Paleoclimatology and provenance of the glaciogenic Gowganda Formation (Paleoproterozoic), Ontario, Canada: A chemostratigraphic approach
The Gowganda Formation (dated as about 2.3 Ga) is widely (but not universally) interpreted as glaciogenic, and our chemostratigraphic study supports such an interpretation. The bulk compositions of diamictite matrix materials and associated argillites are considered to have resulted from the mixture of two detrital components; one is glacial flour, similar in composition to the unweathered Archean upper crust, whereas the other is chemically weathered detritus resembling average Proterozoic shale in composition. The first component has a chemical index of alteration (CIA) value of ∼50, reflecting the abundance of feldspar and dearth of aluminous clay minerals. The second component has a CIA value of ∼70 and was produced primarily in weathering profiles by chemical weathering, typical of temperate climatic regimes. Diamictite matrix materials (92 samples) have an average CIA value of 57. The argillites have higher values; the average is 62 (97 samples). The lower CIA values of the diamictites reflect a higher proportion of glacial flour, suggesting a stronger influence of frigid climatic conditions than that which prevailed during deposition of the argillites. Systematic trends in CIA values are developed across diamictite-argillite boundaries and within a thick argillite between two major diamictite units. These values reflect climatic variations, with maximum amelioration during deposition of the middle section of the argillite. Rare earth element data and Th/Sc and Ti/Al ratios from the diamictite matrix materials all suggest a provenance that included a large supracrustal component (∼45%), together with lesser amounts of tonalite-trondhjemite-granite and granite. The proportion of supracrustal materials such as volcanic rocks and shales may have been exaggerated by selective comminution during glacial transport. The matrix materials of the Gowganda diamictites are severely depleted in Ca and proportionally enriched in Na, relative to the source. The anorthitic component of plagioclase has been selectively replaced by an albitic component during diagenesis and metamorphism. A chemostratigraphic approach provides high-resolution data for reconstruction of paleoclimatic conditions and valuable information concerning provenance and metasomatic processes.