Hilke Timmerman

A field, metamorphic and U-Pb geochronologic study in the Central Gneiss Belt between Huntsville and Haliburton yields important constraints on the timing of accretion of the 1350-1100 Ma old magmatic arc(s) of the Central Metasedimentary Belt onto the pre-Grenvillian Laurentian craton, and thus on a tectonometamorphic model for the southwestern Grenville Province of Ontario.

The study area includes the Muskoka domain in the immediate footwall of the Central Metasedimentary Belt boundary thrust zone, the crustal-scale thrust zone along which the Central Metasedimentary Belt was emplaced onto the Central Gneiss Belt, and the structurally underlying gneiss assemblages of the Kawagama zone and Algonquin domain. The Pre-Grenvillian evolution is characterised by juvenile plutonism from 1457 Ma to 1394 Ma, giving rise to the predominant granodioritic to granitic orthogneisses of the Muskoka domain. High-grade granulite-facies metamorphism during the Pinwarian orogeny at Ca. 1420 to 1430 Ma affected both the gneisses in the western Muskoka domain and in the structurally underlying Algonquin domain.

The Grenvillian orogeny is characterised in the study area - as in the rest of the Central Gneiss Belt- by the emplacement of lithologically and structurally different thrust sheets towards the northwest, that was accompanied by intense progressive homogeneous deformation. Regional high-grade metamorphism, resulting in both granulite and amphibolite facies assemblages, was dated from 1079 to 1063 Ma in the Muskoka domain. As there is no earlier, Elzevirian tectonometamorphic event recorded in the study area, these data are interpreted to reflect the accretion of the Central Metasedimentary Belt onto the Central Gneiss Belt shortly before 1080 Ma.

Near isothermal decompression P-T-t paths from the study area are consistent with field evidence reflecting continental thickening and later extension along reactivated thrust structures. Initial fairly rapid cooling contrasts with previously proposed models of slow cooling from peak metamorphic conditions for the Algonquin domain. The timing of extension in the study area is interpreted to have happened at approximately the same time as extension in other areas of the Central Gneiss Belt in the range of ca. 1040 to 1020 Ma, predating late-stage thrusting in the Grenville Front Tectonic Zone.

In conclusion, the tectonic evolution in the study area is consistent with the previously proposed tectonic model of the normal northward propagation of the orogen into its foreland. This and the contemporaneous extension within the orogen during the later stages of the Grenvillian orogeny suggests that tectonic processes during the mid Proterozoic operated similarly to those in modern collisional orogens such as the Himalayas.

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Pages: 427
Supervisor: Rebecca A. Jamieson