Whole-rock Rb–Sr and Pb–Pb and zircon U–Pb isotope data yield an age of approximately 2800 Ma for the Atâ granite from northeastern Disko Bugt, West Greenland. Field observations and isotope data suggest that the surrounding gneisses were formed by deformation and recrystallization of granitoid rocks similar to the Atâ granite some 100 Ma after the emplacement of the granite. Rb–Sr whole-rock data on siltstones at low metamorphic grade give an age of 1760 ± 185 Ma, which is interpreted as the time of closure of the isotope systems after metamorphism. The initial 87Sr/86Sr ratio demonstrates that the sediments were probably deposited during the early Proterozoic.Field observations and isotope data show that Proterozoic (Hudsonian s.l.) deformation and metamorphism were weak in the investigated area. The Archaean basement is well preserved, and the metasediments have well-preserved sedimentary structures. The area lies between the Proterozoic Nagssugtoqidian and Rinkian mobile belts of West Greenland, which are thus separated by an area of Archaean rocks little affected by mid-Proterozoic tectono-thermal events. Thus the two belts form separate tectonic units and do not constitute a single contiguous vast area of Archaean rocks reworked by Proterozoic deforma-tion and metamorphism. It is suggested that the Proterozoic mobile belts of West Greenland are orogenic zones of restricted width (a few hundreds of kilometres) that may be interpreted in terms of modern plate tectonic processes.

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