IS THE WENATCHEE RIDGE ORTHOGNEISS A MESOZOIC ANALOG FOR ARCHEAN TTG?

Approximately 90% of 4.0-2.5 Ga continental crust is part of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Despite their abundance and volumetric importance, the petrogenetic processes that formed Archean TTGs are poorly understood due to reworking, removal from their original tectonic setting, alteration, and limited exposure. Thus, the identification of younger Archean TTG analogs would lead to a greater understanding of early continental evolution.

The Wenatchee Ridge Orthogneiss (WRO) is a sheeted pluton in the Nason Terrane of the North Cascades of Washington State. The pluton is dominantly tonalitic containing oligoclase, quartz, muscovite, biotite, and epidote as primary minerals. SiO₂ranges from 60-76% and there is a notable absence of hornblende in the vast majority of samples. Trace and REE data show that the pluton is highly depleted in HREEs and shows a distinct positive Eu anomaly of +1.3 times the expected value on average.

Initial geochemical analysis of the WRO indicates a composition very similar to Archean TTGs based upon characteristic Archean TTG geochemical values including Yb <1, Sr/Y >150, La/Yb >15, Y<6, and HREE depletion. Discriminant analysis comparing suites of Archean and post-Archean TTGs show the WRO rocks plot with Archean samples with reliable data separation of the three discriminant functions based on canonical correlation and the Wilks’ Lambda test.

The WRO is similar in many ways to other TTG plutons throughout the Coast Plutonic Complex apart from the positive Eu anomaly, absence of hornblende, and, in some cases, the presence of magmatic epidote. The HREE depletion, Si enrichment, and tectonic setting indicate that the pluton may have formed as a partial melt of overthickened eclogitic crust. The fractionation of hornblende from the subsequent melt would explain the absence of hornblende as well as additional Si enrichment, HREE depletion, and the positive Eu anomaly. Additional geochemical data will further constrain the details of the Wenatchee Ridge Orthogneiss formation and may provide insight into Archean TTG generation.