ABSTRACT
The lower Miocene volcanic and epiclastic unit at Picacho State Recreation Area consists of, from stratigraphically lowest to highest, the Quechan volcanics, Rojo Grande unit, Marcus Wash unit, White Wash unit, Walker volcanics, and the ignimbrite of Ferguson Wash. Published laser-ablation U-Pb zircon ages for the Quechan volcanics, Walker volcanics, and ignimbrite of Ferguson Wash are 23.4 ± 0.4 Ma, 23.5 ± 1 Ma, and 23.2 ± 0.2 Ma respectively. These ages suggest that the entire lower Miocene volcanic and epiclastic unit was likely erupted in a very narrow time interval about 23 Ma and perhaps in as short of a period as about 1 million years. The Quechan and Walker volcanics and the ignimbrite of Ferguson Wash have been studied extensively by previous students, and the major and trace element chemistry of the Rojo Grande unit was studied most recently by T. Pelbath. Though all previous studies recognized that the lower Miocene volcanic and epiclastic unit was altered extensively by hydrothermal solutions, none evaluated the effects of such alteration on the rare earth elements. Here I report the results of REE analyses of about 30 samples collected from the Rojo Grande unit and from the fine-grained ignimbritic part of the White Wash unit. On chondrite-normalized REE diagrams, samples from the Rojo Grande unit display light REE enrichment patterns that are nearly identical to those determined by H. Olson for the Quechan volcanics. Such patterns are commonly interpreted to be the result of low degrees of partial melting in the garnet stability field. This field is commonly cited as lying between 60 to 80 km depth. In addition, published reconstructions of the Miocene crustal thickness in and around Picacho suggest crustal thicknesses around 45 km. Hence, magma that eventually generated the Rojo Grande unit likely originated within the sub-continental crustal lithosphere. In contrast to the Rojo Grande unit, the fine-grained ignimbrite in the lower part of the White Wash unit, is the most chemically evolved of any unit within the lower Miocene volcanic and epiclastic unit. In addition, chondrite-normalized REE distribution patterns reveal a large negative europium anomaly. These data suggest that magma that erupted as the ignimbrite originated with the plagioclase stability field at depths that were likely less than about 30 km.