Sm [sbnd]Nd age of the Fisken˦sset Anorthosite Complex, West Greenland

Abstract

The Fisken˦sset Anorthosite Complex of southern West Greenland is a sheet-like layered igneous body which has been fragmented by granitoid intrusions, multiply deformed and folded, metamorphosed as high as granulite facies, and in some places retrogressed to lower amphibolite or greenschist facies. In some localities, however, such as at Majorqap qaˆva, some 20 km inland from the coast, these effects are minimal. Here, metamorphic grade reached only amphibolite facies with no apparent retrogression. Igneous textures and structures are spectacularly preserved, detailed stratigraphic sequences have been established, and in some samples primary mineralogy remains. We report a Sm Nd isotopic study on samples from this locality, attempting to constrain the age of crystallization of the Fisken˦sset Complex. This complex represents a useful marker unit with which the age relationships of surrounding units may be compared. A five-point isochron, including data for whole-rock samples of anorthosite, metagabbro, metaperidotite, and separates of calcic plagioclase and mafic matrix from a coarse megacrystic leucogabbro, corresponds to an age of 2.86 ± 0.05 Ga (MSWD = 2.5), with initial ε Nd of +2.9 ± 0.4. The Sm Nd age is significantly older than previous Pb Pb whole rock ages of the Complex, but is sufficiently close to the 2.8 Ga high-grade metamorphic event which affected the area to cause partial resetting of the Sm Nd system to be considered. However, based on the integrity of the whole-rock Sm Nd isotopic system in similar anorthositic complexes of higher metamorphic grade, and the apparent preservation of magmatic REE patterns in the analysed plagioclase megacryst and coexisting mafic matrix, we conclude that 2.86 ± 0.05 Ga represents the time of crystallization of the Fisken˦sset Complex. This would imply a relatively short time interval, on the order of 70 Ma, during which anorthosite formation, tonalite emplacement, and high-grade metamorphism took place, and this must be accounted for in any tectonic model for Late Archean events in southern West Greenland. In any case, the positive ε Nd value of the Fisken˦sset Complex requires that it was derived from a source with long-term depletion in light REE. These results add to the increasing body of data indicating the widespread presence of depleted mantle sources during the Archean.