Titanite petrochronology of the Acasta Gneiss Complex, Northwest Territories, Canada

This study provides new in-situ U-Th-Pb age constraints, trace element data, and petrologic context for late Archean and early Proterozoic metamorphism recorded in titanite within felsic orthogneisses and mafic amphibolites from the Acasta Gneiss Complex (AGC), Northwest Territories, Canada. The AGC records several thermal events spanning 4.03.0 Ga and metamorphism and deformation during the ~1.9 Ga Calderian Orogeny. While the timing of primary magmatic events is recorded by the U-Pb system in zircon, chronologic constraints on the younger events are sparse. Titanite can record magmatic, metamorphic, and hydrothermal events, crystallizing and reacting across a wide range of lithologies and metamorphic facies. We apply in-situ LA-ICPMS petrochronology and high-resolution WDS trace element analysis to micron-scale domains within titanite. Within the felsic orthogneisses of the AGC, titanite has sub- to euhedral morphologies, records varying U-Pb date populations, and exhibits complex chemical domains with highly variable abundances of Ce, Zr, Y, Nd, and Nb. We infer that these titanites are igneous in origin, with subsequent metamorphism leading to partial recrystallization with attendant U-Pb age resetting and chemical zonation. In contrast, titanite from amphibolites has lower trace element abundances, little to no chemical zonation, sub- to anhedral morphologies, and microstructures indicating reactions with biotite, chlorite, and ilmenite. Based on this evidence, we hypothesize that the titanites in amphibolite are metamorphic in origin and formed as a retrograde phase during the Calderian orogeny. In discrimination plots comparing Ce, Zr, Y, Al, and Fe, titanite of different lithologies tend to plot in separate clusters except where overlap exists between the mafic titanite and some zones in felsic titanite. This implies that titanite of mafic lithologies define the metamorphic overprint signature(s), allowing deconvolution of the more cryptic, possibly 34 Ga records that are preserved in zoned titanite of felsic lithologies. Our results demonstrate that detailed petrochronology of titanite maximizes the information on early Earth evolution that can be extracted from limited exposures of Archean rocks.