Crystal plasticity and small fluid amounts govern the ability of titanite to record the age of deformation: the case of the Anzola shear zone (NE Italy)
Corvò, S., Maino, M., Piazolo, S. 3, Kylander-Clark A. R.C. 4, Langone, A.2
The age at which deformation localizes into shear zones is key information needed to constrain timing and approximate duration of the sequence of events associated with crustal processes such as rifting or mountain building. Titanite is considered a useful accessory mineral for understanding crustal processes since its properties make it a powerful petrochronometer that allow determination of both age (U–Pb isotopes), as well as deformation (microstructural features), geochemical composition (i.e., trace-element composition) and fluid-mineral interaction (e.g., Kohn, 2017).
In this contribution, we investigate the relationships between local rock composition, grain scale chemical variation, microstructure and fluids with the aim to decipher the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We study titanite grains from a post-Variscan mylonitic shear zone developed at middle to lower continental crustal levels under upper amphibolite-facies conditions (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating.
Titanite is selected from mm- to cm-scale mylonites showing compositional variation alternating between ‘amphibole-rich’ (i.e., amphibolites) and ‘clinopyroxene/plagioclase-rich’ domains (i.e., calc-silicates). Titanite from amphibole-rich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive core-to-rim increasing of lattice distortions and local dislocation densities associated with the development of subgrains. Minor neoblasts along grain edges occur as evidence of fluid-assisted nucleation in high dislocation density sites.
In the clinopyroxene/plagioclase-rich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. Moreover, we document the fundamental role of small amounts of fluids circulating along the grain boundaries in controlling, locally but significantly, the chemistry of titanite within amphibole-rich domains. Fluid-mediated replacement reactions are either rare or absent in clinopyroxene/plagioclase-rich domains, as also indicated by weak chemical variations within and among grains.
U–Pb data present correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where high lattice distortion/dislocations and subgrains/neoblasts define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and limited chemical variation define a population providing a lower intercept age comparable (within error) with that obtained from titanites within amphibole-rich domains.
Our findings demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provide valid ages of shear zone activity, only in case of small local interaction of titanite with fluids and dominance of crystal-plastic processes. In conclusion, the ability of titanite to record deformation under upper amphibolite-facies conditions, depends on the composition of the microdomain and is governed (locally) by the crystal-plasticity and the amount of fluids.
