VOLCANOES OF JOSHUA TREE NATIONAL PARK, SOUTHERN CALIFORNIA

Late Tertiary volcanic centers in Joshua Tree National Park lay near and along the left lateral Blue Cut and Victory Pass faults, within the San Andreas fault system. All of the centers are composed of alkali olivine basalt, enriched in high charge elements compared to nearby basalt suites of similar age such as Cima and Amboy. Most samples are depleted in Mg, Cr, and Ni and are probably significantly fractionated compared to primitive basalt in this region. Centers in the western part of the park are massive and contain ultramafic xenoliths, which are predominantly lherzolite. Xenolith and xenocryst olivine range from Fo90-71; the interiors are Fo90-83 and enriched in NiO. As the rims are approached, NiO decreases and Mg content decreases to Fo78 -71. Phenocrysts and matrix olivine are chemically similar with a composition of Fo84-69. Although the xenoliths are magnesium-rich, the host basalts are iron-rich in comparison to xenolith-free, vesicular basalts in the eastern part of the park. Measured SrIC in basalts range from 0.7031 to 0.7089, and positively correlate with Sr, Ba, and P. Measured carbonate concentrations are uncorrelated with Sr concentrations and SrIC, indicating that isotopic variability is not the result of weathering contamination. Although the variable and locally high SrIC could reflect crustal contamination, the undersaturated basalt compositions and the lack of correlation between SrIC and silica severely limits crustal assimilation. We infer that the eastern, more radiogenic samples had a source in comparatively old lithospheric mantle, which is generally inferred to have been excised beneath the crust in this region in Laramide time. The basalts lie across the southern extension of the isotopic boundary identified in Mojave Miocene volcanic rocks by Miller et al (2000), suggesting that the variations we observe are imaging the same north-south oriented mantle compositional boundary.