THE MYTH OF "AVALONIA": DID IT CONSTITUTE A SINGLE TERRANE OR SEVERAL DIFFERENT TERRANES IN THE EARLY PALEOZOIC?

In spite of years of controversy, the nature of the traditional "Avalon zone" during the Early Paleozoic remains a major unresolved problem in the northern Appalachian orogen. The traditional school of thought interprets Avalon as a composite terrane (variously known as Avalonia, Avalon Zone, or Avalon Composite Terrane) assembled during the Late Proterozoic and later accreted to the Gander margin. The opposite explanation is that the components of "Avalonia" were accreted to North America as individual terranes beginning in the Silurian and ending (except for relatively minor reactivation of terrane boundaries) in the Devonian. In the latter interpretation, favoured by the present authors, the number of terranes that accreted may have been as many as three - Avalon terrane sensu stricto, Bras d'Oria, and the New River terrane (Ganderia). An intermediate interpretation is that "Avalonia" was assembled in the Late Precambrian but broke apart in the Early Paleozoic (when it departed from Gondwana?) to form separate terranes, which then accreted individually to the Gander margin in the mid-Paleozoic.

The reasons for controversy are mainly the major faults that juxtapose belts of contrasting rocks, both within and between Avalon terrane sensu stricto, Bras d'Oria, and the New River terrane. The significance of these contrasts and hence the number of distinct "basement blocks" are at the centre of the controversy. Mapping and petrochemical studies have clarified the character of geological components within the inferred terranes. Precise U-Pb geochronology is essential because units that have similar petrochemical features may be very different in age. These studies have identified Silurian arc terranes between Late Precambrian terranes, providing evidence for Silurian terrane accretion. They have also identified high-pressure metamorphic belts separating Avalon terrane sensu stricto in southern New Brunswick from adjacent Bras d'Oria, and also separating Bras d'Oria from the New River terrane, which may be part of the infrastructure of "Ganderia".

However, such data have not yet resolved the controversy. Detrital zircon age data may reveal differences in the provenance of Precambrian and Paleozoic units in southern New Brunswick and hence may provide the additional evidence needed to dispel the "myth of Avalonia".