TEXTURE, COMPOSITION, AND AGE VARIATIONS IN MONAZITE FROM THE LOWER AMPHIBOLITE TO THE GRANULITE FACIES, LONGSTAFF BLUFF FORMATION, BAFFIN ISLAND, CANADA

Abstract

Electron-microprobe data were obtained from monazite along five transects within the Longstaff Bluff Formation, a thick package of Paleoproterozoic turbiditic rocks exposed on Baffin Island, northern Canada, and metamorphosed at conditions of the upper greenschist to granulite facies during the Trans-Hudson Orogeny. During prograde metamorphism, the LREE content of monazite progressively decreased, whereas the U, Th, Y and HREE content increased, with sharp changes at the transition from Sil–Kfs rocks to incipiently migmatitic rocks. In situ chemical dating of monazite yielded a range of ages from 1913 to 1741 Ma. The age data form five distinct clusters (1910, 1880, 1845, 1805, and 1765 Ma), of which at least four are interpreted to represent different episodes of monazite growth. The textural relationship and petrographic context of the grains were used to constrain the significance of the data. The two oldest grains (1910 Ma) could be detrital or metamorphic. The 1880 Ma age is interpreted to reflect monazite growth near the onset of regional deformation. The 1845 Ma age cluster is mostly found in the three southern (higher-grade) transects, the 1805 Ma event is widespread throughout the study area, and the 1765 Ma episode of monazite growth has been recognized mostly in the northern (lower-grade) part of the study area. The results suggest a northward progression of metamorphism from granulite- to lower-amphibolite-facies assemblages, with textural data suggesting that the widespread 1805 Ma event was responsible for migmatite formation in the southernmost part of the study area.