GEOCHEMISTRY OF THE GRANITOID PLUTONS OF THE BROOKVILLE TERRANE, SAINT JOHN, NEW-BRUNSWICK, AND IMPLICATIONS FOR DEVELOPMENT OF THE AVALON ZONE

The Brookville terrane forms a fault-bounded region, underlain mainly by plutonic and high-grade metamorphic rocks, flanked on one side by a typical Avalonian terrane (Caledonia terrane), and separated from a t ypical Avalonian terrane (Mascarene terrane) on the other side by the Silurian Kingston Dike Complex. Mafic plutons of the Brookville terran e were derived from a source similar to that of the continental margin basalts of the Coldbrook Group of the Caledonia terrane. Some sialic plutons were probably derived from mixtures of this basaltic component with a component similar to bulk crust. Major element data for most o f the plutons fall on smooth curves on Harker diagrams, suggesting tha t crystal fractionation is the major process controlling the evolution of individual plutons. Trace element modelling indicates that this fr actionation was largely due to the removal of hornblende, plagioclase, and alkali feldspar. The presence of A(2)-type granites of similar ag e (ca. 550 Ma) in the Caledonia terrane (Bonnell Brook), Brookville te rrane (Fairville), and Mascarene terrane (Lingley) suggest that these terranes were amalgamated by 550 Ma. Subsequent magmatism in the Brook ville terrane was characterized by emplacement of plutons typical of c ontinental margin magmatism. This period of magmatism ended with the e mplacement of A(2)-type plutons, suggesting that magmatism in the Broo kville terrane ceased during post-subduction extension.