Dating of the Ameralik dyke swarms of the Nuuk district, southern West Greenland: mafic intrusion events starting from c. 3510 Ma
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This paper presents SHRIMP U–Pb zircon dating on four Meso- to Palaeoarchaean amphibolite-facies Ameralik dykes of differing compositions scattered throughout the Palaeoarchaean (3850–3600 Ma) amphibolite–granulite-facies Itsaq Gneiss Complex in the Nuuk district, southern West Greenland. In the north of the complex near the Isua supracrustal belt, an east–west-oriented, >50 m thick metanoritic dyke, with abundant relicts of original opx + plag ± ol igneous mineralogy, gave an age of 3512 ± 7 Ma interpreted as the intrusion age (in general agreement with previous isotope dilution thermal ionization mass spectrometry data). In the same area, the best estimate for the intrusion age of a metadolerite dyke with the same orientation and similar degree of preservation is 3265 ± 15 Ma. At the mouth of Ameralik (fjord) 150 km to the SW, a large noritic Ameralik dyke, the Ugpik metadolerite, was largely recrystallized to amphibolite in the later Archaean. Relict magmatic zircon in this body implies an intrusion age of ≥3453 ± 16 Ma. In Itilleq (fjord) 60 km to the east of the Ugpik metadolerite, a dioritic component of a composite, dioritic–gabbroic intrusion cutting older, more deformed Ameralik dykes has yielded an age of 3257 ± 14 Ma for the time of intrusion. These new sensitive high-resolution ion microprobe dates on magmatic crystallization support previous field observations that the Ameralik dykes consist of several unrelated sets of dykes different in age by up to 250 Ma. The magmatic zircons in three of the dykes have 2700–2500 Ma overgrowths, interpreted to have grown during late Archaean amphibolite-facies tectonothermal events in the region. The original name ‘Ameralik dykes’ might propagate confusion that all such bodies can be regarded as the same age. Thus we recommend retaining the original definition of the Ameralik dykes but using the term ‘Ameralik dyke swarms’ to emphasize that unrelated intrusions of more than one age are present.