DEPTH-DEPENDENT VARIATIONS IN TEXTURE AND PHENOCRYST MINERALOGY IN A LARGE DACITE DIKE, LITTLE CHUCKWALLA MOUNTAINS, CALIFORNIA

The Little Chuckwalla Mountains (LCM), located in Riverside County, California, expose Tertiary volcanic rocks ranging from basalt to rhyolite (Crowe et al., 1979; Mayo, 1993; Mayo and Cole, 2001). These rocks demonstrate the widespread volcanism that was prevalent approximately 25 million years ago when the Farallon plate was being subducted beneath the North American plate (Burchfiel and Davis, 1972; Dickinson, 1981). Swarms of dacite and rhyolite dikes crosscut andesitic lavas in the lower part of the section to feed lava and ash deposits in the upper part of the section. The largest dacite dike is approximately 100 m thick and exposed along strike for 2,250 m. Tilting of 30° around an axis perpendicular to the dike permits sampling of dike rock at paleodepths ranging from 0 to about 1,100 m. Plagioclase laths in the trachytic groundmass increase in abundance and size with depth. Common phenocrysts include plagioclase + biotite + opaques + sanidine + augite + hornblende + fayalite. Specific phenocryst assemblage and texture vary with paleodepth. Plagioclase, biotite, sanidine and augite are present at all paleodepths. Fayalite is only observed in the surface lavas. Hornblende is rare and only found at the deepest 1,100 m paleodepth. The apparent replacement of hornblende at 1,100 m by fayalite in surface lavas records the drop in water pressure as the magma approached the surface. Similar phase changes have been observed in experiments with H₂0-undersaturated granitic magma (Naney, 1983).