Abstract

Trace element data, including rare-earth elements, for six granitic rocks and two metasediments from the Archaean granite–greenstone terrain of Yellowknife, Northwest Territories are presented. Three granodiorites from the synkinematic Western and South-east plutons show similar Rb, Sr, Th, Rb/Sr, and K/Rb to many high level granodiorites, but are enriched in Ba. Their rare-earth element (REE) patterns show enrichment of light REE relative to heavy REE, and lack Eu anomalies. These features, together with Sr isotope data, are compatible with their origin by partial melting of mantle depth garnet-bearing basaltic source rocks, and little if any detectable fractionation of the rising magma. Potassic granites from post-kinematic plutons, such as the Prosperous Lake granite, are enriched in Th, depleted in Sr and Eu, and have very high Rb/Sr ratios and low K/Rb ratios. Their REE patterns, except for Eu, are very similar to those of metasediments which they intrude. These features, together with Sr isotope data, suggest that high degrees of partial melting of metasediments, leaving a plagioclase-rich residue, are the most likely origin for the post-kinematic granites. The REE patterns of the Archaean metasediments, in particular their high CeN/YbN ratios, suggest that they contain a high proportion of material derived from earlier sialic crust, and that local metavolcanic rocks are not clearly reflected in the composition of the stratigraphically younger metasediments.

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