Lava flows and dikes from the Bear Creek area within the Neogene Blue Mountains Province (BMP) were erupted in an extensional tectonic environment. Major and trace element variations in the Bear Creek suite constrain models for the petrogenesis of these rocks. Bear Creek basalts, basaltic andesites, and andesites fall into two compositional groupings, high alumina and tholeiitic. The most primitive flow is high alumina in composition with an Mg ♯=0.69. Various phase equilibria tests suggest that magma of this composition may be a hybrid of an eclogite-derived melt and a peridotite contaminant, or it could be a melt derived wholly from a peridotite source. Large ion lithophile element enrichment and high field strength element depletion (especially Nb) in this sample are consistent with its derivation from a fluid metasomatized peridotite source. An alternative source is required for three Bear Creek tholeiites and high-alumina basalts (HABs) that differ from the other Bear Creek samples primarily in having larger Nb contents relative to other incompatible trace elements.

This source can be modeled as partial melts of a peridotite which has been contaminated by a pelagic sediment, presumably from the subducted slab. Least squares mixing calculations with high-pressure mineral phases from peridotites indicate that high-pressure fractionation may have generated the more primitive high-alumina types in the Bear Creek suite. However, using the computer program SILMIN (Ghiorso, 1985) to semiquantitatively model shallow magmatic processes demonstrates that assimilation of greywacke and rhyolite (as a partial melt of the greywacke) and fractional crystallization (AFC) at 1 kbar pressure better explain the major element variation in the high-alumina types. The tholeiites appear to be derived from high-alumina parents via fractional crystallization at shallower levels in the crust.

Semiquantitative modeling using trace element distributions supports a model in which the HAB suite was derived from periodite and underwent AFC in the upper crust and the tholeiites were derived via Rayleigh fractionation from HAB parents. The trace element modeling also indicates that the range of trace element abundances cannot be explained by mixing of melts derived from eclogite and peridotite. Therefore a model in which dehydration of the Farallon plate metasomatizes mantle wedge peridotite and where pelagic sediment is incorporated into the peridotite, which subsequently melt to yield magmas that are modified by contamination and fractionation in the upper crust, explains all of the geochemical variation in the Bear Creek area rocks. This model may in part be applicable to the origin of the Picture Gorge and Slide Creek volcanic sequences in the BMP. ¿ American Geophysical Union 1989