THE FISH LAKE COMPLEX, NE OREGON: ESTIMATING MELT COMPOSITIONS USING CLINOPYROXENE TRACE ELEMENT DATA FROM A CUMULATE-RICH PLUTON

The Fish Lake Complex (FLC) is a cluster of small gabbroic bodies that intruded the Wallowa terrane. Zircons yielded a Pb/U (SHRIMP-RG) age of 157.5 ± 1.3 Ma, making it one of the oldest Late Mesozoic plutons in the Blue Mountains province. The FLC comprises two-pyroxene (± relict olivine) gabbro that is cross-cut by andesitic and tonalitic dikes. Gabbros commonly exhibit igneous lamination, and have geochemical characteristics suggestive of a cumulate origin, including high Al₂O₃ (up to 21 wt.%), low SREE, and pronounced positive Eu-anomalies. The FLC represents one of only a few examples of demonstrably mantle-derived magmatism in the Blue Mountains province during Late Jurassic time; knowledge of FLC magmatism and mantle processes is therefore crucial for understanding other Late Jurassic plutons in this region.

Trace element abundances in unexsolved clinopyroxene were determined by laser ablation-ICP-MS. Chondrite-normalized REE patterns of clinopyroxene are concave-downward and have slight negative Eu-anomalies. Melt compositions in equilibrium with clinopyroxene were calculated using these data and the cpx-melt partition coefficients of Klein et al. (2000, GCA v.64, pp.99-115). The calculated REE patterns are LREE-enriched, exhibit negative Eu-anomalies, and are similar to those of andesitic dikes in the FLC. These results are in agreement with major element data.

The calculated melt compositions are similar to those of gabbroic rocks and basaltic enclaves in other Late Jurassic plutons in the Blue Mountains province. Therefore, the calculated FLC magma compositions can conceivably be used as a mantle-derived end member in petrogenetic studies of other magmatic suites in this area, for which information had previously been lacking. These results underscore the potential of using trace element data of cumulate phases in petrogenetic studies.