COMPLEX PATTERNS OF CRUSTAL GROWTH IN THE SW GRENVILLE PROVINCE

Nd isotope mapping has revealed large areas of the Grenville Province with homogeneous isotope signatures that are interpreted as accreted juvenile arc terranes. Other areas, especially in the SW part of the province, have more complex Nd isotope signatures. However, by comparing the range of Nd model ages, mean model age, and oldest U-Pb age for each area, relationships between them are suggested.

Orthogneissic rocks from the Muskoka and Algonquin domains in Ontario have complex isotope signatures, but their similar range of TDM model ages (summarised here) suggests that they are related:

Muskoka domain, E of Gravenhurst 1.31-1.70, mean=1.54, n=14 (1.46)

Southeast part of Algonquin terrane 1.38-1.78, mean=1.55, n=21 (1.44)

The oldest published U-Pb age in each area (in brackets) is slightly older than the youngest model age, but the range of model ages reaches back considerably further. This suggests that crustal formation was occurring over an extended period of time, probably in an ensialic arc. This episode of crustal formation was clearly older than the Grenville orogen; hence the designation of this crust as polycyclic.

Orthogneissic rocks from the monocyclic belt (CMB) in Ontario also have complex age signatures, but falling in a much younger range. Again, two or more recognised domains/terranes have similar signatures:

CMB Boundary Zone + Bancroft 1.17-1.47, mean=1.29, n=12 (1.34)

CMB: Elzevir + Mazinaw domains 1.10-1.46, mean=1.25, n=12 (1.30)

In this case the oldest published U-Pb age is considerably older than the youngest TDM model age, which is Grenvillian. This suggests little earlier crustal formation, consistent with the designation of these terranes as a monocyclic ‘composite arc belt’.

In the northeastern Unites States we have obtained the following Sm-Nd results from orthogneissic rocks:

Green Mtns (Vermont Appalachians) 1.36-1.77, mean=1.54, n=10 (1.36)

Adirondack Highlands (N. Y. State) 1.39-1.60, mean=1.50, n=08 (1.33)

These Nd data strongly resemble the polycyclic terranes of Ontario and Quebec, and show much older ranges of TDM model ages than the monocyclic domains of the CMB. Hence, we suggest that the crust in these areas is related to the allochtonous polycyclic belt in Ontario and Quebec, rather than the monocyclic belt. Tectonic implications will be examined.