An Investigation of the Igneous-plutonic Setting of Poway, CA

The Peninsular Ranges Batholith (PRB) occurs along the eastern boundary of the Pacific Plate, along its margin with the North American Plate. The PRB was emplaced between 128 and 112 Ma, as a result of subduction of the Farallon Plate. Many plutons within the PRB are unstudied, including those in the Poway Valley, CA. This study is limited to the city of Poway: Van Dam Peak in the west; Twin Peaks; and Tooth Rock in the east. The valley features felsic plutonic rocks that include a variety of enclaves—they vary in size, abundance, and distribution throughout the area and were studied through field, petrographic, and geochemical analysis. Based on mineralogy and bulk composition, there appears to be a systematic differentiation with diorites at the lowest elevations, granodiorites at middling sections, and granites at top. There are three types of enclaves: globular, salt & pepper, and blocky. Globular enclaves are found everywhere in the study area below 380 m asl, are homogenous and fine-grained. They have sharp transition zones with the host rock. Salt & pepper enclaves are heterogenous, and tend to be smaller than 3 cm in radius. They are present up to 380 m, and have very gradual transition zones with the host rock.  Blocky enclaves are rectangular and only found between 330 and 360 m. Although no samples of blocky enclaves were collected, field characterizations suggest that they may be xenoliths. The mineralogy and transition zones indicate that there was likely more than a single mafic injection—a hot one to form the globular enclaves and a cool one for the salt & pepper. Country rock may have entered the melt to form blocky enclaves. The summit of Twin Peaks, which has no enclaves, different bulk chemistry, and a dissimilar visual character, could represent a separate body of magma from the main pluton, which consists of Van Dam Peak and the main section of Twin Peaks. It’s also possible that the summit is the same pluton but was not subject to a mafic injection. The dissimilar REE pattern of Tooth Rock (lower concentrations of LREEs, and a negative europium anomaly in its enclaves) compared to the other sites suggests an additional pluton. The presence of multiple plutons in addition to the diversity of enclaves inside the Poway Valley demonstrates that plutons in the PRB can have gradual contacts, and that the mountain of Twin Peaks represents a complete cross-section of the plutonic faces in the Poway Valley.