MICROPROBE MONAZITE ANALYSIS: INTEGRATING COMPOSITION, TEXTURE, AND CHRONOLOGY TO CHARACTERIZE HOST-ROCK TECTONICS

Monazite is a mineral very much in the spirit of Charlie Guidotti. It has an extremely variable composition, reflecting its crystallography combined with host rock geochemistry and P-T conditions, allowing incorporation into geochemical and petrological calculations and models. It has distinctive textures that can be linked to, and place constraints on, deformation histories. It is a robust geochronometer that can preserve crystallization ages through a long history of geological events. Finally, the greatest potential of monazite can only be fully realized when analyzed and interpreted in-situ, in the context of host compositions and host textures. Monazite crystals typically contain distinct compositional domains, many on the order of 5-10µm, representing successive generations of mineral growth, which record details of the geologic history of its host rocks. The electron microprobe can be used to characterize the geometry and quantitative composition of domains, and when carefully configured, determine the U-Th-Total Pb age for domains as small as 5µm in width, with precision on the order of 10m.y. One current focus of research involves integrating monazite into metamorphic reactions that in turn have been linked to tectonic events. Examples will be presented from the east Lake Athabasca region, Saskatchewan, a large terrane of deep crustal granulites. Monazite from a large region of high-P-T migmatites records the evolution of host-rock composition through melting, plagioclase depletion and reintroduction, and retrograde metamorphism. In the terrane-bounding shear zones, monazite records Archean high-grade metamorphism and then a detailed record of Proterozoic exhumation and rehydration. Analysis of monazite composition, texture, and geochronology when integrated with silicate petrology (in the manner that Charlie Guidotti would do it) is a powerful tool for extracting P-T paths and tectonic histories from complex tectonites.