PETROLOGY AND GEOCHEMISTRY OF THE KINGSTON COMPLEX - A BIMODAL SHEETED DYKE SUITE IN SOUTHERN NEW-BRUNSWICK

The lower Silurian Kingston complex of southern New Brunswick consists of metamorphosed sheeted bimodal dykes, exposed over a strike length of more than 100 km and a width of 3 to 8 km. The southwestern portion of the complex is bounded by major mylonite zones and the northeaster n portion is bounded by brittle faults. Mafic dykes consist of actinol ite-plagioclase rocks with tholeiitic chemistry, typical of continenta l margin basalts. Salic dykes are rhyolitic or microgranitic, and have a chemistry typical of A2-type granitoids which are emplaced in post- collisional or post-subduction tensional environments. Despite the obv ious extensional setting, the mafic dykes are chemically similar to ol der basalts in the Saint John region, which were emplaced in a subduct ion setting. Their chemistry apparently reflects the source of the mel ts (underplated lithosphere) rather than the tectonic setting. The Kin gston complex marks both the initiation and southeastern limit of majo r Siluro-Devonian magmatism in southern New Brunswick. Modelling sugge sts that emplacement of the complex took at least 2 to 3 million years during which heat input into the crust remained relatively low. Subse quent increase in the heat input during Silurian and Devonian time pro duced large-scale melting, pluton emplacement and metamorphism of the Kingston complex. Heating probably resulted from delamination of the c rust which allowed hot asthenosphere to impinge on the lower crust. Th e Kingston complex is a unique remnant indicating the origins and natu re of magmatism along major transcurrent faults near the edge of the A valon zone of the Appalachian orogen.