IN SITU RB-SR DATING, MICROSTRUCTURAL CHARACTERIZATION AND MINERAL CHEMISTRY ACROSS A SHEAR ZONE WITHIN A SYNTECTONIC PLUTON: AN EXAMPLE FROM THE SHANNON FAULT, YUKON, CANADA

Quantifying deformation timing is fundamental to characterizing shear zone evolution and understanding microstructures that result from tectonic processes. Direct dating of ductile deformation is non-trivial with current methods and typically targets accessory phases, such as U-Pb dating of dynamically recrystallized titanite and/or fluid mediated apatite (re)crystallization. Recent analytical advances that make use of reaction chambers in triple quadrupole ICP-MS systems have enabled in situ investigation of deformed rocks through Rb-Sr geochronology. Critically, the Rb-Sr system enables dating of major mineral phases participating in the deformation such as mica and K-feldspar. Mica, being a mineral phase that readily (re)crystallizes during deformation, is a prime candidate to target for dating moderate to low temperature deformation.

In this study, in situ Rb-Sr geochronology, microstructural analysis and mineral geochemical investigation are applied to a transect of specimens from the Shannon pluton in southeastern Yukon, Canada. Field relationships, including a mappable strain gradient are interpreted to indicate that the ca. 100 to 99 Ma pluton (U-Pb on zircon) was intruded synchronous with movement along the adjacent, northwest trending, strike-slip Shannon fault. Quartz microstructures across the strain gradient associated with the fault are dominantly subgrain rotation dynamic recrystallization and the proportion of recrystallized quartz generally decreases with distance from the fault, indicating a decrease in total strain with distance. Differential stresses calculated using quartz grain and subgrain piezometers are ca. 127 MPa near the fault center and decrease to ca. 39 MPa at 3.6 Km away from the fault within the deformation zone. Mica cores and rims across textural positions (porphyroclasts, shear bands) have overlapping chemical compositions, including Cl, indicating no detectable fluid alteration or chemical changes. Finally, in situ Rb-Sr dating of micas across the strain gradient yield ages that range between 104 ± 1 to 92 ± 3 Ma with an overall increasing-age trend away from the fault center.