Melanie J.Haggart

M. Sc. Thesis

Thermal History of the Grenville Front Tectonic Zone, Central Ontario

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The Grenville Front marks the boundary between the ca. 1.2 -1.0 Ga Grenville Province and its foreland. Abrupt increases in metamorphic grade across the Front in central Ontario,and pervasive high-strain, Front-parallel fabric in the adjacent tectonic zone (GFTZ), imply that it accommodated substantial differential uplift relative to the foreland during the Grenville orogeny. Knowledge of the timing, duration and significance of tectono-metamorphic events at the Front is fundamental to developing models of the evolution of the orogen.

U-Pb and 40Ar/39Ar dating of a suite of minerals from a transect across the Front and GFTZ, along the north shore of Georgian Bay, provides new insight into the thermal history of the area. After metamorphism at about 1446 Ma, probably related to voluminous granitoid plutonism, the GFTZ did not undergo significant metamorphism again until about 990 Ma. The second event caused partial resetting of U-Pb systematics in titanite. All titanites from along a

15-kilometre transect perpendicular to the Front fall along a single discordia, and degree of resetting is correlated with distance from the Front. The second metamorphism was therefore a rapid event, during which temperatures exceeded those required to cause Pb diffusion in titanite, for a short time. Temperatures increased with distance from the Front toward the Grenville interior. Peak temperatures were not high enough to generate the granulite-grade mineral assemblages observed within 10 kilometres of the Front. Granulite grade metamorphism is therefore 1446 Ma, or older. Tectonism in the GFTZ accompanying the 1446 Ma event, and later tectonism related to thrust events occurring in the Grenville interior during 1160 to 1025 Ma, may have caused initial differential uplift of the GFTZ relative to the foreland.

40Ar/39Ar dating of hornblende from the same transect confirms the trend of increasing temperature with distance from the Front, and demonstrates very rapid cooling through hornblende closure following the thermal event. 40Ar/39Ar ages from biotite and multiple diffusion domain modelling of K-feldspar provide additional evidence for rapid cooling.

Telescoping of ca. 985 Ma mineral assemblages toward the Front, documented by other workers, demonstrates that thrusting continued beyond the peak of the thermal event. The association of the temperature increase and cooling with thrusting implies that the event was the result of burial by overthrusting. The event partially reset titanite and hornblende near the Front in the GFTZ but failed to reset 40Ar/39Ar systematics in muscovite within 5 kilometres of the Front. Syntectonic erosion accompanying the thrusting event, resulting in rapid unroofing and cooling, best explains these observations.

The event post-dates the peak of metamorphism in the adjacent Britt domain of the Grenville interior by about 50 My. Metamorphism near the Front during 990 and 970 Ma, later than the peak of events in the interior, is well documented in Labrador. This implies that the event occurred on the scale of the whole orogen, and must be explained by an orogen-wide phenomenon.

The multiple diffusion domain history of K-feldspar Ar diffusion behaviour appears to provide realistic cooling histories, which are more useful in constructing low temperature T-t paths than estimated closure temperatures assigned to plateau-like parts of apparent age spectra.

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Supervisors: Rebecca Jamieson and Peter Reynolds