Multi-stage zircon and monazite growth revealed by depth profiling and in situ U–Pb geochronology: Resolving the Paleoproterozoic tectonics of the Trans-Hudson Orogen on southeastern Baffin Island, Canada

Abstract

Understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust. We present extensive in situ U–Pb monazite geochronology and U–Pb zircon depth profiling analyses from the mid-crust exposed on southeastern Baffin Island, Canada, in the internal zone of the Paleoproterozoic Trans-Hudson Orogen. In situ LA-ICP-MS U–Pb monazite dating on ten metasedimentary rocks (totalling >300 analyses) from different paleodepths reveals a dominant ca. 1860–1820Ma monazite population that records regional mid-amphibolite- to granulite-facies metamorphism with coeval fabric development. Subordinate monazite with ages between ca. 1800–1750Ma are interpreted to have formed through dissolution-reprecipitation. The SIMS U–Pb depth-profiling technique allowed dating of ca. 1861–1833Ma zircon rims and less abundant ca. 1800–1764Ma zircon rims that would be unresolvable using conventional spot dating. Trace element depth profiling via LA-ICP-MS shows that zircon rims typically have lower Th/U values, higher Hf contents and different REE compositions than zircon interiors. Zircon rim compositions are highly variable, with no apparent link to U–Pb age, and are interpreted as the combined result of metamorphic zircon growth and dissolution-reprecipitation. Our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon, and that they are both sensitive to U–Pb age resetting at lower temperatures during late-orogenic tectonism. Regional mid-amphibolite- to granulite-facies metamorphism and east-directed crustal shortening are bracketed between ca. 1860–1820Ma. These events are interpreted to result from the accretion of cratonic blocks in the internal zone of the Trans-Hudson Orogen. The integrated geochronological approach on regionally-distributed samples enabled us to document ca. 1800–1750Ma monazite domains and zircon rims, which we interpret to result from the terminal collision with the lower-plate Superior craton followed by late- to post-orogenic tectonothermal activity that continued for at least 70 Myr after thermal peak. This prolonged history is characteristic of the northeastern Trans-Hudson Orogen, and implies that orogenic collapse may not be a hallmark of all hot, thickened orogens.