Investigating compositional variation in Cabrillo Formation conglomerate volcanic clasts, San Diego County, California
Advisor Dr. David Kimbrough
Conglomerate clasts of the Campanian-Maastrichtian Cabrillo Formation, part of the Rosario Group, have been interpreted as locally derived forearc basin detritus shed from the adjacent Peninsular Ranges batholith (PRB) magmatic arc which ranges in age from ~140-85 Ma. Rosario Group sandstone compositions, detrital zircon U-Pb ages, and K-feldspar 40Ar/39Ar ages confirm a tight provenance linkage of sandstone to PRB arc basement. However little work has been done investigating how conglomerate clast rock types link in detail into the PRB magmatic arc history. Clast counts show that Cabrillo Formation conglomerates contain on average 60% volcanic clasts, 25% quartzite clasts, and 15% plutonic clasts. The volcanic clasts were interpreted by prior workers to have been derived from the early Cretaceous Santiago Peak Volcanics (SPV) exposed along the western side of the PRB. Recent work by John Abeid on the Cabrillo Formation volcanic clasts challenges this simple assumption. Abeid analyzed ten volcanic clasts and found that all of the clasts are high-K rhyolites based on X-ray fluorescence (XRF) whole rock composition and range narrowly in zircon U/Pb age from 96.6 ± 2.5 to 103.7 ± 2.5 Ma. These clasts ages are much younger than ages determined from the SPV which are ~135-110 Ma, and the chemistry of the clasts does not match up well against chemistry of rhyolites from the SPV. Variability of volcanic clast types in the Cabrillo Formation have been recognized in hand sample by previous workers, however no geochemical data exist to document the composition of intermediate and mafic volcanic rock types.
The purpose of this study is document the full range of volcanic clast types in the Cabrillo Formation and assesses their chemical affinities. Twenty-one volcanic clasts from Cabrillo conglomerates were collected mainly from the Bird Rock-Tourmaline Beach coastal section. The rocks are mostly plagioclase porphyritic flow rocks with secondary chlorite and epidote, but include two samples of lapilli tuff breccias. Whole rock major and trace element concentrations were determined by XRF and show that these samples define a coherent high-K calc-alkaline magmatic suite clearly distinct from the SPV which are thoeliitic and calc-alkaline. The Cabrillo clasts are also characterized by distinctly higher Ba, Nb, and Zr trace element contents consistent with the high-K calc-alkaline nature of the suite. The Cabrillo Formation volcanic clast suite is tentatively identified as a previously unrecognized younger phase of supracrustal volcanic arc activity associated with the Peninsular Ranges magmatic arc.