A MOJAVE PERSPECTIVE ON VOLCANIC-PLUTONIC SYSTEMS IN CALIFORNIA’S MIDDLE JURASSIC ARC

We examine variations in volcanic-plutonic character in California’s inboard Middle Jurassic continental arc segment (Barth et al. GSA Sp. Pap. 438, 2008) in the southern Mojave with new zircon O (laser fluorination and single crystal SIMS analyses), U-Pb, and Hf isotope analyses (both by LA-MC-ICP-MS). The work considers how coeval volcanic and plutonic systems were related, especially regarding the influence pre-arc crust. Use of zircon is essential to unravel this history because Jurassic volcanic rocks are severely altered throughout the Cordillera, and tectonic dismemberment of the Jurassic arc in the Mojave has allowed extensive hydrothermal alteration of plutonic rocks (Solomon and Taylor, GC Sp. Pub. 3, 1991). In addition, we have examined Proterozoic (U-Pb ages of 1682–1775Ma, n=4) tonalitic orthogeneiss, amphibolite, and paragneiss in the West Ord Mountains to characterize Mojave crust into which the Jurassic arc was emplaced. Bulk δ¹⁸O zircon values for 164–148 Ma plutons are: 5.0–5.9‰ (n=10); Proterozoic gneisses 3.3–6.8‰ (n=7); 181–148 Ma Sidewinder volcanics 4.7–6.5‰ (n=5); 148 Ma felsic (>73 wt.%) Independence Dikes 5.34–5.39 (n = 3). In situ δ¹⁸O analysis confirms the average δ¹⁸O values, but metamict domains in gneissic zircons show clear exchange with low-δ¹⁸O meteoric fluids. Many concordant zircons are also low δ¹⁸O (3.3-4.8‰), a finding suggesting some samples acquired low-δ¹⁸O low values in the Proterozoic. In situ Hf isotope data show εHf_{(148 Ma)} of ­–9.6 to –13.4 for 148-149 Ma Jurassic gabbro, and εHf_{(148 Ma)} of –29 to –33 (εHf_(t) of 1.4 to 6.3) for Ord gneisses. Collectively, our findings show that most Middle Jurassic plutons and volcanic rocks, and latest-Jurassic Independence Dikes, are from sources dominated by partial melts of aged mafic crust or lithospheric mantle, with minimal upper crustal melts. Coeval plutonic and volcanic rocks the Sierra Nevada, White Mountains, and Owens Valley, show greater crustal input, highlighting important trans-arc differences in magma composition. Lastly, we note that volcanic rocks in all parts of the arc are often lower δ¹⁸O than plutonic counterparts, a consequence of the their generation in caldera settings, where assimilation of hydrothermally altered rocks lowers magmatic δ¹⁸O, akin to processes seen at Yellowstone.