REGIONALLY EXTENSIVE, LOW-δ18O SILICIC VOLCANISM IN THE LATE JURASSIC CALIFORNIA ARC: EXTENSION LEAVES ITS MARK

Oxygen isotope ratios were measured in zircon by laser fluorination and by SIMS from in over 50 silicic (dacite–rhyolite) volcanic rocks of Triassic to Cretaceous (230-98 Ma) age from the Sierra Nevada batholith, White-Inyo Mountains, and Mojave Desert. These data give broad geographic and temporal context of volcanism in the Mesozoic California arcs system that was previously unobtainable because of secondary hydrothermal exchange that routinely alters original magmatic δ¹⁸O values in these rocks (c.f., Sorensen et al. GSAB 1998). SIMS analysis of δ¹⁸O using 10 µm spot size, augmented by U-Pb dating by LA-ICP-MS, further allows appraisal of variability within zircon grains and grain populations as potential evidence of assimilation or magma mixing within caldera systems, including mixing during eruptions.

Values of δ¹⁸O(zircon) in most rocks studied are 5.0–7.5‰, and overlap with values of δ¹⁸O in plutonic rocks (6.80±1.85‰, S.D., Lackey et al. 2005,2006,2008,2012). A subset of Late Jurassic (152-148 Ma) tuffs distributed from the Mojave Desert to Mt. Goddard pendant that contain zircons whose δ¹⁸O values commonly are <5.0‰; such low values are not found in coeval plutonic rocks or dikes of the ca. 148 Ma Independence Dike Swarm. Among these rocks, δ¹⁸O values are as low as 2.6‰ and average 4.4±0.8‰; porphyries associated with caldera complexes in the Mojave Desert are similarly low.

The restriction of these lower values to volcanic rocks of Late Jurassic age suggests that caldera systems at this time interacted with low-δ¹⁸O surface water (meteoric or marine). Such infiltration into caldera environments has not been recognized before or after. The Late Jurassic corresponds to a pronounced but brief transtension event in the arc, which is hypothesized to have impelled mafic mantle melts high into the crust, and also encouraged low-δ¹⁸O surface water exchange with arc wall rocks (especially volcanic crust). These two factors thus created a period of low-δ¹⁸O magmatism in the upper arc crust that is strikingly mismatched with the δ¹⁸O record in coeval plutons. Moreover, the volcanic δ¹⁸O record may provide previously unrecognized information about tectonic stress regimes in arc systems as they respond to plate-tectonic reorganizations.