EVIDENCE FOR SEPARATE EVOLUTIONARY PATHS OF THE NORTHBROOK AND WESLEMKOON PLUTONS, CENTRAL METASEDIMENTARY BELT, ONTARIO, CANADA

The Northbrook and Weslemkoon plutons were intruded into the Grenville Province in Ontario, Canada between 1270 and 1245 Ma. They are part of a larger tonalitic suite intruded into the Central Metasedimentary Belt, a presumed magmatic arc, and have traditionally been interpreted to be genetically related based on similar ages, mineral content, and major element compositions. However, until now, a detailed chemical comparison has not been made. This study provides a detailed trace element comparison of the Northbrook and Weslemkoon plutons, in order to evaluate their petrogenetic relationship. Major and trace elements obtained by XRF and high resolution ICP-MS show that the Northbrook and Weslemkoon plutons are chemically similar with regard to their major elements, but chemically distinct with regard to their trace element compositions. Comparisons of trace elements Zr, Nb, Y, and Sm show that the Northbrook and Weslemkoon plutons have different trace element signatures with almost no overlap of the data. In the case of Sm vs. Y and Y vs. Nb the trace element data produce parallel, non-overlapping trends indicating that the plutons cannot be related by crystal fractionation. A chemical comparison of the plutons to the surrounding wall rock indicates that they cannot be related by assimilation. Based on trace element comparisons, we conclude that the Northbrook and Weslemkoon plutons are chemically distinct. Furthermore, they do not seem to be related by crystal fractionation, or assimilation of wall rock. The most reasonable emplacement model for the Northbrook and Weslemkoon plutons is that they were emplaced as distinct magmas and evolved separately.