HAFNIUM ISOTOPE ANALYSIS OF ZIRCON FROM SIERRA NEVADA GABBROS: IDENTIFYING POTENTIAL MANTLE SOURCE REGION HETEROGENEITY

Hafnium isotope (εHf ) analysis of zircon from arc gabbros may reveal the extent of compositional heterogeneity within the associated mantle wedge. Such heterogeneity may have a variety of origins, including metasomatism of the mantle wedge by slab-derived fluids, and tectonic assembly of the constructive margin prior to subduction initiation. The majority of zircon εHf studies focus on granitoids, and results are commonly interpreted as reflecting crustal inputs. In contrast, gabbros are better suited for discerning geochemical variations originating in the mantle source region because mafic magmas experience less differentiation and geochemical modification prior to solidification. A challenge in working with gabbroids is the low probability that samples will contain sufficient zircon for analyses, thus large samples (~28 kg) are required.

This study focuses on zircon separated from seven gabbros collected from two locations on an approximate west-to-east transect across the Sierra Nevada batholith (SNB) at ~36° N latitude. The first location -- the Stokes Mountain region of the western Sierra Nevada foothills -- is the ideal site for an εHf study because previous work has documented variations with whole rock O-Sr-Nd that are likely attributable to small-scale mantle heterogeneity and correlated to mapped intrusion structures. Preliminary εHf data from Early Cretaceous granitoids of the Stokes Mountain region range from +2.0 to +12.5 ε-units, the highest εHf values found in the SNB to date. In order to maximize the potential of documenting large-scale compositional variability in the SNB mantle source region, Stokes Mountain region data will be compared to data from the Summit Gabbro, located ~100 km ESE on the eastern SNB Kern Plateau.

The extent to which crustal assimilation modified zircon εHf signatures will be assessed by analyzing oxygen isotopes (δ¹⁸O) of the same zircon populations. Lastly, this study will examine the potential for using coupled Hf-O data from zircons in coeval gabbroids and granitoids to track the cumulative history of differentiation in arc magmas.