RARE-EARTH ELEMENT CHARACTERISTICS AND ORIGIN OF THE SUDBURY IGNEOUS COMPLEX, ONTARIO, CANADA

Systematic geochemical studies have been carried out on the Sudbury Igneous Complex (SIC) and its country rocks. The two main rock types of the SIC, norite and granophyre, show contrasting chemical features. The norite contains 54-58 wt% SiO2, whereas the granophyre has a SiO2 content of 65-71 wt%. All rocks are characterized by high absolute rare-earth element contents (SIGMAREE = 63-314 ppm), the granophyre having more than twice the REE contents of the norite, and by LREE-enriched chondrite-normalized distribution patterns. Huronian volcanic rocks and Huronian plutonic granites have the highest REE contents and HREE enrichment among the country rocks. Archean rocks are characterized by very steep REE patterns and positive Eu anomalies. Nipissing diabase contains the lowest REE contents, and has a flat REE pattern. Within the SIC, the REE values are more or less constant through the norite, but the abundances of some elements such as Ce and Yb decrease slightly upward with stratigraphy. REE contents increase sharply in passing from norite to granophyre, and the Eu/Eu* ratio decreases from around unity to 0.66. The contrasting REE contents of the norite and granophyre, combined with their distinctive major- and trace-element contents, are considered to indicate that they resulted from two separate batches of magma, probably emplaced at about the same time. The previous models, in which the SIC was considered to originate from a single magma, either contaminated mantle magma or crustal melt, are not adequate. A new hypothesis is developed based on the chemical features observed in this study. The SIC is considered to have comprised two independent intrusions: the norite was derived from a heavily contaminated mantle magma (30-60%), the main contaminant being Archean granulite-facies rocks of the lower crust; the granophyre represents a crustal melt (75% Archean rocks and 25% Huronian granite) induced by meteorite impact. The new hypothesis better accounts for the large proportion of granophyre in relation to norite and for the Ni-Cu-sulfide mineralization associated with the SIC. It is also favored by the Rb-Sr isotopic data.