MONAZITE AGE MAPPING AND CHEMICAL DATING ON THE ELECTRON MICROPROBE: LINKING PETROLOGY, MICROSTRUCTURES, AND GEOCHRONOLOGY

High-resolution compositional mapping and dating of monazite on the electron microprobe is a powerful and essential tool for tectonic analysis. Because of its rapid and inexpensive nature, this can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains. However, its in-situ nature and high spatial resolution offers a new type of geochronologic data, unobtainable by other techniques. Monazite inclusions in metamorphic porphyroblasts can be used to put specific time constraints on P-T paths and rates of metamorphic and deformational processes. For example, phase relationships in Archean/Proterozoic rocks of northern Saskatchewan suggest a relatively typical, clockwise P-T-t path involving granulite facies metamorphism followed by exhumation. However, monazite analysis indicates that the 1.8 Ga exhumation event may have culminated a multi-stage or protracted history, involving an extended residence at deep-crustal levels. Because monazite is commonly a fabric forming (and inclusion bearing) mineral, microfabrics and microtextures associated with monazite can constrain the age of deformation event, providing new links between metamorphism and deformation. Monazite inclusions in staurolite and andalusite from Proterozoic rocks of New Mexico allow tectonic events at 1.65 and 1.40 Ga to be distinguished even though they involved nearly identical grades. Age mapping and dating can also provide insight into complex results from other geochronologic techniques. Previous monazite dates from the Lower Gorge of the Grand Canyon tend to spread over several tens of millions of years. Age mapping reveals that most monazite grains have a euhedral core domain that is ca. 1.69 Ga with overgrowths ranging from 1.67 to 1.64 Ga. The euhedral core may represent the time of migmatization, with overgrowths representing subsequent events during slow cooling. Single whole-monazite dates yield an average of these domains. Monazite age mapping and dating on the microprobe allow geochronology to be an integral part of the petrological and microstructural analytical process. Dates can be obtained from a large number of samples, and have immediate petrologic and structural context.