Paleoclimatic control on the composition of the Paleoproterozoic Serpent Formation, Huronian Supergroup, Canada: a greenhouse to icehouse transition

Abstract

Unlike other feldspathic arenites in the ∼ 12 km thick Huronian Supergroup, strata of the 250–350 m thick Serpent Formation contain abundant plagioclase. Deposited in a distal alluvial setting, the Serpent Formation consists of arenites with minor shale and siltstone. The plagioclase abundance could reflect unroofing of a unique source relative to other Huronian units, or could be due to less intense chemical weathering. Major-, trace- and rare earth-element (REE) geochemistry of the Serpent Formation are used to evaluate these two possibilities. Serpent Formation source rocks form part of the Archean Superior Province, which mainly consists of tonalitic, granitic and supracrustal assemblages and their metamorphosed equivalents. In Al 2O 3 CaO* + Na 2O K 2O (A CN K) space, shales define a linear array which indicates that the unweathered source contained plagioclase and K-feldspar in the ratio of 5:1. This particular array indicates that Serpent fluvial systems tapped a variably weathered carapace developed on the Superior Province, rather than purely fresh bedrock. A modelled mixture of 80% tonalite- and 20% granite-group rocks reproduces REE and trace element characteristics of the Serpent Formation sandstones and shales: average Eu/Eu*= 0.87, La N/Sm N= 4.4, Gd NYb N= 1.8 and Th/Sc= 1.65. Mass balance considerations indicate that less than 5% supracrustal-group rocks can be added at the expense of the granite-group. A unique source for Serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition. Plagioclase:K-feldspar ratios in Serpent sandstones range from 2:1 to 1:2, which achieves mass balance among source, mudstone and sandstone compositions when plotted in A CN K space. Achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles. These findings indicate that plagioclase preservation in the Serpent Formation is related to less intense (compared with other Huronian arenites) paleoweathering conditions, probably a harbinger of the widespread continental glaciation recorded in the overlying Gowganda Formation.