Geochemical and Isotopic Characteristics of Granitoids of the Avalon Zone, Southern New Brunswick: Possible Evidence for Repeated Delamination Events

Abstract

Nd, O, and Pb isotope data from southern New Brunswick suggest that 625 to 360 Ma granitoid plutons contain a major juvenile component and small amounts of Middle Proterozoic or older crustal material. Plutons fall into four major age groups: 600-630, 540-560, 422-430, and 367-396 Ma. The first and third groups can be related to subduction in other parts of the Appalachian orogen, but most of the second and fourth magmatic pulses have lithologic associations and chemistry not readily reconciled to subduction. The timing and volume of magmatism, presence of mafic end-members, and juvenile signatures in the plutons suggest that crustal delamination may have been involved. Little evidence exists for extensive Middle Proterozoic or older continental crust beneath the Avalon Zone, which consists mainly of Late Proterozoic continental margin arc material intruded by younger granitoid plutons that include a major mantle component. This differs from contiguous Gander and Meguma Zones in which plutons contain a major volume of recycled old (>1.5 Ga) crust. A region with a greater old crustal component in the northwest part of Avalon Zone (Brookville Terrane) may represent a link to the enigmatic "Central Crustal Block" beneath Gander Zone.