ELECTRON MICROPROBE MONAZITE AGE CONSTRAINTS ON THE TIMING OF METAMORPHISM AND DEFORMATION IN THE FOXE FOLD BELT, MELVILLE PENINSULA, NUNAVUT, CANADA

The Paleoproterozoic Foxe Fold Belt (FFB) consists of Paleoproterozoic metasedimentary rocks of the Penrhyn Group and underlying Archean basement gneisses of the Prince Albert Group, interfolded during the 1.88-1.865 Ga Foxe Orogeny (part of the Trans-Hudson Orogeny). A separate, older Archean terrane, the Repulse Bay Block, is exposed immediately south of the FFB and ~10-15 km south of the southernmost exposures of the Penrhyn Group. The Penrhyn Group may have been extruded to the NNW during collision between the Prince Albert Group and the Repulse Bay Block, forming the NNW-verging FFB. The Penrhyn Group is a passive margin sequence of amphibolite-facies pelitic and psammitic gneiss, calc-silicate gneiss, massive marble, and minor amphibolite. The Prince Albert Group consists of ~2.76-2.70 Ga supracrustal rocks and granitoid intrusions.

Henderson (1983) and Mazurski (1980) determined peak metamorphic conditions for Penrhyn Group units at two localities ~130 km apart at opposite ends of the FFB in Melville Peninsula of ~700^oC and ~5 kb in the SW FFB and ~650^oC and ~4.5 kb in the NE FFB. The respective peak assemblages of Penrhyn Group metapelites which were sampled for geochronology from the N, NE, and SE-central FFB are Sil - Bt - Kfs - Pl ± leucosome; Bt - Kfs - Pl ± Sil, and Sil - Grt - Bt - Kfs - Pl ± leucosome. The assemblages are consistent with Henderson’s and Mazurski’s interpretation of near-constant metamorphic grade throughout the FFB.

Petrographic analysis, Y-zonation patterns of monazite, and in-situ electron microprobe U-Th-Pb geochronology from the sampled units were used to determine the age and significance of metamorphic events. Two ~1857 Ma dates on monazite cores from different samples are interpreted as the age of initial monazite growth. An ~1843 Ma date on the rim of a monazite inclusion in a K-feldspar porphyroblast yields a lower age constraint on the peak metamorphic assemblage. A ~1776 Ma high Y monazite rim is interpreted as the age of garnet breakdown to Bt + Pl. The ~1857-1843 Ma dates are consistent with monazite growth from crustal thickening initiated during the Foxe Orogeny. The ~1776 Ma date is consistent with retrograde monazite growth during uplift after the 1.83-1.80 Ga docking of the Superior Province to the SE.