ELECTRON MICROPROBE ANALYSIS OF MONAZITE AND PRELIMINARY CHIME AGES FROM METAPELITES OF THE TRANS-HUDSON OROGEN, NUNAVUT

The metamorphic grade in the metapelites of the Longstaff Bluff Formation,Trans-Hudson Orogen, ranges from upper greenschist to granulite facies. Electron microprobe analysis of monazite was carried on grains of various grade and setting. Major element and trace element contents as well as mapping were used to investigate effects of progressive metamorphism on chemical composition of monazite. Analyses of major elements were used to document chemical variation with metamorphic grade and mineral association. Y is one the main elements of interest. Monazite composition shows an increasing concentration of Y from low grade to high grade with a significant jump at the sillimanite isograd. Other elements such as Th and REE also show variation depending on metamorphic grade or petrological setting. Based on these results, we will attempt to pinpoint key chemical relationships between monazite and other metamorphic minerals and reaction assemblages. The trace element analyses combined with chemical mapping allow calculations of chemical ages (the CHIME method). The study area is thought to have experienced at least two metamorphic events: a older contact metamorphism event (at least partly related to the Cumberland Batholith emplacement) between 1.89-1.86 Ga and a younger regional event from ca. 1.85-1.81 Ga. The dating of monazites will provide better constraints on metamorphic evolution of this part of the Trans-Hudson Orogen. Preliminary results reveal that zoned monazites show older core and younger rim ages that agree with the expected age range. Calculated ages, chemical data, and textural observations will be combined to provide time constraints on microstructural development and porphyroblast growth.