LINKING TITANITE U-PB, MICROSTRUCTURAL AND TRACE ELEMENT DATA TO DEFORMATION AND METAMORPHISM IN A LATE-GRENVILLIAN SHEAR ZONE, SAGUENAY-LAC-ST. JEAN, QUÉBEC

The use of U-Pb geochronology on titanite to date shear zone deformation is promising, but uniquely linking grain portions or populations to specific deformation and/or metamorphic events remains complex. In this study, we investigated the age, trace element and microstructural characteristics of titanite in three ca. 1075 Ma mangerite samples containing mylonitic to ultramylonitic bands to date a late-Grenvillian shear zone. Titanite outside of the ultramylonite portion in the first sample analysed is rare, partially replaces ilmenite and is interpreted as metamorphic titanite. These titanite grains yield a lower intercept age of 1016 ± 11 Ma. The microstructural characteristics of titanite within the ultramylonite (i.e. increased grain size, intracrystalline deformation, polysynthetic deformation twins and asymmetric magnetite wings) are consistent with syn-shearing (re)crystallization. These grains yield a lower intercept age of 1002 ± 10 Ma. There are no systematic variations in geochemical compositions between the two textural settings. The second sample investigated is wholly from within a high-strain zone. The titanite age populations therein correlate with trace element contents (based on Eu anomaly, Th/U, LREE/MREE and Zr), grain morphology and/or within-grain position. The youngest population (1036 ± 5 Ma) is characterized by individual asymmetric grains and overgrowths in low-strain quadrants on ca. 1060 Ma igneous and/or metamorphic titanite, and is therefore interpreted as syn-shearing. In the third sample analyzed, titanite populations identified based on Th/U, Zr and LREE/MREE do not yield singular dates, consistent with decoupling of U-Pb dates and trace elements. The geochronological data, further, do not correlate with grain size, morphology or position relative to the ultramylonite band, which impedes identification of syn-shearing titanite based on microstructural evidences. The results of investigation of these three samples highlight the necessity to use a combination of trace element and microstructural characteristics to interpret dates as deformation ages, and they demonstrate the common decoupling between U-Pb dates and trace element signatures.