JURASSIC MAGMATISM IN SOUTHERN CALIFORNIA: NEW RESULTS AND ATTEMPTS AT A REGIONAL SYNTHESIS

Jurassic arc igneous rocks mark the intermediate stage of Mesozoic magmatism in California and Arizona. First order regional variations in rock and zircon chemistry may reflect regional differences in mantle sources (Miller and Glazner, 1995) and/or heterogeneities in crustal components. These source signatures are overprinted by mixing and fractionation processes associated with ascent and emplacement. New zircon geochemical and U/Pb isotopic data for fifteen samples of Jurassic plutonic rocks, integrated with a data base of ~400 whole rock analyses, suggests that distinctive age and compositional groups of Jurassic intrusive rocks and ignimbrites characterized geographic provinces in this region. Early Jurassic igneous rocks are rare; the oldest well-dated Jurassic rocks are 190-183 Ma ignimbrites, though detrital zircons suggest magmatism may have commenced ~197 Ma. Middle Jurassic intrusive rocks are abundant, typically mesocratic and potassic relative to adjacent Late Jurassic and mid- to Late Cretaceous intrusive suites, and are locally associated with remnants of coeval ignimbrite sheets. Middle Jurassic rocks in the north-central Mojave comprise two contemporaneous (176-171 Ma, with granites as late as 168 Ma) but geochemically distinct groups – the high K calc-alkalic Fort Irwin sequence and higher K alkalic rocks to the south and east, from Devil’s Playground to the Ship Mountains. Further south, the Bullion Intrusive Suite (of Howard, 2002) and Trigo Peaks and Cargo Muchacho super units (of Tosdal et al., 1989) are distinctly younger (167-158 Ma) than well-dated rocks in both the Fort Irwin and alkalic sequences. Whole rock compositions in the younger Bullion suite are similar to the high K Fort Irwin sequence, but zircon data suggest Bullion suite melts had markedly higher Th/U and were LREE-enriched. Ignimbrites in the Sidewinder sequence (181-160 Ma) were contemporaneous with and compositionally akin to the Fort Irwin sequence and Bullion suite, and may have been separated from them by Miocene extension. Late Jurassic calc-alkalic and alkalic intrusive suites (157-149 Ma) crop out along the western margin of the Middle Jurassic arc in the eastern and central Transverse Ranges and western Mojave Desert, and were associated with eruption of lavas and at least one Sidewinder ignimbrite.