Plutonic ages and tectonic setting of the Algonquin and Muskoka allochthons, Central Gneiss Belt, Grenville Province, Ontario
The Central Gneiss Belt comprises parautochthonous gneisses overlain by northwest-transported allochthonous terranes originating from the pre-Grenvillian Laurentian margin or from farther outboard as inferred for the Parry Sound allochthon. In the Huntsville region, orthogneisses of the Algonquin allochthon yielded U-Pb zircon igneous crystallization ages at 1444 +12-8, 1442 +9-8, and 1432 +54-98 Ma. In absence of direct evidence for older intrusions, the association of these plutonic rocks with gneisses giving Nd crustal residence ages of ca. 1.7 Ga sets the Algonquin allochthon apart from the underlying parautochthon, and from the overlying Muskoka allochthon. Orthogneisses from the latter also give zircon igneous ages at 1453 ± 6 Ma and 1427 +16-13 Ma, with no older memory. These ages closely correspond to the Nd model ages of associated gneisses, testifying to the juvenile nature of this terrane. Plutonic ages from the Algonquin and Muskoka allochthons are older than those of the Parry Sound allochtho...