THE HETEROGENEITY OF SOUTHERN CALIFORNIA MANTLE: PETROLOGIC CLASSIFICATION AND GEOCHEMICAL ANALYSIS OF MANTLE XENOLITHS FROM JOSHUA TREE NATIONAL PARK

Southern California has been in an active plate margin setting since the Permian, and is likely to have had a very dynamic mantle history. Episodes of normal subduction, shallow subduction, slab breakoff, and the transition to transform faulting may have imparted different scales of heterogeneity to the lithospheric mantle and asthenosphere. A series of xenolith-bearing basalt dikes and flows exposed along the San Andreas fault in Joshua Tree National Park provide a northwest to southeast transect of southern California mantle in Pliocene time. The sites contain Group I xenoliths, dominated by spinel lherzolite (54%), but are multi-modal including harzburgite (31%), dunite (7.5%), and wehrlite (7.5%). Mantle-derived olivine ranges Fo_91-87, and clinopyroxene (cpx) ranges En_88-83.

We analyzed the minor and trace element contents of 115 cpx grains from 30 mantle xenoliths using the SHRIMP-RG ion microprobe; this type of analysis is utilized at different scales to determine the extent of chemical heterogeneity of these samples using Principle Components Analysis (PCA). Multiple analyses determine on what scale(s) the xenoliths differ; site to site (regional heterogeneity), xenolith to xenolith within a site or grain to grain within a xenolith (local heterogeneity and/or reequilibration during ascent), or within a grain. PCA of preliminary data detects few differences in cpx grains from site to site within the study area. These results are similar to those of other studies (Stull and McMillan, 1973; Wilshire et al., 1991) because three sites contain xenoliths that are different, but no site contains a single type. One site does display heterogeneity on a xenolith to xenolith scale. The other two sites are heterogeneous at grain to grain scale. This result is important because it suggests that southern California mantle is heterogeneous on a small scale (<10 km²), but as a whole is homogeneous (1200 km²).