Abstracts
Abstract
The Brookville terrane forms a fault-bounded region, underlain mainly by plutonic and high-grade metamor-phic rocks, flanked on one side by a typical Avalonian terrane (Caledonia terrane), and separated from a typical Avalonian terrane (Mascarene terrane) on the other side by the Silurian Kingston Dike Complex.
Mafic plutons of the Brookville terrane 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 of the plutons fall on smooth curves on Harker diagrams, suggesting that crystal fractionation is the major process controlling the evolution of individual plutons. Trace element modelling indicates that this fractionation was largely due to the removal of hornblende, plagioclase, and alkali feldspar.
The presence of A2~type granites of similar age (ca. 550 Ma) in the Caledonia terrane (Bonnell Brook), Brookville terrane (Fairville), and Mascarene terrane (Lingley) suggest that these terranes were amalgamated by 550 Ma. Subsequent magmatism in the Brookville terrane was characterized by emplacement of plutons typical of continental margin magmatism. This period of magmatism ended with the emplacement of A2-type plutons, suggesting that magmatism in the Brookville terrane ceased during post-subduction extension.
RESUME
Le terrane de Brookville forme une region, delimite'e par des failles, qui repose principalement sur des roches plutoniques et des roches mltamorphiques a haute teneur, accolte d'un c6t6 a un terrane avalonien caractemtique (terrane de Caledonia) ct s6paree, de 1'autre c8te\ d'un terrane avalonien caractiristique (terrane de Mascarene) par te complcxe de filons paralleles siluricn de Kingston.
Les plutons mafiqucs du terrane de Brookville sont provenus d'une source semblable a cellc des basaltes de la marge continental du groupe de Coldbrook du terrane de Caledonia. Certains plutons sialiques sont probablement provenus de melanges de ce composant basaltique avec un composant analogue a une masse de croutc. Les donnles sur les principaux elements de la majority des plutons d£crivent des courbes egalcs et rlgulieres sur les diagrammes de differenciation, cc qui laisse supposer que la cristallisation fractionn£e constitue le principal processus rdgissant Involution de chacun des plutons. La modulisation des elements traces revele que cette cristallisation <Stait grandement due au retrait de hornblende, de plagioclase et de feldspath alcalin.
La presence de granites de type A-2 d'fige similaire (vers 550 Ma) dans te terrane de Caledonia (Bonnell Brook), le terrane de Brookville (Fairville) et le terrane dc Mascarene (Lingley) permet de supposer que ces terranes se sont fusionnis vers 550 Ma. Le magmatisme subsequent dans le terrane de Brookville a 6t6 caracttrise1par Pintrusion de plutons typiques d'un magmatisme de la marge continentale. Cette p£riodc de magmatisme a pris fin avec une intrusion de plutons de type A-2, ce qui amene a penser que le magmatisme & Pintirieur du terrane de Brookville a cesse" au cours de Ifextension posterieure a la subduction.
[Traduit par la redaction]
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