Geochemical characteristics of the Eden Lake Complex: evidence for anorogenic magmatism in the Trans-Hudson Orogen

Abstract

The Eden Lake Complex intrudes granitic rocks located between the Lynn Lake and Leaf Rapids tectonic domains in the Reindeer Zone of the Trans-Hudson orogenic terrane in northern Manitoba. It consists of a number of magmatic phases. Of particular interest are aegirine-bearing monzonites and late rare earth element (REE)- and fluorine-enriched pegmatites. The aegirine monzonite ranges in SiO2 content from 51 to 76 wt.%, showing a continuous fractionation trend from low to high silica. The CaO, Fe2O3T, and MgO contents are low (about 1-6 wt.%), TiO2 contents range from 0.8 to about 0.1 wt.%, and total alkalis range from about 11 to 13 wt.%. The early evolution of the complex was influenced by pyroxene fractionation; during the later stages, K-feldspar fractionation had an increasing effect on the bulk composition of the magma. Accumulation of K-feldspar has obscured the tectonic signature of the complex, but elevated REE, high field strength element, and fluorine contents are consistent with an A-type granitoid association. The complex is associated with REE-bearing pegmatites. Total REE contents in the monzonites range from 1000 ppm in low-silica phases to 20 ppm in high-silica phases, with La/LuCN values ranging from 80 to 25. REE patterns reflect the fractionation of K-feldspar, pyroxene, and minor amounts of titanite, apatite, and zircon. Y/Nb and Yb/Ta values (relatively constant at ~1 and 1.5, respectively) are consistent with derivation from a depleted- mantle source enriched in REE; moreover, positive εNd values of 0.3-1.7 (with an assumed age of 1750 Ma) are consistent with derivation from an initially light rare earth element (LREE)-depleted source. Elevated Σ REE contents and LREE-enriched characteristics of the complex suggest the source region was altered prior to generation of the magma(s).