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

Abstract

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.

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

“…Previous titanite datasets demonstrate that titanite U‐Pb petrochronology can be a useful tool to constrain the timing of ductile deformation (Bonamici et al., 2015; Gordon et al., 2021; Kavanagh‐Lepage et al., 2023; Moser et al., 2022; Papapavlou et al., 2017). Here, dates from plastically deformed, synkinematic titanite aggregates provide an unequivocal deformation age for the studied shear zones.…”

“…Thermally mediated volume diffusion of radiogenic Pb in titanite is negligible at temperatures <750°C (Holder, Hacker, Seward, & Kylander, 2019; Kohn, 2017; Stearns et al., 2016) and there are few clear examples of Pb volume diffusion in natural titanite data sets (Bonamici et al., 2015; Garber et al., 2017). Titanite further recrystallizes via dissolution–reprecipitation recrystallization, wherein a fluid reacts with a crystal, replaces the original grain with the same or similar phase, alters the trace‐element composition, and variably resets the U‐Pb dates (Holder & Hacker, 2019; Moser et al., 2022; Walters et al., 2022). The development of crystal‐plastic deformation microstructures can also affect U‐Pb dates in titanite and other minerals, with dislocations and/or subgrain boundaries variably trapping or serving as fast diffusion pathways for Pb and other trace‐elements, or forming efficient conduits for fluids to recrystallize grain interiors (Bonamici et al., 2015; Fougerouse et al., 2021; Gordon et al., 2021; Kavanagh‐Lepage et al., 2023; Moser et al., 2022; Odlum et al., 2022; Piazolo et al., 2016; Reddy et al., 2006).…”

“…Each of these processes produces diagnostic intragrain spatial patterns among U‐Pb dates, trace‐element concentrations, and microstructures that can be used to identify the process responsible for the U‐Pb date and trace‐element distributions (Bonamici et al., 2015; Cherniak, 1993; Garber et al., 2017; Holder, Hacker, Seward, Kylander‐Clark, & Holder, 2019; Holder & Hacker, 2019; Kohn, 2017; Moser et al., 2022; Spencer et al., 2013; Stearns et al., 2016; Walters et al., 2022; Walters & Kohn, 2017). We combine backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD) data, laser‐ablation split‐stream ICP‐MS (LASS) analyses of U‐Pb dates and trace‐element concentrations together with electron probe microanalysis (EPMA) measurements of F contents to relate titanite U‐Pb dates to zoning, microstructures, and trace‐element compositions to differentiate among the above processes.…”

“…However, directly constraining the timescales and conditions of deformation, and how the conditions of deformation vary throughout a single deformation phase, is challenging. This is in part because directly dating deformation remains difficult, and constraints on the conditions and timing of ductile deformation typically come from separate minerals, such as quartz and feldspar (Hirth & Tullis, 1992; Tullis & Yund, 1987) versus zircon (Piazolo et al., 2012), monazite (Dumond et al., 2022), titanite (Bonamici et al., 2015; Gordon et al., 2021; Kavanagh‐Lepage et al., 2023; Moser et al., 2022; Papapavlou et al., 2017), apatite (Odlum et al., 2022), etc. There is thus a need for new approaches that combine information on the timing and conditions of deformation (i.e., temperature, pressure, differential stress, strain rate, fluid abundance and composition) from the same mineral.…”

Making sense of shear zone fabrics that record multiple episodes of deformation: Electron backscatter diffraction–derived and crystallographic vorticity axis–enhanced petrochronology

“…We used CSUN's FEI Quanta 600 SEM to collect 1) backscattered electron images (BSEs), 2) EBSD Mis2Mean maps for misorientation analysis, and 3) EDS maps to investigate elemental distributions within individual grains. EDS maps were created to specifically index for the following elements based on methods from Moser et al (2022): Na, Mg, K, Ca, Ti, Zr, Nb, Ce, Fe, Al, Si, P, S, and O.…”