Multiphase Petrochronology of Archean Gneiss Complexes Unraveling polymetamorphic records at the Acasta Gneiss Complex, Northwest Territories, Canada and the Watersmeet Gneiss Dome, MI, USA

Abstract

Archean gneiss complexes represent critical records of the formation and evolution of the continental crust. However, these records are commonly modified by multiple, post- crystallization tectonothermal events, especially during the Archean and Proterozoic. The U-Th- Pb isotope system in titanite and apatite should be advantageous for unraveling the polymetamorphic record within Archean gneiss complexes, but previous studies suggest that these chronometers are partially or fully reset by Paleoproterozoic tectonothermal overprints. This study integrates textures and petrologic context, EPMA WDS major, minor, and trace- element analysis/mapping, and in situ U-Th-Pb isotopic and trace-element LASS-ICPMS data to improve chronologic interpretations of titanite and apatite in Archean rocks with Paleoproterozoic overprints. Titanite from the Acasta Gneiss Complex (AGC), Northwest Territories, Canada and the Watersmeet Gneiss Dome (WGD), MI, USA preserves information about Archean events and Proterozoic overprinting, but apatite is consistently reset by Proterozoic tectonothermal events. At the AGC, four Archean events spanning ~3.6-2.9 Ga are identified in titanite. At the WGD, titanite preserves evidence of metamorphism at ~2.64 Ga interpreted to record the timing of collision and suturing of the Watersmeet terrane to the southern margin of the Superior Province. In both localities, titanite compositions reflect a combination of metamorphic fluid-rock interactions and primary igneous host rock chemistry that can be distinguished through integration of petrologic-geochemical datasets and statistical analysis. These results demonstrate the utility of titanite as an Archean chronometer that can resist Proterozoic overprinting, as well as a petrologic tracer of metamorphic fluids.