TERRANE ACCRETION VS. RE-WORKING: AN EVALUATION BASED ON GEOLOGY OF THE SUDBURY AREA

Terrane accretion is becoming almost universally accepted as the main mechanism by which continents have been formed. As applied to the Canadian Shield, the concept treats entire crustal provinces, such as the Grenville, as accreted terranes.An older view, however, is that continents have evolved by re-working, largely in situ. The Sudbury area is the junction of three provinces: Superior, Southern, and Grenville. This paper reviews the relationships of these to compare terrane accretion with re-working. Terrane accretion in this area is contradicted by the facts that the Grenville Front is demonstrably not a suture, nor has a suture been clearly identified in the Grenville Province to the southeast. In addition, imbricated Superior-age crust has been radiometrically identified 50 to 100 km southeast of the Front.This implies that the Southern Province was deposited on Archean continental crust, as in fact field relations show. Re-working, in contrast, is demonstrated by radiometrically dated tectonic and magmatic events of the Sudbury area, compiled by D. Rousell et al. Starting with the Levack Gneiss Complex, 2.7 Ga ago, there has been repeated magmatism and tesctonism of crust within an area roughly 150 km on a side for at least 1.7 Ga, down to the Grenvillian Orogeny 1 Ga ago. It is concluded that in this area, crustal evolution has been dominantly a process of repeated reworking of a coherent block of crust, not a series of terrane suturing events.