Origin and Provenance of Archean Keewaywin Formation clastic rocks at Sandy Lake, NW Ontario, Canada: Constraints on Archean weathering and depositional processes

Abstract

Mesoarchean supracrustal sedimentary and volcanic rocks reflect provenance, depositional, tectonic and metamorphic conditions in Archean cratons. In the Sandy Lake Greenstone Belt within the North Caribou Terrane of the Superior Province, the Mesoarchean (∼2900 Ma) Keewaywin Formation is a metamorphosed siliciclastic deposit of mudstone, quartz-rich arenite, and volcaniclastic feldspathic litharenites and schists. Analysis of the petrography, mineralogy, and whole-rock and mineral geochemistry of sandstones and pelites indicate that precursor kaolinitic muds, dissolved iron, and quartz-rich sands were developed in soils by moderate to intense, long term weathering of felsic and mafic basement rocks in a tectonically stable environment. The detritus, half or more pelitic, was eroded from those soils and deposited as mud, some aluminous kaolinitic, some ferruginous, and as quartz-rich sands in a low relief setting. Volcaniclastic detritus from contemporaneous felsic volcanism was deposited with continually available kaolinitic mud and quartz- rich sediment. Kaolinitic aluminous muds were variously metasomatised at surface or near surface conditions with K, yielding muscovite-rich schists (as much as 95% muscovite); Ca and Na metasomatism yielded mm-scale, plagioclase rich laminae (up to 89% normative plagioclase) indicative of evaporite conditions. Ferruginous muds yielded chlorite rich schists (as much as 95 modal % chlorite). Reduced iron suggests the original sediments were deposited in an anoxic environment. New detrital zircon U/Pb age for a litharenite sample is 2930–2890 Ma is indicative of the age of an unconformity recognized in the North Caribou Core of the North Caribou Terrane. Detailed analysis of metamorphosed greenstone belt sedimentary rocks enables interpretation of their provenance, climate, weathering, depositional settings and post-depositional alteration. The shale compositions are particularly sensitive indicators of weathering conditions, hence climate. With age data, correlations between greenstone belts may allow recognition of regional large scale climate, weathering and sedimentation events.