Deformation, metamorphism, and time: a multi-method approach to mid-low temperature geochronology in Precambrian cratons

Abstract

Abstract This study examines the microstructural, petrological, and geochemical processes that disrupt date relationships within biotite and apatite. We combine SEM-EBSD, cold/SEM-cathodoluminescence, EPMA and LA-ICP-MS geochemical mapping, with in situ multi-system geochronology ( 40 Ar/ 39 Ar and 87 Rb– 87 Sr in biotite, apatite U–Pb) to investigate grain-scale microstructural controls on isotopic systematics in rocks from the south Rae craton, Canada. Our results show that intracrystalline defects (subgrains, kink bands, twin planes) and intercrystalline decompression-related fabrics (strain-softened and fluid-prone domains) promote fluid-mediated redistribution of major and trace elements (e.g., Rb, Sr, Ar, U, Th, Pb, La, Ce) in biotite and apatite, enabling recrystallization and/or open system behaviour. This results in both older and younger 87 Rb– 87 Sr dates relative to 40 Ar/ 39 Ar and U–Pb systems within the same samples, independently of tectonic domain, reflecting spatially variable isotopic resetting, inheritance, and recycling. These findings underscore how age distributions in biotite and apatite are controlled by spatially variable recrystallization, inheritance, and fluid-mediated reworking rather than simple cooling through closure temperatures. 40 Ar/ 39 Ar, 87 Rb– 87 Sr and U–Pb systems are best interpreted in terms of grain-scale processes rather than cooling paths. Regionally, our youngest 1.73–1.76 Ga ages reflect a late overprint and intracratonic reworking, suggesting that apparent cooling trends in the Trans-Hudson Orogen may sometimes record isotopic disturbance rather than exhumation. Integration of microstructural and geochemical mapping with micro-spatially resolved geochronology provides a more robust framework for interpreting legacy datasets and guiding in situ isotopic dating.