ND MODEL AGES AND U-PB ZIRCON AGES: DATING CRUSTAL FORMATION IN THE GRENVILLE PROVINCE

Crustal formation ages are one of the most useful tools to understand the development of the continental crust over geological time. A crustal formation age map reveals the nature of the crustal blocks which build the continent and allows us to reconstruct the accretionary orogens that joined these fragments together. Nd depleted mantle (TDM) model ages remain one of the best ways to determine the crustal formation age history of the continents. However, they must be interpreted in the context of other geological evidence. In the Grenville Province, there is now a convergence between U-Pb zircon ages, Sm-Nd whole-rock isochron ages and depleted mantle model ages for two of the major crust-forming events in the province.

The first of these events, in the Late Archean, was responsible for much of the crust in the parautochthonous belt of the Grenville Province, where U-Pb dates, Sm-Nd isochrons, and depleted mantle model ages all converge around 2.72 Ga. A second major event, in the Mid Proterozoic, was responsible for a large crustal block in the allochthonous belt of the central Grenville Province, stretching from Trois Riviers to Sept Iles, and also seen in Newfoundland. Here again there is a convergence of ages, with U-Pb dates and Sm-Nd isochrons giving ages of 1.50 - 1.52 Ga, whereas depleted mantle model ages are only slightly older, averaging 1.55 Ga. These convergences show that the two terranes in question represent vast areas of juvenile crust with very little recycling of older continental material into the mantle source. They also validate the depleted mantle model (DePaolo, 1981) that was used to calculate crustal formation ages.

Based on these encouraging results, we can have confidence in Nd crustal formation ages in other areas of the province with more complex geological histories. One such area is the allochthonous belt in Ontario, where the large range of U-Pb ages indicates a long history of plutonism. This history is matched by a large range of Nd model ages. For example, in the southern part of the Algonquin terrane, TDM ages range from 1.38 to 1.78 Ga. However, these values fall close to U-Pb lower and upper intercept ages (ca. 1.43 and 1.71 Ga) determined by Nadeau and van Breemen (1998) on a granitic orthogneiss. This suggests that even in this complex ensialic arc belt, Nd model ages give a good indication of the true range of crustal formation ages. Other examples will be discussed.