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Granite textures are usually assumed to be unmodified igneous features, but titanite petrochronoloy records a progression from magmatic crystallization to fluid-mediated automorphism in the Little Cottonwood stock (LCS). The Wasatch Mountains expose a profile through the 36-25 Ma Wasatch Igneous Belt owing to 20° eastward tilt in the footwall of the Wasatch Fault. The LCS, Alta stock (AS) and their contact aureoles form an integrated magmatic-hydrothermal system that underpinned the cogenetic Keetley Volcanics (KV). The AS (~3-5 km depth) likely formed a conduit from the deeper LCS (~6-11 km) to the KV. The LCS formed in two phases: 1) ~36–33 Ma, coeval with the AS and KV, and 2) ~32–25 Ma, younger than KV and AS but at this time hydrothermal fluid infiltrated the AS­ to form endoskarn. LCS titanite was analyzed by LASS-ICP-MS in 16 samples of unaltered granite (s.l.) collected along transects from the roof on the east to the deepest exposures on the west and from the northern wall to the southern wall. Principal component analysis of titanite trace-element data distinguishes a magmatic group with high REE and a metamorphic group with low REE and high W, Sr, Sc, V, Cr, Fe, Al, and Pb. The metamorphic group forms BSE-dark rims that are variably developed but present in every sample. U-Pb dates indicate that, across the sample suite, there is nearly complete age overlap between magmatic and metamorphic titanite. We interpret chemical zoning of the titanite to record magmatic crystallization followed by hydrothermal modification of primary minerals. The age overlap suggests that solidified increments were infiltrated by fluid released by crystallization of nearby later increments. Infiltrating fluids also affected the feldspars: although apparently intact when examined optically, CL images reveal the feldspars to have been shattered, then healed by dissolution-reprecipitation. Exsolution of Ab component from K-feldspar to form albite selvages against adjacent plagioclase probably was part of the same process, as were biotite chloritization and exsolution of Ti from primary titanomagnetite to grow metamorphic titanite. Taken together, observations from titanite and major phases are consistent with fluid-mediated submagmatic re-equilibration throughout incremental assembly of the LCS. 

The Wasatch Mountains expose an oblique profile through the Alta and Little Cottonwood stocks (LCS) owing to 20° eastward tilt in the footwall of the Wasatch Fault. The cross section spans the upper 11 km of the crust beneath the Eocene paleosurface exposed in Park City, UT. Previous titanite and zircon U-Pb petrochronology established 10 Myr of simultaneous magmatism and hydrothermal metamorphism both in the deeper LCS and in the shallower Alta stock which likely was the conduit between the LCS and cogenetic Keetley volcanic deposits. Hydrothermal metamorphism within and surrounding the Alta stock was synchronous with and most likely driven by emplacement of LCS and migrated from within the Alta stock and contact aureole to margins of the stock suggesting an evolving permeability structure during and after the crystallization of the LCS. New titanite U-Pb petrochronology from the LCS and stock-bounding Wasatch Fault Zone indicate that 1) the LCS was constructed in two phases, an earlier ~36–34 Ma and a younger ~32–25 Ma phase, 2) the presence of both magmatic and hydrothermal titanite as recorded by trace element chemistry, and 3) a pre-Wasatch Fault ductile shear zone likely accommodated magma emplacement at crustal strain rates beginning around 32 Ma. Principal component analysis of LCS trace element data distinguishes two end-member titanite populations along the first component axis: a magmatic population with high REE and a metamorphic population with low REE and high Sr, Sc, V, Cr, Fe, Al, Pb, and particularly W. The second principal component is defined by variance in the REE interpreted to record fractionation by titanite crystallization from melt. The initial ~36–34 Ma phase of LCS construction overlaps with magmatism within the Alta stock conduit and Keetley volcanic rocks and is only found on the western, deepest portion of the LCS. Trace element chemistry of ~36–34 Ma titanites lacks the low REE, high W population suggesting that hydrothermal water released by crystallizing magma did not percolate through these rocks. Low REE, high W titanites are restricted to the structurally higher second phase of the LCS. Despite this relationship, not all samples in the second LCS phase contain the hydrothermal population, which suggests spatially complex magma emplacement and/or later hydrothermal permeability structure. 

Two generations of dikes and sills (earlier granodiorite, later leucogranite) have intruded quartzofeldspathic to semi-pelitic hornfels in the innermost ~200 meters of the southern contact aureole of the Alta stock. Both zircon and monazite are present in the older granodiorite intrusions, and monazite alone is present in the younger leucogranite intrusions, and in biotite-rich reaction selvages formed by hydrothermal contact metamorphism in hornfels adjacent to these dikes and sills. U-Pb dates for zircon (n=532) range from ~38 to 32 Ma, with error on individual measurements of ±1–1.5 Ma, and define a KDE peak at 34.5 Ma. These zircon dates are slightly older than, but consistent with, existing zircon data from the Alta stock (35 to 32 Ma; Stearns et al., 2020), suggesting that the construction of the Alta stock began by emplacement of these granodiorite sills and dikes. Monazite Th-Pb dates (n = 888) range from ~41 to 28 Ma with error on individual measurements of ± 1–1.5 Ma. These dates are complicated by disturbances to the U/Th-Pb systematics by common Pb (Pbc) and excess 206Pb due to 230Th. Dates >38 Ma are disturbed by significant Pbc and do not represent crystallization ages. Dates from the granodiorites range from ~38–32 Ma. In individual samples of granodiorite where the disturbance from excess 206Pb can be rigorously evaluated, the monazite data sets yield concordant 232Th-208Pb and 207Pb/206Pb-corrected dates centered at ~35 Ma, consistent with zircon dates from these same samples. Monazite dates from the leucogranites are younger (<33 Ma), consistent with cross-cutting relationships (leucogranites cross-cut granodiorites). The monazite data from the leucogranite sills and dikes do not record magmatic or hydrothermal activity after ~29 Ma, in contrast to the titanite record of hydrothermal activity to as late as ~23 Ma in the border zone of the Alta stock and its endoskarns (Stearns et al., 2020). This absence suggests that once magma injection and associated contact metamorphism in the hornfels ceased, permeability in the hornfels decreased sufficiently by ~29 Ma to prevent subsequent infiltration of significant fluxes of hydrothermal fluid into these hornfels lithologies in the aureole.