Rb-Sr isotope systematics and contrasting histories of late Archean gneisses, West Greenland

A reconsideration of the Rb-Sr isotope systematics in the late Archean gneisses of West Greenland shows that they can be divided into two groups with contrasting magmatic or metamorphic histories. Around Godthaab the gneisses were emplaced at 3000–3100 Ma, in part as magmas containing previously unrecognized excess radiogenic strontium. Because these rocks also contain varying amounts of early Archean lead, it appears that some of them contain silicate and other components of Amîtsoq-type gneisses that were introduced by anatexis or magmatic contamination. To the south of Godthaab, in the Fiskenaesset region, the Rb-Sr isotope systematics contain no record for a crustal history prior to 2800 Ma. There the gneisses are the products either of a more recent addition of juvenile later Archean calc-alkaline magmas at 2800 Ma or of regional Rb and Sr metasomatism that has profoundly modified the Rb-Sr isotope systematics of 3000–3100 Ma gneisses during the 2800 Ma high-grade metamorphism.