40Ar/39Ar Thermochronology Across a Transect of the Eastern Grenville Front Tectonic Zone and the Western Britt Domain of the Grenville Province, Georgian Bay

Abstract

40Ar/39Ar thermochronology of hornblendes from granitic gneiss and amphibolites in the Central Gneiss Belt (CGB) west of Key Harbour along northern Georgian Bay yields ages ranging from 1016 - 995 Ma. Muscovites from the same are yield ages ranging from 993 - 990 Ma. Hornblendes from two locations in the eastern Grenville Front Tectonic Zone (GFTZ) have similar ages to those found in the CGB; hornblendes sampled further west in the GFTZ portion of the transect have ages ranging from 1120 - 1028 Ma. A single muscovite sample from the western end of the transect is 1112 Ma.

The hornblendes of the CGB and eastern GFTZ are ~35 million years older than hornblendes previously obtained to the southeast in CGB and ~10 million years older than hornblendes previously obtained in the western GFTZ. The ages of the CGB and eastern GFTZ samples seem to be geologically reasonable. The 5 - 11 oC/million years cooling rate of the CGB in the present transect for the 700 - 500 oC temperature range is intermediate between the tectonically-driven, very rapid cooling of the western GFTZ (Haggart 1991) and to the west and the slower rate of ~4 oC/million years in the CGB caused by differential erosion (Check 1989, Culshaw et al. 1991, and Cosca et al. 1991) to the east and south. The ~35 million year difference between the hornblendes in the present transect and hornblendes east of Key Harbour may be the result of extension between 1035 Ma and ~1005 Ma. In this postulated episode of extension, the eastern part of the transect became tectonically denuded following normal faulting with E or SE slip.

Excess argon contaminated the three hornblendes and one muscovite in the western GFTZ, elevating the apparent ages of these samples.

Keywords: 40Ar/39Ar thermochronology, age spectrum, Central Gneiss Belt, excess argon, exhumation, extension, Georgian Bay, Grenville Front Tectonic Zone, Grenville Province, hornblende, inverse isochron, muscovite, Ontario. Pages: 111 Supervisor: Nicholas Culshaw