RED-ROCK COUNTRY HAS BLACK ROCKS TOO: NEPHELINITE, BASANITE, AND BASALT BELOW THE MOGOLLON RIM IN VERDE VALLEY, ARIZONA

Field-based geologic mapping has identified 67 small volcanoes and intrusions and one large central volcano in a 15 km by 40 km area on and near the Mogollon Rim in Verde Valley, Arizona. Ages are middle to late Miocene. Geographic coherency of the volcanic structures and lava compositions that are markedly undersaturated in silica characterize the volcanic area, which extends into the Arizona Transition Zone as an alkaline appendage of the late Cenozoic basaltic volcanic fields on the southern Colorado Plateau.

The small volcanoes are simple structures composed of small-volume lava flows above thin deposits of cinders. Intrusions include plugs and dikes. Plugs range from 80 m to 400 m in diameter, and dikes are up to 1,100 m long and 70 m thick. The most common lithology is basanite (45 percent of the small volcanic structures), followed by basalt (30 percent) and basanitic nephelinite (24 percent); basalts include alkaline and subalkaline types. Felsic derivative magmas range from highly to slightly undersaturated in silica in conformity with respective mafic parent magmas.

The large central volcano is House Mountain, a middle Miocene shield volcano composed mostly of subalkaline basalt cinders and lava flows of basaltic andesite, hawaiite, mugearite, and benmoreite. Some, and possibly all, of the highly alkaline lavas postdate the shield volcano. It erupted near the center of the volcanic area, but southeast of the greatest concentration of vents.

Rows of vent structures and the long axis of the volcanic area are collinear with northwest-trending faults in Verde Valley. Some of the nephelinites erupted coevally with movement on the Verde fault around 8 Ma. A causal relationship of extensional tectonics and volcanism is indicated.