T130 Monazite: The Ultimate Geologic Record

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room 102BC

NEW PETROLOGICAL AND IN-SITU ELECTRON MICROPROBE MONAZITE AGE CONSTRAINTS ON THE TIMING OF THE FOXE OROGENY, MELVILLE PENINSULA, NUNAVUT, CANADA

Paper number: T130-10
Program: Technical Sessions
Time: 3:55 PM – 4:10 PM

Authors

Abstract

The Paleoproterozoic Foxe Fold Belt (FFB) is composed of the Penrhyn Group, a Paleoproterozoic passive margin sequence, interfolded with supracrustal Archean basement units and metamorphosed at upper amphibolite facies conditions during the Foxe Orogeny, a part of the Trans-Hudson Orogeny. The Foxe Orogeny is interpreted to have occurred between 1880 and 1865 Ma on Baffin Island based on the youngest detrital zircon age of a pre-deformational shelf sequence (Lake Harbour Group) intruded by the 1865-1848 Ma post-deformational Cumberland Batholith.

Penrhyn Group metapelites were collected for in-situ monazite electron microprobe U-Th-Pb geochronology. Petrographic analysis and monazite chemistry were used to determine the age and significance of metamorphic events. Sampled assemblages are Crd ± Grt ± Sil ± Bt – Kfs – Pl – Qtz ± leucosome bearing schists and gneisses. A single ~1920 Ma monazite inclusion in a garnet porphyroblast is interpreted as a detrital grain. The date is synchronous with a major detrital zircon population within the Penrhyn Group. Throughout the FFB, high-Y monazite cores yield a ~1855 Ma date, the age of initial monazite growth. Low-Y ~1840 Ma monazite overgrowths are interpreted as the initial age of garnet growth. A very low-Y ~1827-1832 Ma monazite population depleted in HREEs relative to the pre-Grt monazite cores constrains the age of the peak assemblage Grt + Kfs + melt. Here leucosome is folded isoclinally and also occurs in pressure shadows surrounding garnet. A monazite grain with a ~1838 Ma overgrowth in a cordierite porphyroblast provides a lower age constraint on the assemblage Crd + melt. A ~1794 Ma date on a low-Y monazite grain depleted in HREEs is interpreted as the age of retrograde xenotime growth. Deformed garnets are surrounded by coronitic rims of Crd + Bt where a ~1776 Ma high-Y monazite rim enriched in HREEs is interpreted as the age of garnet breakdown.

Dates between ~1855 and ~1827 Ma indicate prograde monazite growth to peak P-T conditions in response to crustal thickening, suggesting that the Foxe Orogeny started by ~1855 Ma and was ongoing until at least ~1827 Ma. Dates of ~1794 and ~1776 Ma are likely retrograde events in response to uplift after the 1830-1800 Ma collision with the Superior Province to the SE.