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The background geology for today is similar to that of Day 1. We remain in the TIS, and focus on the geology of the outer phases (the 93.1 Ma tonalite of Glen Aulin and the 92.8-88.8 Ma Half Dome Granodiorite). Bartley, Coleman, Glazner and their students have remapped the area where we will be today at 1:10,000, providing the foundation for the interpretations we will present (Fig. 16; Taylor, 2004; Coleman et al., 2005).

Three traverses today focus on the margins of the TIS and evidence that individual map units preserve evidence for incremental assembly (Fig. 17). The first traverse of the day crosses the outer contacts of the intrusive suite, from pre-batholithic metasedimentary rocks into the tonalite of Glen Aulin, which contains many wall rock inclusions, and then across the gradational transition from the tonalite into the voluminous equigranular Half Dome Granodiorite. The second traverse is located inside the Half Dome pluton, and crosses one complete map-scale gradational “cycle” of compositional variation within the Half Dome, from granodiorite with a color index (CI) greater than 20 to a leucogranite with CI less than 5. Within this cycle we will see numerous more discontinuous internal contacts. The final traverse begins in the equigranular phase of the Half Dome Granodiorite and crosses the gradational contact into the porphyritic phase and then into the main central pluton of the TIS, the Cathedral Peak Granodiorite.

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