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 Fe3+/Fetot ratio of 0.33–0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 °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 °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.
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Research Article|
July 28, 2020
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
Erik Duesterhoeft
;
a
Christian-Albrechts-Universität zu Kiel, Institute of Geosciences, Ludewig-Meyn-Straße 10, 24098 Kiel, Germany.Corresponding author: Erik Duesterhoeft (email: erik.duesterhoeft@ifg.uni-kiel.de).
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Peter Raase
;
Peter Raase
a
Christian-Albrechts-Universität zu Kiel, Institute of Geosciences, Ludewig-Meyn-Straße 10, 24098 Kiel, Germany.
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Manuel Duguet
;
Manuel Duguet
b
Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario, Canada.
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Robert Michael Easton
Robert Michael Easton
b
Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario, Canada.
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Canadian Journal of Earth Sciences (2021) 58 (1): 21–37.
Article history
received:
16 Dec 2019
accepted:
15 Jul 2020
first online:
19 Jan 2021
Citation
Erik Duesterhoeft, Peter Raase, Manuel Duguet, Robert Michael Easton; 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. Canadian Journal of Earth Sciences 2020;; 58 (1): 21–37. doi: https://doi.org/10.1139/cjes-2019-0238
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