High-P metamorphism in the Mesoproterozoic: Petrochronological insights from the Grenville Province

In the Grenville Province, high-P rocks that are discontinuously exposed along the margin of the allochthonous belt preserve the record of deep crustal processes and are an essential puzzle piece to understanding Mesoproterozoic geodynamics. The Manicouagan Imbricate zone (MIZ) is one of the three high-P domains from the Grenville Province. It is located in the Central Grenville between the Parautochthonous Belt to the North and the orogenic hinterland to the South, that were metamorphosed during the 1005–980 Ma Rigolet and 1080–1020 Ma Ottawan orogenic phases, respectively. In the western MIZ, the Lelukuau Terrane (LT) mostly consists of Labradorian-age (~1650 Ma) mafic suites with a fringe of aluminous rocks at its southern edge. Metamafic samples from the Western and Eastern parts of the MIZ display a peak assemblage of garnet, clinopyroxene, plagioclase, rare pargasite or edenite, and quartz ± kyanite. Pseudosection modelling suggests high-P granulite peak conditions at ca. 14 to 16 kbar and 800–900°C, with the scarcity of hydrous phases and quartz explaining the lack of evidence for partial melting. Zircon cores from the Western LT sample show a maximum magmatic age of ca 1.6 Ga. Lu–Hf and Sm–Nd dating on garnet from this sample yield ages of 1020 ± 7 Ma and 1005 ± 13 Ma, respectively, overlapping within error and inferred to represent peak metamorphic conditions followed by fast cooling. In the Eastern LT sample, garnet Lu–Hf dating yields two ages that are consistent with a petrographically preserved two stage growth, at 1033 ± 6 Ma and 1013 ± 6 Ma, while the Sm–Nd age indicates cooling at 1003 ± 8 Ma. The recorded high-P granulite facies conditions highlight a late Ottawan to Rigolet-age localized crustal thickening at the margin of the hinterland during the propagation of the orogen to the NW, with a possible younging of the high-P granulite-facies metamorphism from the Eastern to Western LT. These new results indicate that the high-P belt in the Central Grenville does not represent the exhumed base of an Ottawan age orogenic plateau, as previously proposed, and that no tectonic hiatus exists between the two orogenic phases, as generally thought. Finally, this publication highlights the diversity and diachronicity of the high-P domains in the Grenville Province.

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