Neoarchean evolution of differing convergent margin assemblages in the Wabigoon Subprovince: geochemical and geochronological evidence from the Lake of the Woods greenstone belt, Superior Province, Northwestern Ontario

Abstract

The Neoarchean Lake of the Woods greenstone belt is subdivided into three lithostratigraphic assemblages: a lowermost mafic volcanic assemblage: a compositionally diverse, predominantly volcanic, middle assemblage; and a predominantly sedimentary uppermost assemblage. The secular evolution and geodynamic settings of these assemblages are constrained with detailed geochronological and geochemical data. The Lower Keewatin is the oldest assemblage with a zircon UPb age of 2738 ± 2.0 Ma. It consists of Mg and Fe tholeiitic basalts (Mg number = 30–65) with flat to slightly enriched rare earth elements (REE) [( La/ Yb) CN = 0.9−1.6], elevated large ion lithophile elements (LILE), and undepleted high field strength elements (HFSE). The tholeiites do not appear to have exact modern analogs but do have some similarities to primitive arc tholeiites and back-arc basin basalts suggesting an Archean destructive margin setting. The Upper Keewatin assemblage disconformably overlies the Lower Keewatin assemblage. It consists of volcanic and sedimentary groups. Two geochemically distinct volcanic suites are recognized. The dominant suite consists of compositionally diverse calc-alkalins basalt to rhyolite. Felsic end-members have yielded zircon UPb ages ranging from 2712 to 2723 Ma. The suite exhibits highly fractionated REE [( La/ Yb) CN = 4−35], elevated LILE and Nb and Ta depletion anomalies which are very similar to modern island arc volcanic rocks developed in proximity to continental margins. A coeval suite of mafic to ultramafic volcanic rocks consists of Mg tholeiite, komatiitic basalt and ultramafic komatiite (Mg numbers = 45–80). The suite exhibits depleted LREE [( La/ Yb) CN = 0.6−1.0], slightly elevated LILE and undepleted to slightly enriched HFSE suggesting that this geochemically primitive suite was produced by partial melting of a refractory mantle source, perhaps as second-stage melts of the same mantle source from which the Lower Keewatin assemblage basalts were previously derived. The Electrum assemblage unconformably overlies the Upper Keewatin assemblage in the northern part of the belt. It consists of conglomerates, sandstones and minor volcanic rocks having detrital ages indicating deposition after 2699 Ma. The volcanic rocks consists of andesite to rhyolite with shoshonitic to calc-alkaline geochemical affinity as indicated by highly fractionated REE [( La/ Yb) CN = 12−112] highly enriched LILE and pronounced Nb, Ta and Ti depletion anomalies. Samples from the syenitic Falcon Island pluton appear to be coeval with Electrum assemblage volcanics having an U-Pb isotopic age of 2694 Ma and similar but even more alkaline chemistry consisting of highly fractionated LREE and LILE trace elements with very pronounced Nb, Ta and Ti depletion anomalies. These late tectonic suites are geochemically similar to modern shoshonites with a continental influence formed in the late stages of a mature island arc or a continental arc setting.