Major and trace element geochemistry and genesis of supracrustal rocks of the North Spirit Lake Greenstone belt, NW Ontario, Canada

Abstract

Supracrustal rocks of the North Spirit Lake (NSL) greenstone belt comprise three stratigraphic units. The basal unit (Unit 1) consists of pillowed, massive and tuffaceous, tholeiitic basalts and komatiitic basalts (approximately 3.1 Ga) with minor interbeds of greywacke and black pelite displaying characteristics of turbidites and mass flow deposits. REE patterns indicate basalt genesis at comparatively shallow depths (spinel-lherzolite field). Although REE patterns and other trace element characteristics are similar to N-MORBs, concentrations of REE's and Groups IV and V incompatible elements are somewhat lower than in average N-MORB, suggesting comparatively high degrees of partial melting of a fertile mantle. The evidence for an active hydrothermal system (black smoker and “Lost City” types) during the late stages of subaqueous volcanism, the lack of calc-alkaline characteristics for the NSL volcanic rocks, and the absence of geological evidence of a convergent margin setting, together suggest that the rocks of Unit 1 originated at a spreading centre, and represent ocean floor, generated at about 3 Ga. Whether derived from a mantle plume or through upwelling mantle at a spreading centre, the source was more fertile than depleted MORB mantle. Sedimentary rocks of Units 1 and 2 provide evidence of a mixed compositional source that included intra-basinal basaltic material and an exotic tonalitic component. Sedimentary rocks of Unit 3 are derived almost exclusively from a tonalitic source, with derived sediments ranging from highly mineralogically mature to very immature. In spite of the highly active tectonic setting (i.e., rapid denudation rates), alumina-rich pelites are present in all the three Units, implying exceptionally rapid chemical weathering rates in the (tonalitic) source lands. Rapid weathering may be explained by a high CO 2 content in the Archean atmosphere. There is about a 300 Ma interval between Unit 1 volcanism at approximately 3.1 Ga, and igneous activity represented by the Bijou Point Complex which intrude rocks of Unit 3 at about 2.7 Ga. The basal volcanic rocks resemble MORBs whereas the young volcanic rocks are potassic and of calc-alkaline affinity. Although the rocks of the two basal units appear to have formed in or near a spreading center, the rocks of Unit 3 likely were produced in a subduction environment such as those found in island arcs or some continental margin settings. Together, the three units resemble products of a Wilson Cycle, operating between 3.1 and 2.7 Ga, with the calc-alkaline Bijou Point Igneous Complex representing terminal volcanism. Proof or rejection of this hypothesis requires additional detailed mapping, geochemical and geochronological investigations.