GEOCHEMISTRY OF THE SOUTHERN CALIFORNIA BATHOLITH: NEW DATA FOR THE NORTHERN PENINSULAR RANGES AND TRANSVERSE RANGES

Estimates of the overall composition of the Earth's upper crust have been based largely upon Archean and post-Archean sediments with little input from batholith-scale plutonic rocks. Such data are quite scarce and new information suggests average upper crustal values for Rb/Cs, Zr/Nb, Hf/Ta, Ce/Yb, K/U are too low, and Nb/U, U/Th are too high. Here, we report on a regional data set of precise trace element and Sr isotopic determinations on 491 composite samples (representing > 4000 samples collected on a grid system) from the Transverse and Peninsular Ranges of the Southern California Batholith. Trace elements were determined by isotope dilution laser ablation ICP-MS. Rb, Sr, ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr were determined by thermal ionization mass spectrometry. Plutonic rocks within the Transverse Ranges are composed principally of quartz diorite, granodiorite and quartz monzonite with rarer gabbroic plutons in the Peninsular Ranges. The majority of plutons were emplaced between 140-80 Ma. The data set is divided into geologic blocks bounded by major faults. These are: San Gabriel, San Bernardino, Little San Bernardino, Santa Ana, Perris, San Jacinto. The Santa Ana plutons are distinctive in Rb/Cs, , Zr/Nb, Hf/Ta, Ce/Yb, La/Sm, U/Th, K/U and also fall upon a well-defined whole rock isochron with (⁸⁷Sr/⁸⁶Sr)_I=.70373. The Transverse Ranges plutons, particularly the San Bernardino block, have a markedly higher scatter in most lithophile element ratios cf. Peninsular Ranges plutons and also have higher (⁸⁷Sr/⁸⁶Sr)_I ratios (.7083 - .7086). This information is consistent with the premise that the western parts of the Peninsular Ranges plutons have a major component of subducted oceanic crust and do not involve significant continental crust anatexis. The eastern parts show geochemical evidence for both mantle and crustal components in the plutons. The Transverse Ranges remain enigmatic as they do not continue the geochemical trends of the Peninsular Ranges, northward. They apparently have a larger component derived by crustal anatexis in their genesis than any of the Peninsular Ranges rocks.