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

   https://doi.org/10.1007/s00410-022-01906-9  

   Moser, Amy C. ; Hacker, Bradley R. ; Gehrels, George E. ; Seward, Gareth G. E. ; Kylander-Clark, Andrew R. C. ; 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.

    

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2. Titanite Petrochronology Records Secular Temperature and Fluid Evolution During Ductile Deformation: An Example From Late Cretaceous Shear Zones in the Eastern Transverse Ranges

   https://doi.org/10.1029/2022GC010855  

   Moser, Amy C. ; Lusk, Alexander D. ; Attia, Snir ; Garber, Joshua M. ; Seward, Gareth G. E. ; Kylander‐Clark, Andrew R. C. ( May 2023 , Geochemistry, Geophysics, Geosystems)

   Quantifying the timing and conditions of ductile deformation is essential for quantitative models of lithospheric deformation. Yet, directly constraining these variables and documenting how they change during a single deformation event remain difficult. We present titanite microstructural, zoning, trace‐element, and U‐Pb data from  

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3. 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. 

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4. Titanite petrochronology records multiphase construction of the Little Cottonwood stock, Utah U.S.A. accommodated by an upper crustal shear zone

   Stearns, M.A. ; Bartley, J.M. ; Bowman, J.R. ; Beno, C.J. ; Udy, N.D. ; Garber, J.M. ( January 2022 , Transactions American Geophysical Union)

   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. 

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5. The Black Belt Shear Zone records the earthquake cycle at the brittle-ductile transition: implications for the SCEC Community Rheology Model

   Elena Miranda, Joshua Schwartz ( September 2023 , Poster Presentation at 2023 SCEC Annual Meeting)

   Abstract. The inception of the Laramide Orogeny in Southern California is marked by a Late Cretaceous arc flare-up in the Southern California Batholith (SCB) that was temporally and spatially associated with syn-plutonic development of a regionally extensive, transpressional shear zone system. This ~200 km-long system is the best analog for the shear zones that extend into the middle crust beneath the major lithotectonic block-bounding faults of the San Andreas Fault system. We focus on the Black Belt Shear Zone, which preserves an ancient brittle-ductile transition (BDT), and is exposed in the SE corner of the San Gabriel lithotectonic block. The mid-crustal Black Belt Shear Zone forms a ~1.5-2 km thick zone of mylonites developed within hornblende and biotite tonalites and diorites. Mylonitic fabrics strike SW and dip moderately to the NW, and kinematic indicators from the Black Belt Shear Zone generally give oblique top-to-SW, sinistral thrust-sense motion (present-day geometry). U-Pb zircon ages of host rock to the Black Belt mylonites demonstrate crystallization at ~86 Ma and metamorphism at ~79 Ma at temperatures ~753 ¡C. Syn-kinematic, metamorphic titanite grains aligned with mylonitic foliation in the Black Belt Shear Zone give an age of ~83 Ma. These data indicate syn-magmatic sinistral-reverse, transpressional deformation. The BDT rocks in the Black Belt Shear Zone are characterized by a ~10 m-thick section of high strain mylonites interlayered with co-planar cataclasite and pseudotachylyte (pst) seams. Microstructural and electron backscatter diffraction (EBSD) analysis shows that the mylonites and cataclasites are mutually overprinted, and pst seams are overprinted by mylonitic fabric development. Pst survivor clasts show the same shear sense as the host mylonite, and this kinematic compatibility demonstrates a continuum between brittle and ductile deformation that is punctuated by high strain rate events resulting in the production of frictional melt. EBSD analysis reveals a decreasing content of hydrous maÞc mineral phases in host mylonite with increasing proximity to pst seams. This suggests that pst was generated by melting of hornblende and/or biotite, implying that coeval development of mid-crustal mylonites and pst does not require anhydrous melting conditions. Rather, the production of pst may liberate water, implying that BDT rock rheology is affected by transient pulses of water inßux and strain rate increases. 

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