LOCATING THE VOLCANIC SOURCE ROCK OF PREHISTORIC COGSTONES FROM SOUTHERN CALIFORNIA
Four volcanic cogstone fragments, unearthed in Orange County, were analyzed together with volcanics from in or near Orange County, specifically El Modena, Santa Rosa, Catalina Island, Santa Monica Mountains, and from the Van Winkle Mountains, and Fossil Falls in the Mojave desert, to compare and test for shared origins. Analyses included macroscopic observations of composition and texture/vesicularity, petrographic observations, whole rock XRF major oxide and trace element geochemistry, and Sr, Nd, and Pb isotope geochemistry.
Results from petrographic analysis of thin sections indicate that basalt from El Modena, Santa Rosa, Catalina Island, and Fossil Falls are potential matches with cogstones 2 and 3. The samples are all plagioclase rich with phenocrysts of olivine, pyroxene, Fe-oxides, and iddingsite and vary only slightly in grain size and texture. XRF analysis shows that major oxides and trace elements of particularly El Modena samples are similar to those of cogstone 2, while cogstone 3 analysis best matches with Santa Rosa and Fossil Falls basalts. Isotope analysis further tested these hypotheses and revealed that only El Modena basalt (87Sr/86Sr=0.70366) and cogstone 2 (87Sr/86Sr=0.703971) as well as cogstone 3 (87Sr/86Sr=0.703167; εNd=5.31) and Santa Rosa basalt (87Sr/86Sr=0.703318; εNd=4.27) are isotopically similar. Isotopic values for Fossil Falls (87Sr/86Sr=0.705855, εNd=-1.97) clearly indicate a different magma source. Pb isotope results yield similar conclusions.
Our results suggest that the basaltic rocks Native Americans used for carving cogstones were preferentially collected nearby, and that there might have been no importance to the collection location beyond proximity.