Rare earth distribution in Archean granitoid plutons of the Wabigoon volcanic–plutonic belt, northwestern Ontario

Abstract

Rare earths (La to Lu, Y) are investigated for five Archean granitoid stocks of Kenoran age that intrude the Wabigoon volcanic–plutonic belt. Homogeneous granodiorites are characterized by low total rare earth concentrations (ΣREE), with chondrite-normalized REE patterns that show steep negative slopes, no Eu anomalies, and enrichment of Lu. A hypabyssal porphyry of possible volcanic affiliation displays similar REE patterns, but is more depleted in heavy REE. Zone plutons yield patterns of steep slope, no Eu anomalies, with or without Lu enrichment. REE concentrations decrease from monzodioritic margins, to granodioritic cores, to aplitic apophyses.These plutons carry REE concentrations similar to the Canadian Shield average, but notably lower than some published averages for granitoids. No secular change is evident for the Archean interval.ΣREE decreases during differentiation with no appreciable fractionation of heavy over light REE, until the end stages. Late differentiates suffer depletion in heavy REE by hornblende fractionation. Lu enrichment correlates with deuteric metasomatism, as evidenced by microcline megacrysts. Eu anomalies are absent because fractionation of divalent Eu is prevented by high concentrations of Sr and Ba.Quantitative source modeling should consider the complete magma history of emplacement, crystallization, and deuteric metasomatism.