The Galiuro Volcanics occurs in the Galiuro, Winchester, and Little Dragoon Mountains, east and northeast of Tucson, Ariz. The sequence comprises lava flows and ash-flow tuffs ranging in composition from andesite to rhyolite. In general they can be subdivided into two parts separated by a major erosional unconformity. The lower part is predominantly lava flows ranging from andesite to rhyodacite but contains three ash-flow tuffs. The upper part is chiefly ash-flow tuff but also includes two areas of rhyolite-obsidian flows and domes and rhyolitic to andesitic flows. Conglomerate separates many if not all of the rock units in the upper part. The individual flows and tuffs are lenticular, and the stratigraphic relations are complex. Chemical variation diagrams suggest consanguinity among all the volcanic rocks, but the complex intercalation of rhyolite and andesite and of lava flows and ash-flow tuff indicates more than one magma chamber, different stages of differentiation in separate magma chambers, and several eruption centers. Chemical analyses indicate that the magmas were normal calc-alkaline. Eleven potassium-argon age determinations indicate that the Galiuro Volcanics accumulated from about 29 to 23 million years ago, which is in the middle of the mid-Tertiary volcanic and plutonic event in Arizona.
