**3. Results**

## *3.1. Petrologic Observations*

The Alta and Little Cottonwood stocks are texturally [28,40] and compositionally variable [12]. Internal intrusive contacts were locally visible and sugges<sup>t</sup> that the stocks are also highly composite. Granitic and aplite dikes were common both within and outside the intrusions (Figure 2A,B). Additionally, wall rocks adjacent to both intrusions recorded many generations of melt and fluid infiltration (Figure 2C–F). Where dikes intruded the wall rocks at the margins of the Alta and Little Cottonwood stocks, the dikes were commonly deformed and suggested syn-deformation emplacement (Figure 2C,D).

At the thin section scale, titanite in both intrusions ranged from euhedral with no inclusions to anhedral and overgrowing ilmenite (Figure 3). Titanite was commonly associated with both ilmenite and magnetite, and with amphibole, biotite, apatite, and sometimes plagioclase. This assemblage suggests a titanite-forming reaction similar to that seen in amphibolite. Titanite was commonly included within quartz, alkali feldspar, and amphibole (Figure 3), but titanite included within plagioclase was not observed. Titanites in both intrusions contained internal chemical zoning, including oscillatory zoning, sector zoning, and patchy zoning indicative of a range of processes from neocrystallization from a granitic melt, neocrystallization from pre-existing phases, and fluid-aided interface-coupled dissolution-reprecipitation recrystallization [51]. Zircons were typically euhedral and were observed to be included within most major minerals, including plagioclase (Figure 3A). Zircons ranged from nearly equant to acicular and oscillatory zoning was common. Zircons in the Little Cottonwood stock often contained distinct mantles and rims that were typically cathodoluminescent dark and bright, respectively. Resorbed, inherited cores were observed and are interpreted to have traveled from the melt source and served as nuclei for further zircon growth (Figure 4).
