Age of the Peach Springs Tuff, southeastern California and western Arizona
- Geological Survey, Menlo Park, CA (United States)
- Univ. of Maine, Orono (United States)
- Univ. of North Carolina, Chapel Hill (United States)
Sanidine separates from pumice of the early Miocene Peach Springs Tuff (PST) are concordantly dated at 18.5 {plus minus} 0.2 Ma by two isotopic techniques. The PST is the only known unit that can be correlated between isolated outcrops of Miocene strata from the central Mojave Desert of southeastern California to the western Colorado Plateau in Arizona, across five structural provinces, a distance of 350 km. Thus the age of the PST is important to structural and paleogeographic reconstructions of a large region. Biotite and sanidine separates from bulk samples of the PST from zones of welding and vapor-phase alteration have not produced consistent ages by the K-Ar method. Published ages of mineral separates from 17 localities ranged from 16.2 to 20.5 Ma. Discordant {sup 40}Ar/{sup 39}Ar incremental release spectra were obtained for one biotite and two of the sanidine separates. Ages that correspond to the last gas increments are old as 27 Ma. The {sup 40}Ar/{sup 39}Ar incremental release determinations on sanidine separated from blocks of PST pumice yield ages of 18.3 {plus minus} 0.3 and 18.6 {plus minus} 0.4 Ma. Laser fusion measurements yield a mean age of 18.51 {plus minus} 0.10. The results suggest that sanidine and biotite K-Ar ages older than about 18.5 Ma are due to inherited Ar from pre-Tertiary contaminants, which likely were incorporated into the tuff during deposition. Sanidine K-Ar ages younger than 18 Ma probably indicate incomplete extraction of radiogenic {sup 40}Ar, whereas laser fusion dates of biotite and hornblende younger than 18 Ma likely are due to postdepositional alteration.
- OSTI ID:
- 5010144
- Journal Information:
- Journal of Geophysical Research; (United States), Journal Name: Journal of Geophysical Research; (United States) Vol. 95:B1; ISSN 0148-0227; ISSN JGREA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580000* -- Geosciences
AGE ESTIMATION
ALKALI METAL ISOTOPES
AMPHIBOLE
ARGON 39
ARGON 40
ARGON ISOTOPES
ARID LANDS
ARIZONA
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
BIOTITE
CALIFORNIA
CENOZOIC ERA
COMPARATIVE EVALUATIONS
CORRELATIONS
DESERTS
DEVELOPED COUNTRIES
DISTANCE
EVALUATION
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FEDERAL REGION IX
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC STRATA
GEOLOGIC STRUCTURES
HORNBLENDE
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MEASURING METHODS
MICA
MINERALS
MIOCENE EPOCH
NORTH AMERICA
NUCLEI
POTASSIUM ISOTOPES
RADIOISOTOPES
SILICATE MINERALS
STABLE ISOTOPES
TERTIARY PERIOD
TUFF
USA
YEARS LIVING RADIOISOTOPES