HOW TO READ THE OXYGEN ISOTOPE SYSTEM IN CORDILLERAN ARCS: LESSONS FROM 190 MILLION YEARS OF SIERRAN MAGMATISM

Since the first oxygen isotope (δ¹⁸O) studies of zircon in the Sierra Nevada in 2005, a far more extensive picture of spatial and temporal patterns of magmatism has emerged from a tenfold increase in geochronologic coverage, and many new radiogenic isotopes (whole rock Sr and Nd; zircon Hf) Models of Cordilleran-type arc systems have interpreted magmatism as cyclic, with radiogenic isotope “excursions” tracing variable input of crust and mantle into magma sources [1]. Such models haven't incorporated oxygen isotopes to full advantage because of apparent complexity in the signals they record [2]. Here we describe many new single-zircon, SIMS δ¹⁸O analyses of plutonic, volcanic, and detrital zircon, which triples the original data set [3], giving far greater coverage of key magmatic eras of 120–100 and 180–275 Ma. These new data re-emphasize the power of δ¹⁸O of zircon to detect relatively fast (<10 myr) recycling of subducted supracrustal rock and accreted terranes in forearc settings. Such recycling is not resolved by radiogenic isotope systems that require time for radiogenic ingrowth. A wealth of new volcanic δ¹⁸O zircon data, along with δ¹⁸O of hydrothermal minerals like skarn garnet [e.g., 4], also record periods of significant δ¹⁸O “pull-downs,” where surface rocks that exchange with low-δ¹⁸O hydrothermal waters are buried and assimilated, embedding the surface δ¹⁸O signals in both plutonic and volcanic systems. Such low-δ¹⁸O events are brief (< 5 myr) and small volume, and have been overlooked, but are becoming more widely recognized in other arcs [5], suggesting that Cordilleran systems reprocess much of their crustal column [5]. Moreover, discovery of fossil low-δ¹⁸O systems in wallrock screens in mid-crustal levels [4] documents wholesale rapid burial of these domains in arcs, during episodes of shortening or transpression. Altogether, δ¹⁸O(zircon) uniquely traces surface-to-source transport and recycling in Cordilleran arcs as it relates to changing arc stress-regime, at periods that may fail to be recorded in excursions of radiogenic isotopes, such as relaxation of stress regimes in upper plate domains.

[1] DeCelles, P. G. et al. Nat. Geo. (2009); [2] Chapman, J. B. et al. Lithos (2021); [3] Lackey, J. S., et. al. J. Pet. (2008); [4] Ryan-Davis, J. et al. CMP (2019); [5] Turnbull, R. E. et al. Gond. Res. (2023).