ZIRCON HAFNIUM-OXYGEN ISOTOPE EVIDENCE FOR LATE JURASSIC INTRA-ARC RIFTING OF THE SOUTHERN SIERRA NEVADA, CA, PART 1: ORIGIN OF ENRICHED MAFIC ARC MAGMAS

Zircon hafnium isotope (εHf) data document intrusion of depleted-mantle-derived magmas into the southeastern Sierra Nevada arc at the end of the Jurassic. Zircon εHf₁₅₀ (n=226) values of ten Kern Plateau gabbroids exhibit an ~40 ε-unit range from +16.2 to -24.0, requiring the coexistence and intermingling of partial melts of both depleted mantle and of old continental crust. Gabbros are concentrated along the Troy lineament, which trends subparallel and adjacent to the Kern Plateau shear zone. Previously documented normal-sense, down to the NE motion along this structure has been interpreted to record Late Triassic to Early Jurassic extension related to the development of an intra-arc rift graben in which thick volcano-sedimentary sections accumulated. Zircon εHf data document renewed extension in the latest Jurassic and suggest opening of a crustal-scale structure that permitted mantle-derived magmas to reach the upper crust.

Zircon εHf data also document the existence of relatively homogenous, isotopically enriched mafic magmas with low Mg-numbers, similar to previously described mafic rocks in the Sierra Nevada arc. We model coupled zircon εHf-δ¹⁸O values as forming during a two-stage mixing process: (1) lower crustal interaction of depleted-mantle-derived magmas with partial melts of Paleoproterozoic Mojave Province rocks; followed by (2) mixing in the middle crust between isotopically evolved mafic magmas and partial melts of Neoproterozoic-Paleozoic passive margin sediments. Besides the newly-documented presence of depleted-mantle-derived magmas, zircon δ¹⁸O values below globally invariant mantle values demonstrate the enriched mafic magmas are not simply mantle-derived melts but instead require interaction with hydrothermally altered crust. The combined presence of depleted-mantle-derived melts and modeled isotopic enrichment in chemically evolved mafic melts precludes the need for an enriched mantle source region below the Mesozoic Sierra Nevada arc segment. The trench-ward migration of mantle-like values with decreasing age, as documented by comparison with zircon εHf-δ¹⁸O data from the Early Cretaceous Stokes Mountain region located ~35 km to the NW, is consistent with a hypothesized slab roll-back mechanism for rifting and upwelling of magmas from the deep arc crust.