CLINOPYROXENE TRACE ELEMENT CHEMISTRY OF GABBROIC ROCKS FROM THE BLUE MOUNTAINS PROVINCE: IMPLICATIONS FOR CRUSTAL THICKNESS
Melt compositions in equilibrium with clinopyroxene were calculated using the partition coefficients of Klein et al. (2000) for the REE and Hack et al. (1994) for Y. Melt compositions in equilibrium with clinopyroxenes from the Bald Mountain, Wallowa, and Mountain Home gabbros have REE patterns characterized by LREE-enrichment (La=90-2200 x chondrite; La/Yb=3.2-14.5) and negative Eu-anomalies (Eu/Eu*=0.19-0.81). Calculated melt compositions for the Fish Lake gabbros show much less LREE-enrichment and less pronounced negative Eu-anomalies. Depth to the Moho was estimated using Ce/Y values following the method of Mantle and Collins (2008). The Moho was calculated to be ~35 km beneath the Mountain Home complex, ~40 km beneath the Bald Mountain batholith, and between 47 and 50 km beneath the Wallowa batholith. We recognize that these depths are dependent upon the partition coefficients used but regardless, they demonstrate significant differences in crustal thickness between different regions of the Blue Mountains province during the Late Jurassic/Early Cretaceous. The calculated melt compositions will also serve as useful parental melts for petrogenetic studies of plutons for which a mafic end member is lacking.