Patterns of volcanism along the southern margin of the Colorado Plateau: The Springerville Field

Abstract

The mostly 2.1‐ to 0.3‐m.y.‐old Springerville volcanic field is the southernmost of the Colorado Plateau‐margin fields and encompasses −3000 km2, with a volume of −300 km3. Mapping documents 409 flow units, most emanating from one of the −400 predominantly pyroclastic cones. Alkali olivine basalt constitutes about one‐half the outcrop area, with hawaiite and tholeiite each composing about a quarter of the area, and mugearite and benmoreite each covering less than 1% of the area. The composition of the lavas shows a progression away from tholeiitic and toward alkalic affinities with decreasing age, in marked contrast to that found in both the southeast and western margins of the Colorado Plateau. Estimates of volume effusion rates show an early eruptive episode dominated by large volumes of tholeiitic lavas, a middle period of alkali olivine basalt eruption, and a late pulse of evolved alkalic rocks. Changes in chemistry [Na2O + K2O, MgO/(MgO + FeO*), K/P] with respect to location of lava sources that might reflect thermal and/or chemical differences appear to be minimal. Eastward migration of alkali olivine basalt volcanism within the Springerville volcanic field is essentially identical to that seen in the San Francisco volcanic field for the same time period, supporting the suggestion of Tanaka et al. (1986) that it may reflect the westward motion of the North American continent above relatively fixed mantle sources. The late Cenozoic migration of volcanic fields toward the Colorado Plateau suggested by Best and Hamblin (1978) and by Luedke and Smith (1978) does not seem to be reflected by vent migrations from 2.1 Ma to the present within the Springerville and San Francisco fields.