GEOCHEMICAL TRENDS IN IGNEOUS ROCKS IN THE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA

Baird et al. (1984) collected 128 composite rock samples in the San Bernardino Mountains which are bounded on the southwest by the San Andreas Fault, the southeast by the Pinto Mountain Fault and the north grading into the Mojave Desert. The samples were analyzed for major and trace elements using XRF and LA-ICP-MS. The samples were split into 7 groups based upon similarities in their major element compositions. Five of these groups form a straight line differentiation series on Harker diagrams and appear to come from a common magma source.

The rocks were classified under the standard IUGS nomenclature using CIPW norm calculations for mineralogy. The West Mojave group samples were all granites with 55-65% plagioclase and 30-35% quartz. The Lake Arrowhead group granites had the highest quartz [35-40%] and lowest plagioclase contents [40-55%]. The rest of the groups were more scattered, each with some granites, granodiorites and a few quartz monzodiorites. A few samples from the Running Springs and Forest Falls groups plot as quartz monzonites.

The Forest Falls group in the central part of the San Bernardino Mountains appears to be the least evolved in the differentiation series. The remaining four groups are arranged with two on the west [Lake Arrowhead with 63-70% SiO₂ and Running Springs with 70-75% SiO₂] and two on the east [East Mojave with 63-70% SiO₂ and Pipes Canyon with 70-75% SiO₂]. Most samples have Na₂O between 3 and 4%, except for West Mojave samples with Na₂O above 4% and the high Sr_i (0.719-1.04) samples with Na₂O below 3%.

The two remaining groups [West Mojave and high Sr_i] fall one on either side of the differentiation series line and are interpreted to be derived from different magma sources. West Mojave samples are characterized by 1% MgO compared to the main series' 3.5%. Most of these samples also have uniquely high Sr of >900 ppm. The high Sr_i samples are located in the NE Pipes Canyon region and have a uniquely low Sr of <200 ppm. They also have a low Sr/Y ratio of <8 which suggests a source in a different tectonic region than the other groups.

On Pearce's (1984) Ta .vs. Yb plot all samples plotted as volcanic-arc granites, except for most of the Running Springs samples, which plotted as syn-collision granites. All samples have relatively high La/Yb ratios (>9) indicating that melting originally occurred at depth.