TRACKING BASEMENT DOMAINS WITH ZIRCON LU-HF AND TRACE ELEMENT GEOCHEMISTRY IN THE NORTHERN SIERRA NEVADA BATHOLITH, CALIFORNIA

The northern Sierra Nevada batholith was emplaced into and across a series of accreted crustal belts that vary considerably in their ages and lithologies. Unlike batholitic segments to the south, where modern exposures are composed almost entirely of voluminous mid to Late Cretaceous intrusions, the northern Sierra comprises smaller, spatially separated plutons mainly of Jurassic through mid-Cretaceous age. In many cases, geologic relations with the host basement can be observed. Intermediate to felsic plutons were sampled along an arc-perpendicular transect at the latitude of Lake Tahoe and zircon Lu-Hf and trace element analysis was performed in order to assess the relative impacts of temporal and spatial variability on the zircon record. Initial epsilon Hf weighted means of northern Sierra Nevada plutons range from -0.6 to +14.4. Trends through time in the Hf data are complex, whereas there is an abrupt step from uniformly juvenile values in plutons intruding western belts (+12.3 to +14.4) to more evolved values in those intruding the Northern Sierra terrane to the east (-0.6 to +5.2). A similar pattern is observed in several zircon trace element signatures, including pronounced steps toward higher U/Yb, Ce/Y and ∑LREE/∑HREE from the western belts into the Northern Sierra terrane. The step is approximately coincident with the Feather River terrane, which is interpreted to mark the suture between oceanic Panthalassan lithosphere to the west and North American continental lithosphere to the east. Using data from (mostly Jurassic) plutons where intrusive relations with basement type can be observed in the field, bivariate zircon trace element plots are used to discriminate the crustal belts. Intrusions with uncertain basement affinity (mostly Cretaceous) are then assessed using those discrimination diagrams. The observed links between variation in zircon Lu-Hf and trace element concentration and basement domain indicate that northern Sierran zircons incorporate, and are sensitive to, the crustal tracts into which they’re emplaced. Our results have important implications for provenance studies, as zircons of similar age, but deriving from different parts of the California arc system, may be distinguishable.