELKHORN MOUNTAIN?

A chemically coherent swarm of mafic dikes occurs in the area between Lookout Mountain and West Point in Skamania County of southwestern Washington, ~ 35 km south of Mount St. Helens. The dikes range in composition from basalt to andesite, are uniformly tholeiitic, and have K₂O contents between 0.1 to 0.8 weight percent. They are moderately to highly porphyritic, with phenocrysts of plagioclase and olivine; many also contain phenocrysts of augite and (or) hypersthene. Groundmass ranges from microcrystalline to medium-grained; is intergranular, pilotaxitic, or trachytic in texture; and consists of plagioclase, augite, and Fe-Ti oxide. Primary mineralogy is variably replaced by minerals such as quartz, calcite, zeolites, and clays; typical of the local character of zeolite-facies burial metamorphism. An average magnesium number of 45 suggests a moderately evolved magma, while incompatible-element ratios indicate that the dikes are probably related by fractional crystallization from a common parental source.

Chemical and mineralogical similarities suggest that these dikes may have been feeders for a nearby package of Oligocene lava flows: the basaltic andesite of Elkhorn Mountain. Feeder dikes for this extensive unit have not been previously identified. These flows have been removed from the study area due to an apparent local uplift of greater than 1,000 meters since their emplacement between 27 to 25.5 Ma. The distribution of the feeder dikes thus exposed by this exhumation indicates that the original extent of the basaltic andesite of Elkhorn Mountain was in excess of 1,200 km² and 530 km³, and that it was erupted from a multitude of coalesced vents; herein informally named the Elkhorn Mountain Volcanic Field. Contemporary Cascadian volcanic fields of similar size and chemistry, such as the Medicine Lake Volcanic Field, are thought to form in extensional environments due to decompression melting of shallow MORB-source mantle, rather than by slab-fluid-flux melting that is typical of continental volcanic arcs.