Paired oxygen and hafnium isotopic analysis of zircon from gabbros: Identifying potential Mesozoic mantle heterogeneity in the Sierra Nevada arc

The composition of the mantle below the Mesozoic Sierra Nevada arc is a topic of debate; the presence of enriched mantle has major implications for modeling the magnitude and mechanisms of the Cordilleran crustal growth event. Multiple lines of whole-rock geochemical and isotopic evidence indicate the presence of depleted mantle below the western arc. Similar whole-rock isotopic studies suggest a more complex origin of eastern Sierra magmas, and the presence of an enriched mantle source.;This study represents a significant contribution to the enriched-mantle debate because (A) the new isotopic data are from gabbros, which closely represent the composition of mantle-derived parental magma; (B) ages determined during this study are late Jurassic and Early Cretaceous, making these rocks coeval with arc magmatism; and (C) zircon are better suited for isotopic studies because they are not susceptible to open-system process that impact and potentially compromise whole-rock analyses.;Coupled zircon hafnium-oxygen isotopes provide mantle signatures that suggest arc magma generation in two distinct source regions, with western samples derived from depleted mantle and eastern samples possibly extracted from enriched mantle. Three potential interpretations are proposed for the eastern gabbros: extraction from depleted mantle with subsequent crustal assimilation; extraction from an enriched mantle source region; or derivation from a tectonically assembled deep crustal mafic magma source.