Metamorphic and geochronological study of anatectic aluminous gneisses: insights on evolution of the hinterland in the central Grenville Province.

Lasalle, Stéphanie (2014) Metamorphic and geochronological study of anatectic aluminous gneisses: insights on evolution of the hinterland in the central Grenville Province. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis is an integrated petrographic, metamorphic, and geochronological investigation of granulite-facies aluminous gneisses from the central Grenville Province in Québec, Canada. The project aimed at characterizing the gneiss protoliths and reconstructing the metamorphic history of the region. The results provide insight into the evolution of the Laurentian margin upon which the Grenville orogen is built, and contribute to our understanding of the Grenvillian orogeny. The rocks come from a metasedimentary sequence and a volcanogenic bimodal (felsic-mafic) sequence, both exposed in the orogenic hinterland. They consist of garnet–biotite–quartz–K-feldspar– plagioclase with kyanite in the north (high-pressure segment) and sillimanite in the south (mid-pressure segment), and display microstructural evidence of anatexis. The nature of the protoliths was constrained by zircon textures and in situ U–Pb ages obtained via laser ablation–inductively coupled plasma mass spectrometry (LA– ICPMS). Detrital zircon grains from the metasedimentary samples show Laurentian affinities, with ages between 2700 and 1500 Ma. In contrast, well-preserved igneous zircon grains from a sample of the bimodal sequence suggest a deposition age of 1238 ± 13 Ma, coeval with an episode of crustal extension in the central Grenville orogen. The metamorphic evolution of the aluminous rocks is inferred by interpretation of microstructures and mineral chemistry within the framework of isochemical pressure– temperature (P–T) diagrams (i.e., pseudosections), and in situ (LA–ICPMS) U–Pb dating of monazite. Data from a kyanite-bearing rock record a steep P–T path peaking at ~ 14.5 kbar and 860–900 °C, as in other parts of the high-P segment. In contrast, sillimanite-bearing rocks record large variations in T relative to P with a peak at ~ 9.5 kbar and 850 °C. Monazite mainly occurs in the rock matrix, where it commonly shows complex internal textures. Areas dark on backscattered electron (BSE) images record ages of 1070–1020 Ma with a main cluster at 1060 Ma. The large spread of ages in individual samples is consistent with a protracted residence of the host rocks under high-T conditions. In contrast, BSE-bright rims within matrix grains recorded ages of 1010–990 Ma, attributed to (re)crystallization that was promoted by fluid infiltration of the host rocks. This late event is coeval with ultra-potassic magmatism in this area and may record the orogenic collapse.

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