PALEOCLIMATIC CONTROL ON THE COMPOSITION OF THE PALEOPROTEROZOIC SERPENT FORMATION, HURONIAN SUPERGROUP, CANADA - A GREENHOUSE TO ICEHOUSE TRANSITION

Unlike other feldspathic arenites in the similar to 12 km thick Huroni an Supergroup, strata of the 250-350 m thick Serpent Formation contain abundant plagioclase. Deposited in a distal alluvial setting, the Ser pent Formation consists of arenites with minor shale and siltstone. Th e plagioclase abundance could reflect unroofing of a unique source rel ative to other Huronian units, or could be due to less intense chemica l 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 Prov ince, which mainly consists of tonalitic, granitic and supracrustal as semblages and their metamorphosed equivalents. In Al2O3-CaO+Na2O-K2O( A-CN-K) space, shales define a linear array which indicates that the u nweathered source contained plagioclase and K-feldspar in the ratio of 5:1. This particular array indicates that Serpent fluvial systems tap ped 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 character istics of the Serpent Formation sandstones and shales: average Eu/Eu= 0.87, La-N/Sm-N=4.4, Gd-N/Yb-N=1.8 and Th/Sc=1.65. Mass balance consid erations indicate that less than 5% supracrustal-group rocks can be ad ded at the expense of the granite-group. A unique source for Serpent d etritus is rejected because underlying and overlying units have approx imately the same provenance composition. Plagioclase:K-feldspar ratios in Serpent sandstones range from 2:1 to 1:2, which achieves mass bala nce 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 findin gs indicate that plagioclase preservation in the Serpent Formation is related to less intense (compared with other Huronian arenites) paleow eathering conditions, probably a harbinger of the widespread continent al glaciation recorded in the overlying Gowganda Formation. (C) 1997 E lsevier Science B.V.