COMPARISON OF THE PETROLOGY OF BASALTS ALONG A LEFT LATERAL FAULT, JOSHUA TREE NATIONAL PARK, SOUTHERN CALIFORNIA

Olivine basalt stocks and flows, locally xenolith-bearing, crop out in Joshua Tree National Park along the left lateral Blue Cut fault, within the San Andreas fault system. A basalt flow from Pinto Well, at the east end of the Blue Cut fault, is dated at 4.5 ± 0.29 Ma (Calzia et al., 1986); none of the other stocks or flows are dated. The objective of this project is to determine the diversity of mineral and rock chemistry within this suite of volcanic rocks. Based on normative mineralogy, the stocks and flows are alkali olivine basalts, enriched in high charge elements compared to nearby suites of similar age such as Cima and Amboy. Overall, chemical compositions of these basalts are most similar to ocean island basalts such as Hawaii rather than mid-ocean ridge or arc basalts. Of the five study sites, two contain xenoliths, which are mostly lherzolite. Olivine in xenoliths and xenocrysts ranges from Fo90 to Fo71; chemical analyses indicate that the interiors of these crystals are Fo90 to Fo83 and are enriched in NiO. As the olivine rims are approached, NiO decreases and magnesium content decreases to Fo78 to Fo71. Phenocrysts and matrix olivine are chemically similar, with a composition of Fo84 to Fo69. These data suggest that the xenoliths are a magnesium-rich contaminant; however, although similar to other sites along the Blue Cut fault, basalts at the two localities that contain lherzolite xenoliths are actually relatively iron rich. Further field work and chemical analyses are being conducted in order to understand this relationship.