More Like this

1. Linking titanite U–Pb dates to coupled deformation and dissolution–reprecipitation

   Moser, A; Hacker, B; Gehrels, G; Seward, G; Kylander-Clark, ARC; Garber, Joshua M (March 2022, Contributions to Mineralogy and Petrology)

   Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains. 

   more » « less
2. Titanite zonation records magmatic to autometamorphic transition in the Little Cottonwood stock, Utah

   Bartley, J.M.; Stearns, M.A.; Bowman, J.R.; Beno, C.J.; Saunders, E.; Nicholas, S. (April 2023, Abstracts Geological Society of America)

   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. 

   more » « less
3. Insights into chemical mobility in titanite driven by low-temperature crystal-plastic deformation

   https://doi.org/10.5194/egusphere-egu21-7327  

   Udy, Nicholas; Stearns, Michael A. (March 2021, EGU General Assembly)

   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. 

   more » « less
4. Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

   https://doi.org/10.2138/am-2022-7967  

   Stowell, H.H. (June 2022, The American mineralogist)

   Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures. 

   more » « less
5. Petrochronology of Wadi Tayin Metamorphic Sole Metasediment, With Implications for the Thermal and Tectonic Evolution of the Samail Ophiolite (Oman/UAE)

   https://doi.org/10.1029/2020TC006135  

   Garber, Joshua M.; Rioux, Matthew; Kylander‐Clark, Andrew R. C.; Hacker, Bradley R.; Vervoort, Jeffrey D.; Searle, Michael P. (December 2020, Tectonics)

   Abstract Ophiolite metamorphic soles preserve important records of ophiolite emplacement, but there have been few detailed investigations into their non‐mafic portions. We present new thermobarometric and petrochronologic data from a metasediment and mafic restite in the upper Wadi Tayin sole exposure in the Samail (Oman‐UAE) Ophiolite. Thermodynamic modeling suggests metasedimentary garnet nucleation at ~4 kb, ~550°C and final growth at 7.5 ± 1.2 kbar, 665 ± 32°C, occurring by 93.0 ± 0.5 Ma (Lu‐Hf isochron). Zircon U‐Pb dates of 106.9 ± 2.3 (detrital) and 98.7 ± 1.7 to 94.1 ± 1.6 Ma (metamorphic) bracket the initiation of metamorphism, and monazite U‐Pb dates from ~97–89 Ma suggest a lengthy period of growth or recrystallization. A mafic titanite U‐Pb age of 92.2 ± 1.8 Ma records the earliest possible juxtaposition of high‐ and lower‐grade sole rocks. These and other data suggest that (i) the Wadi Tayin sole preserves an inverted metamorphic, metasomatic, and age gradient,(ii) metasediment metamorphism occurred during, or soon after, crystallization of the overlying ophiolite (≤96.5 Ma); and (iii) sole metasediments define a thermal gradient continuous with hotter, higher‐Pamphibolites. Some of these data conflict with existing models for sole formation, and we propose several hypotheses to explain them. Cooling of the sole below Ar closure by ~92 Ma suggests that strain rapidly partitioned away from the sole, leading to large‐scale, thin‐skinned thrust emplacement of the ophiolite >100 km across the continental margin and the late, cool underthrusting of the continental margin. 

   more » « less