Independent ages of magmatic and hydrothermal activity in alkaline igneous rocks: The Motzfeldt Centre, Gardar Province, South Greenland

The Motzfeldt Centre is one of four major alkaline centres belonging to the Igaliko complex, part of the Mid-Proterozoic Gardar province of South Greenland. Motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories. Ta, Nb and REE-rich zones are associated with a pervasive hydrothermal alteration process in the North and East of the complex, part of a subdivision of the Centre called the Motzfeldt So Formation (MSF). The zircons from the MSF show textures that are both magmatic and altered, whereas the textures of pyrochlore have a predominantly subsolidus chemistry. We compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes. Selected analyses of magmatic zircons are concordant with an age of 1,273 +/- A 6 Ma. Altered zircons are normally discordant with one intercept at 1,261 +/- A 28 Ma and the other at 349 +/- A 49 Ma. This younger age would appear on face value to date the hydrothermal event at Motzfeldt to similar to 900 Ma after the magmatism. However, the Pb-Pb isotope systematics from altered pyrochlore define an isochron age of 1,267 +/- A 6 Ma for isotopic closure of the pyrochlore following alteration during the subsolidus. There is no known magmatic event in Southern Greenland at similar to 350 Ma and discordant points have high common lead. We therefore infer that the younger intercept age is spurious. The overlap between the magmatic U-Pb zircon and the Pb-Pb pyrochlore age suggests that alteration of the Motzfeldt centre occurred shortly after magmatism.