Composite dike of andesite and rhyolite at Klondyke, Arizona

A composite dike of probable Tertiary age intrudes Precambrian granodiorite 6 miles north of Klondyke, Arizona. The dike is exposed discontinuously for about 1500 feet along the strike and has a core of porphyritic rhyolite 15-20 feet thick flanked by coarsely porphyritic andesite 1-2 feet thick. Field evidence indicates that the rhyolite is later than the andesite but that the core of the original andesite dike was still hot and unconsolidated at the time of intrusion of the rhyolite. Chemically, the rhyolite is nearly identical to a large alkali granite pluton of Tertiary age exposed 1 mile east. The andesite component is similar both petrographically and chemically to lavas exposed in the region, but a direct relationship could not be established. Meager evidence suggests that the two dike components were derived from separate magma bodies rather than being differentiates of a single magma.