PETROGENETIC ANALYSIS OF THE WENATCHEE RIDGE ORTHOGNEISS IN THE NORTH CASCADE MOUNTAINS, WASHINGTON STATE

Petrogenetic analysis of the Wenatchee Ridge Orthogneiss (WRO) (Magloughlin & Evans, 1987) in the Nason Terrane of the North Cascade Mountains has been undertaken in order to gain insight into epidote-bearing TTG plutonism associated with mid-Cretaceous orogenesis in the North American Cordillera. Discriminant analysis indicates the WRO is very similar to Archean TTGs based upon characteristic geochemical values (Yb <1, Sr/Y >150, La/Yb >15, Y<6) and thus may provide insight into Archean crustal generation processes.

Samples were taken from within the pluton and from within the surrounding banded gneiss (Tabor et al., 1987). The pluton is chemically heterogeneous and samples all show some degree of foliation which is concordant with the foliation in country rocks. Samples range from leucotrondhjemite to granodiorite and contain oligoclase, quartz, muscovite, biotite, and epidote. SiO₂ is 56.3-76.8% and REE data show that the samples are highly depleted in HREEs, variably depleted in LREEs, and have an average Eu/Eu* of 1.36±0.5. Though positive Eu anomalies are typically associated with plagioclase accumulation, the WRO appears to lack any correlation between plagioclase and Eu/Eu*.

Geochemical results and the tectonic setting of the WRO indicate the initial magma may have formed as a partial melt of overthickened eclogitic crust. The subsequent LREE depletion and high positive Eu anomalies in the most evolved samples may be controlled by amphibole, epidote, and/or titanite fractionation. LA-ICP-MS analyses will indicate whether these phases had significant control on the REE signature of the WRO.

Deformation-driven differentiation would have controlled any fractionation of amphibole, epidote, and/or titanite in the WRO magma which has viscosities ≥10^6.8 Pa·s at 1000°C calculated using the method of Giordano et al. (2008). Differentiation likely occurred simultaneously with intrusion into a lower crustal zone of plastic strain, resulting in the WRO’s heterogeneity, sheeted nature, and syn-tectonic fabric.