MICROSTRUCTURE OF ACCESSORY PHASE CHRONOMETERS: INTEGRATING TIME INTO STRUCTURAL AND PETROLOGIC ANALYSIS

Microstructural analysis is one of the key tools for linking deformation events with metamorphic reactions and for defining a relative chronology of tectonic events. Compositional mapping is an essential step in the microstructural and micrometamorphic analysis. Full-section or area mapping allows integration of assemblages, textures, and fabrics, and also allows accessory chronometer phases to be placed into metamorphic and structural context. High-resolution (0.5µm) mapping (WDS mapping for metamorphic minerals, and monazite/xenotime, CL mapping for zircon) reveals textures and fabrics critical for linking stages of mineral growth, interpreting relative timing and kinematics, and for interpreting chronometer generations to specific reaction and deformation events. That is, the combination of large-scale and small scale compositional mapping is an essential tool for integrating metamorphic texture, deformational fabric, and timing constraints. Individual monazite or xenotime grains can be fabric elements and kinematic indicators in their own right. Many show asymmetric overgrowths, inclusion trails, or broken and dispersed grains in-filled with later monazite or xenotime. Kinematic chronometers are particularly valuable when linked with specific metamorphic reactions, i.e. “reaction dating”. This allows age constraints to be placed on other fabric-forming phases (reactants and products) and thus, on other fabric-forming events. Y has most commonly be used to link monazite to garnet or xenotime growth or breakdown; Eu and Ca variation has been related to plagioclase and allanite growth/consumption; and U variation has been linked with K-feldspar recrystallization, as well as changes in oxidation/reduction. Absolute timing constrains also provide a means of constraining one of the most difficult problems in microstructural analysis, relating structures and assemblages in one rock to those in another, and thus, building heterogeneous strain and strain partitioning into structural analysis. Heterogeneous strain during progressive exhumation of the Athabasca granulite terrain provides a superb example of the use of accessory phases to illuminate microstructure and the use of microstructure to illuminate accessory phase petrogenesis and geochronology.