Abstract

In situ U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of monazite from upper amphibolite-facies paragneiss of the Committee Bay supracrustal belt, central Rae domain, Canada, reveal three age populations: ca. 2350, 1850, and 1780 Ma. The ca. 1850 Ma age also corresponds to growth of low Th/U zircon as indicated by U-Pb SHRIMP analyses of zircon separates from melanosome and leucosome. The contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution. The region is dominated by a northeast-striking S2 (±S1) fabric, axial planar to tight, northwest-vergent F2 folds. Early garnet is enveloped by this biotite-sillimanite ± cordierite S2 fabric. GarnetI hosts ca. 1850 Ma monazite inclusions (with ca. 2350 Ma cores), placing a maximum age on garnetI growth and S2 development. D2 metamorphic conditions progressed through ∼3.5 kbar (1 kbar = 100 MPa) and 600 °C to near-peak conditions of ∼5 kbar and 675 °C. A minimum age for S2 is provided by unstrained ca. 1820 Ma monzogranite that locally, and regionally, truncates S2. Dominantly ca. 1780 Ma matrix monazite is interpreted to date post-S2 garnetII and cordierite, which record ∼5 kbar and 675 °C. These data indicate that the Committee Bay region experienced penetrative D2 tectonometamorphism at ca. 1850–1820 Ma, with a subsequent static overprint. The absence of a ca. 1.85 Ga plutonic suite in the region suggests that low-pressure metamorphism was a response to thick-skinned crustal thickening initiated at ca. 1870 Ma. The new data highlight the importance of Paleoproterozoic reworking of the central Rae domain in the hinterland of the Trans-Hudson orogen.

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