Snowflake troctolite in the Hettasch intrusion,Labrador: Evidence for magma-mixing and supercooling in a plutonic environment

The snowflake troctolite (SFT) in the Hettasch intrusion is a thin (0–10 m) zone of melatroctolite concordant with the normal leucotroctolites on one limb of the Hettasch intrusion. The textures of this unit are strikingly different from the normal cumulate textures of the Hettasch leucotroctolites and include comb-layered plagioclase, skeletal megacrysts of plagioclase, and spherulitic plagioclase 5–15 cm in diameter (snowflakes). These supersaturation plagioclase morphologies are set in a matrix of fine-grained, layered and unlayered melatroctolite. Because the mineral compositions are more primitive in the SFT than in the surrounding Hettasch cumulates, crystallization models involving supersaturation of the basaltic Hettasch magma fail. However, over a short distance below the texturally-defined SFT and a shorter distance above it, the mineral compositions of the surrounding cumulates gradually merge with those of the SFT. It is concluded, therefore, that the SFT formed from a separate magma that irrupted onto the floor of the Hettasch magma chamber and was supercooled by the basaltic Hettasch magma. Thus the SFT magma is inferred to have been more primitive than basalt. Assuming that fractionation in the SFT was minimal or that no fractionated material has escaped the SFT, an estimate of the bulk composition of the unit should approximate the magma composition. A picritic magma is thereby inferred, and a search for comparable rocks or magma types indicates that these rocks have the most similarities with picritic rocks of the Brito-Arctic province which formed during the early opening of the North Alantic. This similarity supports the hypothesis that the anorthositic Nain complex in Labrador also formed in a rifting environment about 1.4 Gyr ago.