A geochemical and petrologic study of Poway clasts in San Diego and the Channel Islands, California

A geochemical and petrologic study was conducted to investigate the relationship of Poway clasts from the northern Channel Islands off the southern California coast to Poway clasts from the San Diego region. These clasts are variably metamorphosed rhyolite crystal vitric tuff, are Jurassic in age, and have distinctive lithologic characteristics. These clasts have been deposited in several Paleogene conglomerate units in southern California and their distribution has been used in paleogeographic reconstructions that juxtapose the northern Channel Islands and the San Diego coast. The objective of this study was to re-evaluate whether the clasts from these conglomerate units were mineralogically, texturally, and geochemically similar and thus could be utilized in paleogeographic models. Clasts meeting the petrographic criteria established by others for Poway clasts were collected in the San Diego area (Poway Conglomerate and Stadium Conglomerate), Santa Cruz Island (Jolla Vieja and Vaqueros Formations), San Miguel Island (Canada Formation), and San Nicolas Island (Units 2, 4, and 10). Thin-sections were studied for mineralogy and texture, and geochemical studies were conducted to determine the concentration of major oxides and trace elements. IX Clasts from the Sespe Formation in the Santa Ana Mountains and the Blanca Formation on Santa Cruz Island were collected to act as possible control clasts. The Scape clasts were deposited contemporaneously with the Poway clasts but represent a river system north of the Ballena River which deposited the Poway clasts. The Blanca Formation is a younger formation and is believed to be local in origin. The mineralogic and textural characteristics of clasts collected at the five Poway collection sites are strikingly similar. Quartz makes up 6% to 19% of samples studied. Quartz phenocryst contacts with groundmass are mostly sharp; the phenocrysts are commonly embayed and contain little pitting or vacuolization. Feldspar makes up 22% to 40% of samples studied. Feldspar contacts with groundmass are usually sharp with dissolution present in variable proportions. Crystals are usually pitted (vacuolized) and frequently exhibit regions of calcite replacement. While plagioclase commonly exhibits sericite replacement, potassium feldspars do not. Biotite is usually altered to piemontite or to sericite or some other colorless mica. Altered biotite is characterized by the presence of abundant, small opaque grains that are aligned with remnant cleavage. Chlorite is commonly present in San Nicolas Island clasts, uncommonly present in the Vaqueros clasts from Santa Cruz Island, and not present in clasts from the other collection sites. Hornblende is present in low abundance in San Miguel and San Nicolas Island clasts. The hornblende is highly altered and opaque grains outline each crystal. Piemontite is present in a few clasts from Santa Cruz and San Miguel Islands, the San Diego area, and the Sespe Formation. It is not found in San Nicolas Island clasts. It is present mostly as fibrous crystals, rarely as euhedral crystals. Calcite is commonly present as a replacement mineral in feldspar grains and in the groundmass. Opaque minerals, mostly hematite, magnetite, and minor iron sulfides, are frequently observed. The groundmass makes up 42% to 61 % of clast volume. Groundmass constituents are mostly glass and variably devitrified glass. Remnant glass shards are present in a few samples and indicate the tuff origin of the clasts. The devitrified material, likely composed of microcrystalline quartz and feldspar, is bimodal in size. Geochemical analysis shows complete overlap of the five Poway clast collection sites. Most clasts are classified as rhyolite. The clasts are all subalkaline, and belong to the calk-alkaline magma series. All clasts are slightly enriched in light rare-earth elements and exhibit a small Eu anomaly. On spider diagrams, negative spikes are exhibited by the elements Ba, Nb, Sr, P, and Ti. When Poway clast sites are standardized to the average chemical composition of the San Diego clasts, the patterns of all clasts cluster in a horizontal pattern around the value of one. The greatest variability is seen in San Diego clasts. Clasts from the Santa Ana Mountains fall within the same geochemical range exhibited by Poway clasts. These data support the previous proposal of a possible overlap of the drainage basins for the Owl Creek and Poway clasts. The geochemical composition of Blanca Formation clasts is distinctly different from that of the Poway clasts, suggesting a different magma source. Modern models, based on a variety of geological and geophysical evidence, suggest that the western Transverse Ranges block (which includes the northern Channel Islands) formerly possessed a north-south orientation and was located adjacent to what is now the San Diego coast. The present distribution of Poway clasts has been a key component of this model. This study confirms that clasts previously identified as belonging to the Poway clast family, are in fact, virtually indistinguishable from each other with respect to their mineralogy, texture, and geochemistry, and that their use in paleogeographic models is justified.