*3.2. Petrochronology*

#### 3.2.1. U-Pb Dates

We presently have many more isotopic dates from titanite (n = 17 samples, 766 analyses) than from zircon (n = 9 samples, 254 analyses), particularly in the Little Cottonwood stock (titanite: n = 8 samples, 326 analyses; zircon: n = 3 samples, 129 analyses) (Figure S1 and Table S1). Isotopic dates are

reported as 207Pb-corrected dates projected to concordia from an initial 207Pb/206Pb ratio for each sample (Data Repository). For discussion of this method, see the Materials and Methods Section above. The mean square weighted deviations (MSWD [88]) of the Little Cottonwood stock zircon date populations ranged from ~6 to 21. Dates from three samples (MS15-01, 02, and SC16-02) ranged from 34–26 Ma, 35–25 Ma, and 33–27 Ma, respectively, with Proterozoic (~1000 Ma) inherited cores. Older Cenozoic dates (~35–34 Ma) came from distinct cores with zircon mantles that produced dates from ~33–30 Ma. The younges<sup>t</sup> dates, ~30–25 Ma, came from cathodoluminescent dark zircon rims, most notably in sample MS15-01 (Figure 4). MSWD values from LCS titanites ranged from ~1.2 to 7.2. Analyses from two of the eight samples, namely LCS-01 and 02, formed single populations (MSWD = 1.2). Titanites from these samples were concentrated by mineral separation techniques and analyzed in a 25-mm-diameter epoxy mount rather than with in vivo thin section analysis. It is possible (or likely) that mineral separation steps and grain picking biased the analyses toward a single titanite population. Little Cottonwood stock titanite grains from all eight samples produced dates from 37–27 Ma with modes at 34.5 and 31.5 Ma, and a di fferent distribution for the zircon dates (Figure 5C). The oldest titanite dates ranged from ~37–32 Ma and came from structurally deeper western samples (MS15-03 and DR18-01, 02, and 03; Figure 5) near the Wasatch fault and from the satellite Ferguson stock. Samples from the center and structurally higher portions of the stock defined the middle and younger portions of the range from ~34−27 Ma. However, the incomplete spatial coverage of presently dated samples led us to postpone interpretation of an emplacement pattern pending collection of more spatially complete data.

Both zircon (125 analyses) and titanite (316 analyses) were analyzed in six samples from the Alta stock, including four samples from the border phases (D310b, B4, 89-I-11, and 88-I-9) and two samples from the central phase (C4 and C7). Zircons within the Alta stock range from an equant with a diameter of ~15 μm to elongate and ~100 μm in the long dimension. Uranium-lead dates from zircon in the AS border phases ranged from 35–31 Ma with a mode slightly younger than 34 Ma. Dates from zircons in the Alta central phase ranged from 34.5–32 Ma with a mode of 33.5 Ma. Modes of titanite dates from each Alta stock phase were younger than the zircon modes, and the ranges of titanite dates exceeded the ranges of zircon dates. The majority of titanite in the Alta border phases ranged from ~35–30 Ma. However, sample 88-I-9 from near the southern Alta stock contact contained titanites that spanned 35−25 Ma (excluding four analyses; Figure 4).

Titanites were also dated from a metasomatized Alta stock granitic rock (endoskarn sample 12-12-A2; 67 analyses) and from a wollastonite-bearing skarn (sample 11-1; 45 analyses) in the inner Alta contact aureole. Dates from the endoskarn sample ranged from 35–23 Ma (excluding three analyses) and had a complicated polymodal distribution (Figure 5A). The modes did not correlate with modes from sample 88-I-9 or in any of the Little Cottonwood stock distributions. The majority of titanite dates from the wollastonite skarn ranged from 37–29 Ma with modes at 34 Ma and 32 Ma and a single analysis at ~26.5 Ma. These modes did not correlate to the Alta stock zircon or titanite date modes, but did correlate both with modes of Little Cottonwood stock titanite dates and with the mode of Ferguson stock titanite dates (34 Ma; Figure 5).

**Figure 4.** (**A**) The typical Tera–Wasserburg plots of U-Pb isotopes in zircon were nearly concordant with some outliers. The zircons in the Alta and Little Cottonwood stocks contained dates that reflected multiple generations of growth. In contrast, the Tera–Wasserburg plots of U-Pb isotopes in titanite have a large range of radiogenic and common Pb [89] and were interpreted to indicate an age range within these two samples from the Alta stock. The grey lines that bound the data were reference isochrons for the age range listed for each sample [82]. Red error ellipses were excluded based on either anomalously high 204Pb or low Si and/or Ca concentrations, which indicated that a mineral other than titanite was sampled by the laser. (**B**) Representative cathodoluminescence (CL) and backscattered electron (BSE) images of zircon and titanite grains show the zoning and range of spot dates (colored by lithologic unit) observed in the Little Cottonwood stock (white data) and Alta stock (grey data) border phase and endoskarn samples. Laser sampling locations are labeled with the 207Pb-corrected date and show that neocrystallized and reaction-rim grains were typically younger than recrystallized titanites. (**C**) Hand sample and petrographic thin section photographs show the unaltered macroscopic appearance of MS15-01, 88-I-9, and the gradational replacement of igneous rock by endoskarn (light purple data) in sample 12-12-A2.

**Figure 5.** Kernel density estimates (KDEs) colored by lithologic unit of 40–20 Ma 207Pb-corrected dates that were calculated and plotted using IsoplotR [81] for the (**top**) Alta aureole and endoskarn titanite dates (solid purple), (**middle**) Alta stock border and central phases titanite (dashed dark greys) and zircon dates (solid dark greys), and (**bottom**) Little Cottonwood stock titanites and zircon (solid white and light grey) and Ferguson stock titanite dates (dashed white and light grey). The heights of the KDE curves were normalized per grouping and the maximum abundances are labeled on the y-axis for each curve.

#### 3.2.2. Trace Elements and Thermometry

The concentrations of all rare earth elements (REEs) in the Alta–Little Cottonwood titanites decreased with time (Figure 6A, Figure S1 and Table S1). Ytterbium, for example, decreased from >10<sup>3</sup> times chondrite prior to 30 Ma to <10<sup>2</sup> times chondrite after 30 Ma. The oldest titanites defined a narrow range of REE contents with a slightly higher light (L)REE and lower heavy (H)REE contents. The europium anomaly (Eu/Eu\*) in titanites ranged from 0.3 to 3, excluding outliers, with a mode of ~0.9. Pre-35-Ma titanites generally fell below 1.0, while 35–30-Ma titanites defined the full range of Eu/Eu\* from 0.1–3. Analyses younger than 30 Ma also generally had an Eu/Eu\* of less than 1.0. Many analyses of zircon had REE concentrations near or below the detection limit during LASS analysis and the spider plot does not include concentrations below 1× chondrite (Figure 6B). All zircon analyses showed a typical pattern of low LREE and middle (M)REE concentrations and increasing HREE content with a pronounced cerium anomaly (Ce/Ce\*). Zircons increased in all REE with time, most notably Pr through Dy. The samarium content (Figure 6B) increased from <10<sup>2</sup> times chondrite prior to 30 Ma to generally >10<sup>2</sup> after 30 Ma. The Eu/Eu\* (~0.1–7) in zircon became more scattered through time. Cerium anomalies (Ce/Ce\*) in the Alta–Little Cottonwood zircons ranged from 101–104 and were positively correlated with Eu/Eu\* (*p* < 0.01; n = 22), meaning zircons with a higher Ce/Ce\* tended to have a less negative Eu/Eu\* (closer to 1.0). The Ce4+ and Eu3<sup>+</sup> ions were both favored at higher oxygen fugacity [90,91], which is suggestive of trends in the Alta–Little Cottonwood magma oxidation state, but there was no statistically significant relationship between either anomaly and the 207Pb-corrected date.

**Figure 6.** Semi-logarithmic chondrite normalized spider diagrams for all (**A**) titanite and (**B**) zircon analyses, colored using the 207Pb-corrected dates. Older dates are blue and younger dates are red. Titanite analyses became depleted in lanthanides through time, while zircons became enriched through time. See the Data Repository File for the concentration and normalized data.

The titanium contents of Little Cottonwood stock zircons ranged from 3.8–10 ppm (Figure 7) with a dominant mode at 4.8 ppm. Zircons from the Alta border phase ranged from 2.7–8.4 ppm titanium with a mode at 4.0 ppm. The Ti contents of Alta central phase zircons ranged from 0–8.3 ppm with modes at ~3.0 and ~4.5 ppm Ti. Abnormally high Ti contents (>>10 ppm) were likely caused by the ablation of a Ti-rich phase like ilmenite, rutile, or titanite included in zircon. Inclusions were avoided during spot placement and none were observed during analysis, but mineral inclusions were observed during optical and SEM microscopy.

**Figure 7.** Kernel density estimates (KDEs) of the Ti content of zircon (left column) and Zr content in titanite (right column) colored by lithologic unit and calculated and plotted using IsoplotR [81] for the Alta endoskarn (purple), Alta stock border and central phases (dark greys), Little Cottonwood stock (white), and Ferguson stock (light grey). The heights of the KDE curves are normalized per grouping and the maximum abundances are labeled on the y-axis for each curve.

Zirconium contents of Little Cottonwood stock titanites ranged from 10 to >1000 ppm. The two units had different distributions. Ferguson stock titanites contained less Zr with a mode at ~400 ppm, while the Little Cottonwood stock titanites had a small mode at ~15 ppm and a large mode at ~700 ppm. Zirconium contents of the Alta stock border phase titanites ranged from ~10 to ~2000 ppm with a complex distribution. The two modes at ~15 ppm and ~750 ppm roughly corresponded to the Little Cottonwood stock titanite modes. The lower Zr mode in the Alta stock border phase titanites was found only in sample 88-I-9 from the southern margin of the pluton. The Alta stock central phase titanites had a more restricted range of Zr content from ~500–1500 ppm with a mode at ~700 ppm. The titanites from the Alta endoskarn sample 12-12-A2 contained 20–110 ppm Zr and largely overlapped with the mode defined by sample 88-I-9. Titanites from the wollastonite skarn contained anomalously high Zr contents (>>1000 ppm) that corresponded with sector zoning in the crystals. These titanites likely incorporated a non-equilibrium amount of Zr and other trace elements [92,93] and were not considered in the thermometry discussion below.
