LATE CENOZOIC BASALTIC VOLCANIC STRUCTURES IN AND NEAR PETRIFIED FOREST NATIONAL PARK, NORTHEASTERN ARIZONA

Two small Cenozoic basaltic volcanic structures (Pintado Point and Pilot Rock) are exposed within Petrified Forest National Park (PFNP), and a third (Black Knoll) is exposed just outside the park boundary to the east. These three structures have been interpreted by various workers as outliers of either the Hopi Buttes Volcanic Field to the northwest, the San Francisco—Mormon Mountain Volcanic Field to the west, or the Springerville—White Mountains Volcanic Field to the southeast.

Ar-Ar dating of a massive flow unit from each of these three bodies yields 4.67 ± 0.66 Ma for the Pintado Point structure, 5.0 ± 1.4 Ma for the Pilot Rock structure, and 5.0 ± 1.5 Ma for the Black Knoll structure. These ages do not preclude the PFNP structures being outliers of the San Francisco—Mormon Mountain or Springerville—White Mountains volcanic fields, but they are most consistent with the timing of the major volcanic activity in the Hopi Buttes field.

Geochemical analysis of seven samples from the Pintado Point structure and five samples each from the Pilot Rock and Black Knoll structures, all taken from massive flow units, reveal that these three bodies are remarkably homogeneous. Silica content for these seventeen samples varies only from 40.6 to 45.1 wt.%; CaO varies from 10.9 to 12.2 wt.%; MgO from 10.7 to 14.3 wt.%; total iron as FeO from 10.3 to 11.1 wt.%; and total alkalis from 2.6 to 3.6 wt.%. On standard Harker-style and AFM variation diagrams, these compositions appear consistent with the PFNP magmas being parental to many of the lavas of both the San Francisco—Mormon Mountain and the Springerville—White Mountains volcanic fields, whereas the possible geochemical relationship between these magmas and the lavas of the Hopi Buttes field is more problematic. Analysis of minor and trace elements (Cr, V, Rb, Sr, REEs) yields no additional discrimination.