THE 700 MA RAPITAN IRON FORMATION, A TALE FROM LAURENTIA’S LIPS

The Rapitan iron formation is a ca. 700 Ma glacially-related iron formation that occurs in the northern Canadian Cordillera. It is just over 100 m thick, preserved over 40 km in strike length, and was deposited after a 1 billion year hiatus of iron formation deposition. However, it is unclear where the massive amount of iron was derived. The models either depict sub-aqueous black-smoker related iron discharge into the water column, or iron build-up under a glaciated ocean. An alternative interpretation that is gaining traction is that the iron was continentally-derived. In support of this continentally-derived hypothesis, there are three late Proterozoic mafic Large Igneous Provinces (LIPS) in northwest Laurentia. These intruded and erupted on Archean to Neoproterozoic crust and include the 1267 Ma Mackenzie event, the 780 Ma Gunbarrel event, and the 720 Ma Franklin event. The flood basalts with each of the LIPS were erupted sub-aerially in near equatorial positions and would have been exposed to aggressive weathering; only a fraction of these once voluminous flood basalt provinces are now preserved.

The very extensive Decan traps in India preserve ~500 000 km³ of flood basalt. In northwest Laurentia, if each of the three LIPS were of similar size to the Decan traps, the crust at about 700 Ma would have included ~1.5 million km³ of iron rich flood basalt, and as they are generally around 12-18 wt.% Fe₂O₃, this included ~225000 km³ of total Fe₂O₃. Perhaps it was much larger as the Mackenzie Event alone is one of the largest recognized LIPs in Earth’s history. The Rapitan iron formation is exposed in two dimensions and the lateral extent remains unknown, but if it is presumed that it was initially ~400 km³ and contains ~50% Fe₂O₃, then it contains ~200 km³ of Fe₂O₃ in the iron formation. While this does not include all the iron in the Rapitan Group, this 200 km³ of iron is less than 1% of the total iron that was contained in the basalts of Laurentian LIPs. Aggressive weathering and glacial advance eroded iron-rich basalts and deposited iron-rich sediments into the Rapitan depositional basin. For the iron formation to precipitate, a restricted basin would be necessary. Methods of testing these hypotheses will be presented.