Volcanic rocks of the eastern and northern parts of the San Francisco volcanic field, Arizona

Journal of Research of the U.S. Geological Survey
By: , and 



The eastern and northern parts of the San Francisco volcanic field, between San Francisco Mountain and the Little Colorado River, contain about 175 cinder cones, many with one or more associated lava flows, and one center of silicic volcanism, O'Leary Peak. Basaltic flows and cones are divided into five groups, primarily on the bases of stratigraphic and physiographic relations, degree of weathering and erosion, K-Ar and tree-ring age determinations, and, in part, chemical and petrographic data:

Basaltic rocks of Sunset age --------  A.D. 1064-1065 Holocene.
Basaltic rocks of Merriam age -----  <100,100 years   Pleistocene.
Basaltic rocks of Tappan age -------  0.2-0.7 m.y.           Do.
Basaltic rocks of Woodhouse age --- 0.8-3.0 m.y.       Pliocene and
Basalt of Cedar Ranch --------------- 5.5 m.y.             Pliocene.


The flows and cones are predominantly alkali olivine basalt, commonly nepheline normative, and characterized by a single Ca-rich pyroxene phase and the absence of the olivine-pyroxene reaction relation. By depletion in olivine and pyroxene these basalts grade into alkali-rich high-alumina basalts; the most silicic of which contain both Ca-rich and Ca-poor pyroxene but show no olivine reaction. By relative enrichment in K2O and SiO2, alkali olivine basalts grade into intersertal basaltic andesites that contain, in addition to olivine, two pyroxene phases and minor primary cristobalite. Volcanic rocks of O'Leary Peak consist largely of rhyodacite domes, flows, and minor pyroclastic deposits. An andesite flow underlies the rhyodacites and andesitic cinders mantle the rhyodacite porphyry dome that forms O'Leary Peak. This dome, 0.23±0.04 m.y. old, is both underlain and overlain by Tappan-age basaltic rocks. Variation diagrams that include data from the dominantly andesitic to rhyolitic San Francisco Mountain as well as from the mapped area show the differentiated and consanguineous character of the lavas, which are chemically intermediate between typical calc-alkalic and alkalic suites. Sr87/Sr86 ratios of 0.7026 to 0.7050 suggest a mantle origin with little contamination by upper crustal material. Ultramafic and mafic xenoliths with cumulus texture may represent crustal intrusions related to the volcanic rocks. Eruption of intermediate to silicic magmas was generally localized in a few centers such as San Francisco Mountain and O'Leary Peak. In the same general time interval, basaltic eruptions dominated in the surrounding areas.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Volcanic rocks of the eastern and northern parts of the San Francisco volcanic field, Arizona
Series title Journal of Research of the U.S. Geological Survey
Volume 4
Issue 5
Year Published 1976
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Description 12 p.
First page 549
Last page 560
Country United States
State Arizona
Other Geospatial San Francisco Mountain, Little Colorado River
Online Only (Y/N) N
Additional Online Files (Y/N) N
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