Eoarchean–Paleoproterozoic Tectonothermal History of the Acasta Gneiss Complex Constrained by Titanite and Apatite Petrochronology

Abstract

Deciphering Earth's earliest geodynamic processes requires temporally constrained chemical and structural data from rare exposures of ancient rocks. One of the most well-studied of these exposures is the Acasta Gneiss Complex (AGC) in the Northwest Territories, Canada, which contains the oldest known felsic rocks on Earth. Like other Archean terranes, the AGC has experienced billions of years of magmatism, metamorphism, and deformation, which make it difficult to tie chemical and structural information to specific events. To construct a reliable tectonothermal history, U-Th-Pb isotope and trace-element data from titanite and apatite were collected from two mafic amphibolites and four felsic-intermediate gneisses along a regional traverse through the AGC. The U-Th-Pb isotope data are augmented by petrography, backscattered electron and X-ray element maps, and trace-element compositions to better contextualize the polymetamorphic history preserved in these phases. Titanite records multiple metamorphic stages from ≥3.6 to 1.9 Ga, whereas apatite only preserves 1.9 Ga metamorphism. The ubiquity of ∼2.9 Ga titanite growth at amphibolite-facies or higher temperatures suggests widespread metamorphism in response to early tectonism. Partial to complete recrystallization of titanite and apatite at ∼1.9 Ga in the AGC is coeval with metamorphism during the Calderian orogeny. The preservation of Archean titanite in most samples suggests that the influence of the Calderian orogeny was not pervasive across the AGC. Our accessory phase petrochronologic data present a more complete geologic history for the AGC and inform models for early Earth geodynamics, formation of cratons, and the evolution of tectonics in the Slave craton.