Dating Deformation in the Hinterland: Linking Monazite Dates to Tectonic Events

The ages of deformation events in multiply deformed and metamorphosed terranes are typically inferred from relationships with igneous rocks or metamorphic textures. These can be obscured or compromised by deformation partitioning, and difficulties in interpreting “syntectonic” granitoids. In-situ mapping and dating of monazite is a promising alternative for directly dating tectonic processes. Monazite is a fabric forming mineral that is typically aligned with foliations and lineations. Monazite grains contain aligned inclusions or occur as aligned inclusions in other phases. Compositional domains in polygenetic monazite can define sigma and delta tails, strain shadows, or fracture- fillings that can be kinematically linked to deformation. Significantly, the growth or breakdown of monazite (and other accessory phases) can be understood in terms of silicate reactions that are linked to deformation (i.e. low-Y monazite domains associated with garnet growth during crustal thickening). Typically, a small number of monazite grains have unambiguous microstructural relationships. Thus, a “Rosetta Stone” approach has been extremely successful. Full-section compositional maps are used to locate all accessory minerals in structural context. A subset of grains are mapped and analyzed to define the general sequence of monazite growth events in the rock or region. Finally, a high-resolution compositional single-grain images are used to identify the rare “Rosetta” grains that can place specific timing constraints. This technique has been used to constrain the age of deep crustal shearing in the complexly tectonized Athabasca granulite terrane, Canada. A second example involves unraveling the timing of shear-related structures in the eastern Adirondacks. Pervasive east-dipping fabrics with east-plunging lineations may result from shortening or extension during or after high P-T (ca. 1050 Ma) tectonism. Polygenetic monazite within the garnet-rich gneisses of this region can be used to directly constrain this event. Results will be presented to illustrate the general approach and the specific challenges.