Proterozoic plate tectonics, glaciation and iron-formations

Abstract

Glacigenic diamictites and associated rocks are common in Early and Late Proterozoic sequences. Most of the preserved Proterozoic glacigenic rocks formed in marine basins in an extensional tectonic regime at or close to the rift-drift transition. The reasons for glaciation in this tectonic setting are not known but two possible contributing factors, given an initially cool climatic regime, are the formation of new seaways by continental fragmentation and uplift of the rift shoulders during early stages of breakup. The absence of such glacigenic deposits in the classical miogeoclines of the peri-Atlantic region may be due to an anomalously warm global climatic regime in the late Mesozoic. Iron-formations in the Archean and Early Proterozoic formed in many different environments; the critical factor was a dearth of siliciclastic deposition. Starved basins commonly developed during the transition from miogeocline to foreland basin in the Wilson cycle of ocean opening and closure, when drainage from the craton was replaced by that from a rising orogen. Early Proterozoic iron-rich sediments in the Great Lakes region, in southeast Wyoming and in the Northwest Territories of Canada are interpreted as the earliest deposits of such foreland basins. They are considered to be chemical precipitates from iron-rich sea water. Late Proterozoic iron-formations formed under an extensional regime, possibly in small Red Sea-type ocean basins where hydrothermal circulation produced metal-rich brines. These brines were displaced by movement of cold waters derived from sea-going glaciers, leading to dilution and precipitation of Fe and Si. Thus, in the Late Proterozoic, there was a critical association of continental rifting, glaciation and hydrothermal circulation. This combination of environmental parameters led to the production of the unique Late Proterozoic class of hydrothermal iron-formations.