The role of excess oxygen for modeling high-Mn, low-Ca garnets in metapelites from the northern Central Metasedimentary Belt of the Grenville Province, Ontario, Canada

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a [Fe.sup.3+]/F[e.sub.tot] ratio of 0.33-0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 [degrees]C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 [degrees]C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P-T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction. Key words: Bancroft terrane, Central Metasedimentary Belt, Grenville Province, excess oxygen, thermodynamic modeling. Le terrane de Bancroft et la zone de chevauchement bordant la ceinture metasedimentaire centrale associee representent la partie nord de la ceinture metasedimentaire centrale (CMC) de la Province canadienne de Grenville. Seules quelques determinations directes de la pression et de la temperature basees sur des methodes petrologiques d'equilibre des phases existent pour la partie centrale du terrane de Bancroft, et la presente etude applique des approches thermodynamiques comme la geothermobarometrie basee sur les isoplethes de grenat pour combler cette lacune et examiner l'histoire metamorphique de la partie nord de la CMC. Quatre echantillons de metapelite ont ete preleves a proximite de l'enigmatique zone de cisaillement de Bancroft, qui marque l'emplacement approximatif de la limite entre le terrane de Bancroft et le terrane d'Elzevir au sud. Les isoplethes de grenat pour ces echantillons ne se croisent que si une certaine quantite d'oxygene excedentaire est ajoutee a la composition de la roche totale correspondant a un rapport [Fe.sup.3+]/F[e.sub.tot] de 0,33-0,38. L'echantillon preleve le plus au nord donne des conditions du pic du metamorphisme d'environ 1 GPa et 780 [degrees]C, alors que l'echantillon preleve le plus au sud, dans le terrane d'Elzevir, enregistre une pression metamorphique d'environ 0,9 GPa a une temperature de 520 [degrees]C. Ce dernier resultat est en contradiction avec des estimations passees de la pression pour la region et le gradient metamorphique de terrain propose, mais repose sur un echantillon dont le modele thermodynamique est mal contraint. Nous en concluons que le gradient metamorphique de terrain dans la partie nord de la CMC dissimule deux trajectoires P-T differentes, un scenario souvent observe dans des modeles d'epaississement crustal et qui indiquerait que la plaque superieure froide (terrane d'Elzevir) a ete charriee vers le nord-ouest sur la plaque inferieure chaude (terrane de Bancroft). Mots-cles : terrane de Bancroft, ceinture metasedimentaire centrale, Province de Grenville, exces d'oxygene, modelisation thermodynamique.
Introduction The Canadian Grenville Province exposes the western roots of an ancient orogenic belt that surrounded the Laurentian and Baltic cratons, forming the supercontinent Rodinia around 1 Ga (e.g., Cawood [...]