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

Abstract

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 <m‐scale mylonitic shear zones in the Eastern Transverse Ranges, CA (Joshua Tree National Park) that record (a) the timing and conditions of ductile deformation and (b) the evolution of deformation conditions within a single, punctuated ductile deformation event. Titanite grains in sheared plutonic rocks yield a bimodal distribution of dates (∼151 and ∼75 Ma) interpreted as the independently constrained pluton crystallization and shear zone deformation ages, respectively. Synkinematic titanite neocrystallization at ∼75 Ma documents early deformation at ∼640–710°C under hydrous fluid conditions. Recrystallized titanite rims with ∼75 Ma dates indistinguishable from the neocrystallized titanite record Zr‐in‐titanite temperatures of 600–610°C and higher F contents, suggesting that the fluid became a more halogen‐rich brine as deformation progressed and the system cooled. Chemical zoning related to fluid‐driven recrystallization occurs in tandem with lattice misorientations and partially to fully reset U‐Pb dates in bent titanite, implying that reactive fluid flow is the ultimate mechanism by which dates are reset in titanite deformed by dislocation creep. These data document a punctuated (e.g., ∼1–3 Myr) phase of high‐temperature deformation that accompanied local pluton emplacement and demonstrate that titanite petrochronology can capture multiple, discrete reactions associated with crustal weakening and variables that affect the rheological evolution of ductile shear zones.

Long‐Lived (180 Myr) Ductile Flow Within the Great Slave Lake Shear Zone

“…Considering these textural observations in conjunction with the dates yielded by apatite and titanite within the shear zone, we interpret the dates as recording syn‐kinematic (re)crystallization, which encompasses both dynamic recrystallization of, or re‐equilibration of the U‐Pb system in, pre‐existing apatite and titanite as well as syn‐tectonic growth of new grains. Such (re)crystallization may have been influenced by fluids associated with rehydration of the shear zone during retrograde metamorphism (e.g., Moser et al., 2023). It is possible that the apatite and titanite grains that are aligned with shear planes do not all record the timing of the cessation of shearing, however, given that the apatite and titanite data are significantly younger than prograde‐to‐peak metamorphism as outlined above, it is unlikely that these chronometers are recording earlier magmatic or metamorphic events.…”