CLINOPYROXENE TRACE ELEMENT CHEMISTRY OF GABBROIC ROCKS FROM THE BLUE MOUNTAINS PROVINCE: IMPLICATIONS FOR CRUSTAL THICKNESS

Gabbroic rocks in the Blue Mountains province (BMP) are a volumetrically minor component of Jurassic and Cretaceous plutons and batholiths, but provide the only direct information about the underlying mantle. Unfortunately, most (if not all) of these gabbros are largely cumulate in nature, and therefore do not represent true melt compositions. We have attempted to extract parental melt information from cpx + opx ± ol-bearing gabbroic/microgabbroic rocks (2 samples each) from the Bald Mountain (157 Ma) and Wallowa (140 Ma) batholiths and the Fish Lake (158 Ma) and Mountain Home (148 Ma) intrusive complexes, by examining clinopyroxene REE and Y abundances. Chondrite-normalized REE patterns for clinopyroxenes from the Bald Mountain, Wallowa, and Mountain Home gabbros show moderately-steep (Nd/La=1.6-4.8) positive slopes for the LREE, negative Eu-anomalies (Eu/Eu*=0.19-0.61), and slight negative slopes for the HREE (Gd/Lu=1.2-2.9). In comparison, those from the Fish Lake complex show lower SREE, less pronounced Eu-anomalies (Eu/Eu*=0.37-1.22), and relatively flat HREE.

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.