TEXTURE, CHEMISTRY, AND AGE VARIATIONS IN MONAZITE FROM GREENSCHIST TO GRANULITE FACIES, LONGSTAFF BLUFF FORMATION, CENTRAL BAFFIN ISLAND, NUNAVUT, CANADA

The Longstaff Bluff Formation, central Baffin Island, is a thick sequence of psammitic, semi-pelitic, and pelitic metasedimentary rocks that forms the uppermost unit of the Paleoproterozoic (ca. 2.0-1.9 Ga) Piling Group. Metamorphic grade ranges from greenschist facies (biotite + muscovite) in the central part of the study area, through amphibolite facies (cordierite + andalusite or sillimanite), to granulite facies (K-feldspar + cordierite + garnet + melt) in the southern part of the area. The increase in metamorphic grade towards the northern margin of the 1.86-1.85 Ga Cumberland Batholith suggests a relationship between high-grade metamorphism and intrusion. The present study was designed to test this hypothesis, and to investigate systematic variations in monazite texture, composition, and age with increasing metamorphic grade, by analyzing monazite from a series of north-south transects across important regional metamorphic isograds. The work has been carried out using the JEOL 8200 Superprobe recently installed at the Dalhousie Regional Electron Microprobe Laboratory. Results to date show that fine-grained monazite is present in the matrix in the vicinity of the cordierite-in isograd, and persists to the highest grade part of the study area, where it typically forms large inclusions in cordierite in migmatite leucosomes. There is a significant increase in the complexity of chemical zoning in monazite with increasing grade, suggesting multiple stages of growth and resorption, particularly in migmatite samples. Variations in the concentrations of Y and other trace elements can be linked to reactions involving garnet and melt. Preliminary chemical age data fall in the range 2.0-1.8 Ga, compatible with existing constraints from U-Pb isotopic dates. Work is in progress to determine if multiple age domains are present in the analysed monazites at any grade, and to test the hypothesis that regional metamorphism was related to intrusion of the Cumberland Batholith.