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The Tuolumne Intrusive Suite (TIS), Sierra Nevada, California, accumulated magmatic rock from 95 to 85 Ma. Ar-Ar biotite dates require that temperatures within the TIS remained above ~300°C until ~79 million years ago. The protracted thermal history resulted in five texturally and chemically distinct units that young towards the center and was recorded by chemical and isotopic re-equilibration of the minerals. Challener and Glazner (2017) demonstrated that amphibole phenocrysts from the Half Dome Granodiorite (Khd) experienced greenschist-facies metamorphism. Amphibole phenocrysts host abundant inclusions of biotite, chlorite, feldspar, titanite, epidote, and apatite, which are interpreted to have crystallized via breakdown of magnesiohornblende. Additionally, Al zoning suggests fracturing and subsequent healing of the amphibole crystals occurred at near- or subsolidus temperatures. New EPMA and LASS-ICP-MS analyses of texturally related amphibole, titanite, feldspar, and biotite from the equigranular Khd place limits on the timing of amphibole breakdown and contextualize the low-temperature re-equilibration of many of the major minerals in the rock. Most of the amphiboles analyzed contain 0.5–6 wt. % Al2O3 corresponding to actinolite compositions, while feldspar pairs record ~475 ºC apparent temperatures. Titanite grains (re)crystallized between 91–80 Ma and contain 25–825 ppm Zr, which correspond to apparent temperatures between 550–710 ºC (150 ± 50 MPa, aTiO2 = 0.5 ± 0.1). The distribution of Zr in titanites is bimodal with the majority having <200 ppm Zr. Titanites younger than 87 Ma have decreasing Zr content and titanites included within actinolite amphibole contain the lowest Zr content (25–50 ppm) and youngest dates (85–80 Ma). Melt-present crystallization of titanite began at ~91–90 Ma, followed by both near and subsolidus (re)crystallization from ~88–86, concluding with titanite growth via hornblende breakdown from 82–80 Ma. These data taken together with previous investigations provide a continuous record of the rock’s chemical evolution driven by incremental emplacement and subsequent episodic autometamorphism of the equigranular Khd, and critically, any inferences regarding magmatic processes in the TIS must first account for the metamorphic re-equilibration of the rock. 

The U-Pb system in titanite has been shown to be reset during a variety of high-temperature processes including high-temperature deformation, but post-deformation modification and recovery of crystal-lattice strain have so far made U-Pb equilibration mechanism from deformed titanites equivocal. Microstructures, including mechanical twinning and subgrain rotation recrystallization are more likely to be preserved at low-temperatures, but the systematics of chemical equilibration have not been established for these conditions. This study identifies progressive crystallographic misorientation and deformation twins in titanite porphyroclasts from the Wasatch Fault Zone, Utah, USA. The microstructures, mapped using electron backscatter diffraction (EBSD), developed at ~11 km depth during 300–400 ºC crystal-plastic deformation within the ductile fault zone. These microstructural maps were used to guide laser ablation-split stream ICP-MS analysis: U-Pb isotopes measured in tandem with major and trace element contents. Despite the low temperature, U-Pb and trace element contents in titanite equilibrated, at least partially, during deformation. Both major and trace elements in titanite also likely partitioned with a fluid and in response to the (re)crystallization of other mineral phases in the fault zone. Chemical zoning and crystal lattice recovery suggestive of fluid-aided recrystallization are absent, and the main mechanism for this resetting may instead be an enhancement of element mobility along microstructure dislocations. These processes are interpreted to record complex open-system behavior of titanite caused by crystal-plastic deformation during the initiation of the WFZ. This presentation will summarize the comparative analysis of microstructure by EBSD and titanite chemistry by LASS-ICP-MS, and how it bears on the understanding of elemental mobility in titanite during low-temperature crystal-plastic deformation. 

Ian Carmichael wrote of an “andesite aqueduct” that conveys vast amounts of water from the magma source region of a subduction zone to the Earth’s surface. Diverse observations indicate that subduction zone magmas contain 5 wt % or more H2O. Most of the water is released from crystallizing intrusions to play a central role in contact metamorphism and the genesis of ore deposits, but it also has important effects on the plutonic rocks themselves. Many plutons were constructed incrementally from the top down over million-year time scales. Early-formed increments are wall rocks to later increments; heat and water released as each increment crystallizes pass through older increments before exiting the pluton. The water ascends via multiple pathways. Hydrothermal veins record ascent via fracture conduits. Pipe-like conduits in Yosemite National Park, California, are located in or near aplite–pegmatite dikes, which themselves are products of hydrous late-stage magmatic liquids. Pervasive grain-boundary infiltration is recorded by fluid-mediated subsolidus modification of mineral compositions and textures. The flood of magmatic water carries a large fraction of the total thermal energy of the magma and transmits that energy much more rapidly than conduction, thus enhancing the fluctuating postemplacement thermal histories that result from incremental pluton growth. The effects of water released by subduction zone magmas are central not only to metamorphism and mineralization of surrounding rocks, but also to the petrology and the thermal history of the plutons themselves. 

Magmatic and hydrothermal systems are intimately linked, significantly overlapping through time but persisting in different parts of a system. New preliminary U-Pb and trace element petrochronology from zircon and titanite demonstrate the protracted and episodic record of magmatic and hydrothermal processes in the Alta stock–Little Cottonwood stock plutonic and volcanic system. This system spans the upper ~11.5 km of the crust and includes a large composite pluton (e.g., Little Cottonwood stock), dike-like conduit (e.g., Alta stock), and surficial volcanic edifices (East Traverse and Park City volcanic units). A temperature–time path for the system was constructed using U-Pb and tetravalent cation thermometry to establish a record of >10 Myr of pluton emplacement, magma transport, volcanic eruption, and coeval hydrothermal circulation. Zircons from the Alta and Little Cottonwood stocks recorded a single population of apparent temperatures of ~625 ± 35 °C, while titanite apparent temperatures formed two distinct populations interpreted as magmatic (~725 ± 50 °C) and hydrothermal (~575 ± 50 °C). The spatial and temporal variations required episodic magma input, which overlapped in time with hydrothermal fluid flow in the structurally higher portions of the system. The hydrothermal system was itself episodic and migrated within the margin of the Alta stock and its aureole through time, and eventually focused at the contact of the Alta stock. First-order estimates of magma flux in this system suggest that the volcanic flux was 2–5× higher than the intrusive magma accumulation rate throughout its lifespan, consistent with intrusive volcanic systems around the world.