Secular variation in greenstone sequence development emphasizing Superior Province, Canada

Abstract

Abstract Evidence from the Superior Province of the Canadian Shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts: (1) quartz arenite and carbonate-bearing sequences; (2) mafic-ultramafic volcanic sequences; (3) mafic to felsic volcanic cycles; and (4) ‘Timiskaming-type’ sequences of fluviatile sediments and calc-alkaline to alkaline volcanics. Recently recognized quartz-arenite-bearing sequences in the Sachigo and Wabigoon subprovinces, are, from base to top: quartz arenite±stromatolite-bearing carbonate, oxide facies iron formation, and komatiite or tholeiite. Primary structures in the quartz arenites indicate shallow-water deposition and represent mature sedimentation. Granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates. The few dated Superior Province quartz-arenite-bearing sequences are > 2.85 Ga. The shallow-water origin of the primary structures and biota, and the areal extent of the quartz arenite and carbonate sequences, invite comparison with platform sediments at passive margins. Platform sequences unconformably overlie ∼ 3 Ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities. Mafic volcanic sequences contain sheet flows, iron formations, and deep-water argillites. Based on physical volcanology and sedimentology, these sequences are compared to modern deep water mafic plain and Hawaiian volcanism. They generally underlie mafic to felsic volcanic cycles and overlie platform sequences. Mafic sequences are 2740-2700 Ma in age. Mafic to felsic volcanic cycles consist of deep-water tholeiitic flows overlain by shallow-water to subaerial, intermediate to felsic pyroclastics and flows. The facies, style of volcanism, petrography, and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems. This type of lithostratigraphic association varies in age from 2775 to 2700 Ma in the Superior Province with minor exceptions. ‘Timiskaming-type’ sequences consist of fluviatile conglomerate, immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry. Longitudinal and lateral variation within fault-bounded basins is extreme and the sequences have generally been affected by only late stages of deformation. The combination of rock types, facies distribution, the fault-bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest ‘Timiskaming’ sequences are best compared with modern pull-apart basins. Superior Province ‘Timiskaming’ sequences are Within the Superior Province, platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts, followed by mafic plain sequences, arc volcanics, and pull-apart basins. We suggest that platform sequences, based upon limited evidence, are preceded by an orogeny and that pull-apart basins represent a subsequent orogeny. The lithostratigraphic units above have distinct metallogenic associations: (1) platform sequences, based upon compilation of several shields, may contain detrital Au and U, and oxide facies iron formations. Associated felsic volcanics contain no recognized volcanogenic Cu-Zn deposits. In the Superior Province, the following metallogenic associations are noted: (2) mafic plain sequences contain komatiite flow-related Ni deposits Cu, Ni and platinum-group deposits are associated with flows and coeval intrusions: (3) arc volcanic sequences contain volcanogenic Cu-Zn sulphide deposits and scarce deposits of porphyry-type Cu-Mo; and (4) Timiskaming sequences and related plutonism are related to lode Au.