POTENTIAL U-PB DECOUPLING IN TITANITE INDUCED BY DEFORMATION: INSIGHTS FROM THE MANICOUAGAN IMBRICATE ZONE IN THE GRENVILLE PROVINCE

In this study, we investigate the Pressure-Temperature-time-deformation path from the Manicouagan Imbricate Zone (MIZ) of the central Grenville Province, which exposes eclogitic nappes, in order to better constrain previously proposed model of ductile extrusion. We report new in situ U-Pb, Zr and REE analysis for two samples: (1) an amphibolite gneiss with textures indicative of decompression from eclogite facies collected 1.5 kilometers structurally below a normal-sense shear zone; and (2) a mylonitic garnetite with lineation-parallel titanite collected within the shear zone itself. In sample (1) thermodynamic modelling suggests titanite growth during decompression and heating from eclogite facies to high-pressure granulite facies. Zr-in titanite and Zr-in-rutile thermobarometry results for samples (1) and (2) support titanite genesis at 17 kbar-875°C and 17kbar-895°C, respectively. REE profiles and trace element patterns further suggest that titanite from both samples grew at the expense of allanite and rutile, whereas garnet influence was minimal. In situ LA-ICP-MS U-Pb geochronology yields single populations at 1033.7 ± 7.1 (2σ; MSWD = 2.4) Ma and at 950.1 ± 24.0 (2σ; MSWD: 1.1) Ma for samples (1) and (2), respectively. The age of the former is compatible with the age of the sole regional peak documented at ca. 1030-1050 Ma, whereas the age of the later is 40 myrs younger than the end of extrusion recently constrained at ca. 990 Ma.

Thus, to account for: a similar REE and trace element signature, a single petrogenesis event at peak T and the difference in age, we infer that titanite grains of sample (2) were effectively reset during localized deformation associated with a normal-sense shear zone at ~950 Ma. This implies that trace element and REE composition of titanite is somewhat permanent, whereas the U-Pb system remains sensitive to deformation and recrystallization. In agreement with a growing number of studies, we interpret these results as evidence for decoupling of the U-Pb system from other trace elements and REE.