Major, Minor, and Trace Element Composition of Clinopyroxene Grains from Southern California Mantle Determined Using LA-ICP-MS and SHRIMP-RG Geochemical Analysis

Alkali basalts erupted from approximately ten small volcanic centers in the eastern Transverse Ranges. Five of the centers contain xenolith-bearing basalt dikes and flows and provide a 60 km long northeast to southwest transect of southern California mantle during the Late Pliocene to Early Miocene time. The xenoliths thus represent a broad regional sampling of the mantle during the early stages of San Andreas transform tectonism in the Pacific-North American plate boundary zone. The xenoliths are classified as Group I (Wilshire et al., 1991) and are dominated by spinel lherzolite (52%), but are multi-modal including harzburgite (29%), dunite (5%), and wehrlite (14%). Xenolith olivine ranges Fo_87-91, and clinopyroxene ranges Wo_42.5-50.9 En_47.4-53.3 Fs_1.7-4.8. The Fo content of the mantle xenoliths more closely resembles Proterozoic mantle than Archean mantle (Lee et al., 2001).

We also analyzed clinopyroxene grains using LA-ICP-MS and SHRIMP-RG. LA-ICP-MS analysis of a gem quality Russian chrome-diopside allowed calibration of SHRIMP-RG minor and trace element analysis of clinopyroxene. Principle Components Analysis of major and trace element data show no distinct groups but do show progressive variation from northwest to southeast. Northwestern samples that are high in LILEs are typically high in HFSEs. Southeastern xenoliths that are high in REEs are also high in LILEs and HFSEs. Elements which are the most variable include Cs, Sr, P, Zr, Nd, Eu, and Gd. The xenoliths also record an enrichment process indicated by increasing incompatible element content with increasing quenching temperatures. These results suggest that flat-slab subduction of the Farallon Plate did not completely destroy the cratonic mantle in southern California, and the southern San Andreas – eastern California shear zone may have initially nucleated along this boundary between cratonic and Farallon mantle lithosphere.