Granites and granites in the East Greenland Caledonides

Caledonian (435-425 Ma) and "Grenvillian" (950-900 Ma) S-type leucogranites and augen gneisses are prominent in the thrust units that form the southern half of the East Greenland Caledonian orogen, south of 76°N. Such rocks do not occur further north (76°N-81°N), where the bedrock is dominated by Paleoproterozoic orthogneisses and metagranitoid rocks (2000-1750 Ma). More mafic Caledonian granitoid rocks (quartz diorites, granodiorites, quartz monzonites, syenites, etc.) are found only in the southernmost parts of the orogen (∼71°N), side by side with S-type leucogranites. The S-type granites were formed by partial fusion of "fertile" lithologies within the late Mesoproterozoic Krummedal supracrustal sequence prior to or during emplacement of the thrust units and subsequent collapse of the orogen. The lack of similar granites north of 76°N is probably related to the absence of major units of metasedimentary rocks in that area. Among the granitoid rocks in the southernmost area, an early quartz-dioritic to granodioritic intrusion was dated at 466 ± 9 Ma; this is ∼35 m.y. older than most Caledonian S-type granites. Quartz-monzonitic, granitic, and syenitic intrusions have yielded ages of 444-432 Ma. These rocks are geochemically similar to Caledonian granites in Scotland and may be related to subduction of oceanic lithosphere underneath East Greenland. The north-south variation in the occurrence of granites in the East Greenland Caledonides is the expression of an original (pre-thrusting) west-east zonation. It is envisaged that the orogen consists of a number of parallel belts, now telescoped by thrusting: a southeastern belt containing supracrustal rocks (Krummedal sequence) with leucogranites, with more mafic granitoids in the southeast, and a northwestern belt where these rocks do not occur. These belts are envisaged to run from Scotland over the southern parts of the East Greenland Caledonides and, obliquely to the Greenland coast, over the North-East Greenland shelf to Svalbard and Norway, where similar rock units also occur.