TECTONIC SETTING OF BASALT MAGMATISM ALONG LEFT LATERAL FAULTS IN 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 and Victory Pass faults, 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 and flows are dated. Paleomagnetic data (Carter et al., 1987) indicate that, as movement occurred along the dextral San Andreas fault, these smaller now east-west trending faults accommodated clockwise rotation of intervening crustal blocks. 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. Chemical compositions are most similar to ocean island basalts such as Hawaii rather than mid-ocean ridge or arc basalts. Most samples have low Mg, Cr, and Ni and are probably significantly fractionated compared to primitive basalt in this region. Two of the basalts contain ultramafic xenoliths, which are mostly lherzolite. Although the lherzolite xenoliths are magnesium- rich, the host basalts are relatively iron-rich compared to xenolith-free basalts. Minor and trace element abundances suggest that the source of all of these basalts lies within the asthenospheric mantle. Enrichment in large elements such as Ba and Sr, and high charge elements such as Nb, Ti, and P are not representative of a lithospheric source. Isotopic analyses are in progress to test the timing of large and high charge element enrichments, and the extent of contamination by lherzolitic upper mantle or silicic crust that may have occurred during ascent, fractionation and eruption.