Proterozoic continental volcanism in the Belcher Islands: implications for the evolution of the Circum Ungava Fold Belt

Abstract

The Belcher Islands of eastern Hudson Bay expose a continuous sequence of Early Proterozoic volcanics, and continental and shallow-marine sedimentary rocks. The volcanic rocks comprise two sequences of continental basalts: the older Eskimo Formation and the younger Flaherty Formation. The flows of both formations are composed of tholeiitic basalts (MgO < 9 wt.%) which, in each formation, are divided into two groups based on the concentrations of high field strength elements. Both groups of Eskimo flows have fractionated rare earth element (REE) patterns, with negative Nb anomalies. Relative to Eskimo flows, the Flaherty flows have flatter REE profiles and higher Nb contents. The chemical variations observed in the Eskimo flows are consistent with an assimilation – fractional crystallization process, involving a lower continental crust contaminant. The major and most trace element trends of the Flaherty flows are attributed to gabbroic crystal fractionation of two parental liquids, which are interpreted to represent varying degrees of partial melting of a common mantle source. An increase in La/Sm with increasing La, however, requires that the Flaherty flows evolved under open-system conditions, possibly in a replenished magma chamber or by selective contamination. The interbedding of low- and high-Zr flows in the upper portion of the Flaherty Formation combined with the possibility that their parental magmas originated from a common source require a complex melt segregation process in a partially molten ascending mantle.The chemical compositions of coeval basalts exposed along the eastern Hudson Bay coast and in the Cape Smith Fold Belt (western and eastern Povungnituk Group) indicate that they can be divided into two groups that are the chemical equivalents of the Eskimo and Flaherty Formations. Presently, the chemical stratigraphy of the Belcher Islands offers the most reliable means of correlating the isolated segments of Proterozoic supracrustal volcanics that rim the Superior Province of northern Quebec.