New Petrological and In-situ Electron Microprobe Monazite Age Constrainsts on the Timing of the Foxe Orogeny, Melville Penninsula, Nunavut, Canada

Abstract

The Paleoproterozoic Foxe Fold Belt (FFB) is composed of the Penrhyn Group, a Paleoproterozoic passive margin sequence, and supracrustal Archean basement. These units were interfolded and metamorphosed at amphibolite to lower granulite facies conditions during the 1883-1865 Ma Foxe orogeny, a part of the Trans-Hudson orogeny. The purpose of this study was to constrain the timing of metamorphism and deformation within the Penrhyn Group, in order to determine the role of the Foxe orogeny within the Trans-Hudson orogeny. Petrographic analysis, P-T-X pseudosections, monazite composition, and in-situ electron microprobe U-Th-Pb geochronology from sampled metapelites were used to determine the age and significance of metamorphic and deformation events related to monazite age populations. Population 1 is composed of 1926 ± 8 Ma monazite interpreted as detrital. Population 2 consists of 1892 ± 9 Ma monazite, the youngest detrital ages seen in the Penrhyn Group. Population 3 is composed of 1853 ± 5 Ma high-Y + HREE monazite predating an episode of pervasive garnet growth. Population 4 contains 1839 ± 8 Ma lower-Y + HREE monazite related to pervasive garnet growth. Population 5 is 1819 ± 16 Ma lowest-Y + HREE monazite with high LREE and Th/U, linked to the interpreted peak reaction: Bt + Sil + Pl = Grt + Crd + Kfs + melt. Monazite constraints on deformation fabrics indicate that deformation was ongoing locally as early as 1853 ± 9 Ma and continued until at least 1814 ± 14 Ma, pre- to syn-peak metamorphism. Rare 1794-1776 Ma monazite is interpreted to constrain the age of retrograde metamorphism as the Trans-Hudson orogeny waned. These data support interpreted clockwise P-T-t-D paths consistent with metamorphism initiated by crustal thickening in an orogenic belt.