Kimberlitic-carbonatitic dikes of the Saguenay River Valley, Quebec, Canada

Abstract

Dikes of kimberlitic and carbonatitic affinity (age 546 m.y.) cut the Precambrian granulitic and anorthosite terrane in the Saguenay rift valley in the vicinity of Arvida, Quebec. The dikes range from a few centimetres to about a metre in width. They are not kimberlites, sensu stricto, but have features of carbonatitic kimberlites and appear to indicate the separation of a carbonatitic fluid from a kimberlitic magma, albeit in small amount. The rocks are porphyritic and display such textures as chilled margins with quench dendrites of calcite, and fluidal texture in groundmass phlogopite flakes. The dikes are composed of former olivine (now pseudomorphed by talc in some cases and by antigorite in others), phlogopite, apatite, titaniferous magnetite, calcite and minor amounts of aluminous titaniferous salite and aluminous titaniferous pargasite. Three groups are recognized: kimberlitic, carbonatitic and intermediate kimberlitic-carbonatitic. Phlogopite crystals are zoned: Phlog 74Annite 17Ti-biotite 9 (cores), Phlog 80Annite 15Ti-biotite 5 (mantles), Phlog 90Annite 9Ti-biotite 1 (rims and groundmass). Magnetite is zoned from Ulvöspinel 14Magnetite 86 to Ulvöspinel 61Magnetite 39. The zoning of phlogopite toward more magnesian rims is probably due to enrichment of the magma in Mg owing to early separation of magnetite. In general the rocks have a much higher Ca Mg ratio than true kimberlites and this reflects their intermediate character between kimberlites and carbonatites. They appear to lack the pyrope and picroilmenite which are often considered essential to a true kimberlite and in this and other respects they resemble the South African Benfontein sills. The evolution of the kimberlitic magma has been controlled largely by the crystallization of olivine, phlogopite and magnetite, the two latter varying reciprocally with respect to Fe, Ti, Mg and Al. The gradual concentration of Ca and CO 2 and depletion of Fe, Mg, Ti and Al in the magma has led to the development of a carbonatitic magma.