New U-Pb zircon, Nd-Sm, and Sr isotopic data from Gander Zone basement fragments in northern New Brunswick, combined with a review of existing data, support a basement-cover relationship between the Avalonian Bras d'Or-Brookville belt and lower Paleozoic clastic rocks of the Gander Zone. Basement fragments occur as (1) large foliated and unfoliated granodiorite cobbles in a late Arenig to Llanvirn conglomerate of the Vallée Lourdes Formation of the Tetagouche Group and (2) as an allochthonous gabbro sheet (Upsalquitch gabbro), tectonically emplaced on top of the Tetagouche Group during Late Ordovician or Early Silurian thrusting.The granodiorite cobbles of the Vallée Lourdes Formation yielded late Mesoproterozoic (ca. 1.09 Ga) U-Pb zircon crystallization ages and gave an ε Nd (T) of -3.47 and a depleted mantle model age (T DM ) of 2.0 Ga. Xenocrystic zircon grains in the cobbles range in age between ca. 1.16 and 1.55 Ga. Geochemical data from a foliated granodiorite cobble suggest that the magma from which it crystallized formed by mixing of mantle-derived and older (>1.5 Ga) crustal-derived components.The Upsalquitch gabbro has a U-Pb zircon crystallization age of 543 +1/-2 Ma (earliest Cambrian) and is inferred to represent a fragment of the Gander Zone basement. Similar age igneous rocks occur in the Avalonian Bras d'Or-Brookville belt. The Upsalquitch gabbro forms the immediate base to one of the Middle Ordovician basalt suites of the Fournier Group (ca. 464 Ma), which indicates the presence of a significant hiatus. The Fournier Group was deposited in a back-arc basin that formed by splitting a late Arenig magmatic arc built on the Gander Zone. During rifting of the arc the gabbroic basement fragment was tectonically exhumed onto the sea floor by low-angle normal faulting immediately before eruption of the basalts. The gabbro's geochemistry indicates that it was mainly derived from a depleted mantle source. Xenocrystic zircons (ca. 2.6 Ga) in the Upsalquitch gabbro suggest the involvement of old Archean crust, although the presence of the latter is not reflected in the Nd-and Sr-isotopic data.The ages presented in this chapter complement detrital and xenocrystic zircon studies carried out by others in the Gander Zone of New Brunswick and Newfoundland and together indicate that basement to the Gander Zone is mainly made up of Early Cambrian (0.54-0.55 Ga), Neoproterozoic (0.6-0.8 Ga), Mesoproterozoic (1.0-1.55 Ga), and Archean rocks (2.5-2.7 Ga). This range of ages, combined with the isotopic and geochemical data, eliminates Laurentia as a possible basement to the