GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS OF GRANITOIDS OF THE AVALON ZONE, SOUTHERN NEW-BRUNSWICK - POSSIBLE EVIDENCE FOR REPEATED DELAMINATION EVENTS

Nd, O, and Pb isotope data from southern New Brunswick suggest that 62 5 to 360 Ma granitoid plutons contain a major juvenile component and s mall amounts of Middle Proterozoic or older crustal material. Plutons fall into four major age groups: 600-630, 540-560, 422-430, and 367-39 6 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 ma gmatic pulses have lithologic associations and chemistry not readily r econciled to subduction. The timing and volume of magmatism, presence of mafic end-members, and juvenile signatures in the plutons suggest t hat crustal delamination may have been involved. Little evidence exist s 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 includ e a major mantle component. This differs from contiguous Gander and Me guma 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 nor thwest part of Avalon Zone (Brookville Terrane) may represent a link t o the enigmatic ''Central Crustal Block'' beneath Gander Zone.