THE WAKEHAM TERRANE - A MESOPROTEROZOIC TERRESTRIAL RIFT IN THE EASTERN PART OF THE GRENVILLE PROVINCE

The Wakeham Terrane (northeastern part of the Grenville Province) is a thick pile of sedimentary and volcanic rocks that exhibits most of the characteristics of a terrestrial rift sequence (Wakeham Supergroup). A first rift sequence, the basal Aguanus Group in the eastern half of the terrane is dominated by mature arenites with well-preserved sedimentary structures. Interlayered magmatic rocks belong to a bi-modal suite with a high proportion of porphyric felsic volcanic rocks (dated at 1271 Ma) and subordinate basalts and gabbro sills. Major elements and rare earth contents attest the intraplate nature of this magmatism which has some alkaline afrinities. Late granites and monzonites were emplaced between 1239 and 1245 +/- 3 Ma and contain inherited (Labradorian?) zircons. The second rift sequence, the Davy Group, overlies the Aguanus Group and its base is characterised by conglomerate fans with clasts and pebbles of the Aguanus Group. The Davy Group includes mature arenites and feldspathic arenites, with numerous intercalations of thick gabbro sills (continental tholeiites). One of these gabbros contains baddeleyite and zircon, providing a crystallization age of 1177(+5/-4) Ma. As the Aguanus Group is deformed by two folding events, the second of which only is present in the Davy Group, two stages of rifting must be advocated, one approximately 1280 and the other at 1180 Ma, separated by a compressional event. The major part of the Wakeham Terrane is in the greenschist facies, but metamorphic grade increases near the contacts with granitic and charnockitic terranes. Along its western margin the Davy Group is invaded and metamorphosed by granites and charnockitic rocks (previously dated between 1126 and 1062 Ma) associated with the Havre St. Pierre anorthositic complex. Along its northwestern margin, the Wakeham Terrane is separated from underlying 1665 Ma-old (Labradorian?) gneisses and 1079 Ma-old charnockites by a SE-dipping extensional sinistral shear zone. The waning stages of the Grenville orogeny in the area are dated at 987 +/- 8 Ma from a titanite extracted from the 1245 Ma-old granite of the eastern Wakeham Terrane.