Heavy-mineral suites confirm the wide extent of the Peach Springs Tuff in California and Arizona, U.S.A.

Abstract

The Peach Springs Tuff of Young and Brennan (1974) is a voluminous ash-flow tuff of early Miocene age that has been identified in western Arizona and the Mojave Desert of southeastern California. The tuff was recognized and described in the areas of Peach Springs and Kingman, Arizona, on the western Colorado Plateau by Young (1966), and Young and Brennan (1974). Glazner et al. (1986) and Wells and Hillhouse (1986) have recently proposed that the Peach Springs Tuff may correlate with ash-flow tuff occurring to the west across the Mojave Desert as far as the area of Barstow, California. If the correlation is correct, the Peach Springs Tuff would be the only Miocene unit of wide areal extent in this region and would cover an area of approximately 35,000 km 2. Thus, the Peach Springs Tuffwould be invaluable for use as a stratigraphic marker and for determining the timing and amount of Tertiary crustal extension. However, ash-flow tuffs are commonly difficult to correlate due to lateral and vertical variations in welding, mineralogy, and chemistry, complex depositional mechanisms, and other complicating factors (Hildreth and Mahood, 1985). Proposed identification of the Peach Springs Tuff over this extensive area has been based primarily on field relations such as its lower Miocene stratigraphic position, and phenocryst assemblage which includes abundant sanidine and lesser amounts of biotite, hornblende, plagioclase and sphene. Additional criteria used for correlation include phenocryst composition and paleomagnetic directions ( Glazner et al., 1986; Wells and Hillhouse, 1986). Inconsistent K-At dates from localities of the Peach Springs Tuff and its proposed equivalents have left nagging uncertainties regarding this correlation. The source of the tuff is not known. In addition, correlation of some localities has been uncertain because of limited and widely scattered outcrop preservation, and the varied stratigraphic relations of the tuff within the numerous isolated Tertiary stratigraphic sections. We report the results of a study of relative abundances of heavy minerals to characterize and distinguish ash-flow