COUPLED ZIRCON HAFNIUM AND OXYGEN ISOTOPES OF LOW MG# GABBROS: MIXING BETWEEN PARTIAL MELTS OF CONTINENTAL CRUST AND DEPLETED MANTLE IN THE DEEP ARC LITHOSPHERE, SOUTHEASTERN SIERRA NEVADA

Coupled zircon hafnium (eHf; n=347) and oxygen (d¹⁸O; n=188) isotope data from Late Jurassic (ca. 150 Ma) gabbros and associated granitoids of the Kern Plateau document significant interaction between depleted mantle-derived magmas and continental crust in the southeastern Sierra Nevada arc. With one exception (Mg#=65.6), these gabbros are characterized by relatively low Mg-numbers (54.5 to 42.6; n=8) similar to previously described Sierran mafic rocks. Zircon eHf₁₅₀ values (0 to -11.1) from a subset of these gabbros suggest the existence of relatively homogenous isotopically enriched mafic magmas, and would be consistent with the current paradigm suggesting derivation of eastern Sierran magmas from an enriched mantle source. However, the total range of zircon eHf (+16.2 to -24.0) and d¹⁸O values (+4.0 to +9.5‰) require the intermingling of partial melts of both depleted mantle and of old continental crust. Furthermore, zircon with d¹⁸O values lower than the composition of globally invariant mantle values provide strong evidence for recycling of hydrothermally altered crust. The coupled zircon hafnium-oxygen values can be modeled as forming during a two-stage mixing process. During an initial stage, depleted-mantle-derived magmas interact with partial melts of Paleoproterozoic Mojave province rocks in the lower crust. Subsequently, mixing between isotopically evolved mafic magmas and partial melts of Neoproterozoic-Paleozoic passive margin sediments occurs within the middle crust. Zircon xenocrysts (n=29) separated from an olivine-bearing gabbro define ages populations of ca. 1.1, 1.4, and 2.0 Ga that provide independent evidence for assimilation of passive margin sediments. Low Mg# indicate that the gabbros of the Kern Plateau have been modified by cumulate formation, and therefore do not represent minimally-modified partial melts of a mantle endmember. Instead, the isotopic enrichment in both zircon hafnium and oxygen of Kern Plateau gabbros serve as records of their crustal heritage despite their mafic compositions.