ZIRCON HAFNIUM AND OXYGEN ISOTOPIC ANALYSIS OF SIERRA NEVADA GABBROS: EVIDENCE FOR MAJOR COMPOSITIONAL VARIATION IN THE MESOZOIC MANTLE

Hafnium isotope (εHf) analyses of zircon separated from gabbros confirm the existence of compositional variation within the mantle source region of the Mesozoic Sierra Nevada arc. Oxygen isotope (δ¹⁸O) values of these zircons indicate this heterogeneity originated in the mantle. Most zircon isotopic studies focus on granitoids, and non-mantle-like values are often interpreted as reflecting variable crustal input during magma differentiation. In contrast, gabbros are better suited for discerning geochemical variations originating in the mantle because mafic rocks have undergone minimal differentiation.

Zircons were separated from 7 gabbros collected from two locations at ~36° N latitude, close to the western and eastern edges of the Sierra Nevada batholith (SNB) ~100 km apart. The Stokes Mountain region of the western SNB is an ideal site for a zircon Hf-O study because previous work has documented variations within whole rock O-Sr-Nd isotopic values likely attributable to small-scale mantle heterogeneity. Zircon eHf data from these western Early Cretaceous gabbros range from +15.9 to +2.7 ε-units, with the highest e-values approaching that of the modeled Cretaceous depleted mantle. In stark contrast, zircons from the late Jurassic Summit Gabbro of the eastern SNB Kern Plateau have εHf values ranging from +0.3 to -11.4 ε-units. Typical mantle-like zircon δ¹⁸O values (i.e. +5.0‰ to +5.6‰) in both the western and eastern gabbros rule out the possibility that the ~27 ε-unit Hf variation is due to variable crustal assimilation. The disparity in εHf values suggests heterogeneity was present in the SNB mantle source region as early as the latest Jurassic, and supports earlier proposals for an enriched mantle end-member below the eastern arc flank.

Minor zircon δ¹⁸O variation suggests the limited role of hydrothermally- or metasomatically-altered source material in the production of these gabbros. Within the western zircon population, δ¹⁸O varies from typical mantle to values elevated by ~1‰. In contrast, the δ¹⁸O of the eastern zircon population varies from typical mantle signatures to values lowered by ~1‰. The magnitude and depth of such an interaction is constrained by the undifferentiated character of the gabbroic host magma.