Dating agpaitic rocks: A multi-system (U/Pb, Sm/Nd, Rb/Sr and Ar-40/Ar-39) isotopic study of layered nepheline syenites from the Ilimaussaq complex, Greenland

摘要

The Ilimaussaq complex in southern Greenland is a shallow crustal composite intrusion comprising augite syenite, peralkaline granite and volumetrically dominant agpaitic nepheline syenites. Previous studies indicated a baddeleyite U-Pb age of 1160 +/- 5 Ma for the augite syenite, the earliest intrusive unit of the complex. A similar crystallization age, within error, is inferred for the main sequence of agpaitic nepheline syenites. However, direct age determination of these units has been challenging because agpaitic rocks characteristically lack robust phases for in situ U-Pb dating (e.g. zircon/baddeleyite). An additional challenge is the pervasive subsolidus alteration, of which the isotopic effects are poorly constrained. Here we present new U-Pb. Sm-Nd and Rb-Sr isotopic data from whole rocks and mineral separates and a Ar-40/Ar-39 amphibole age of three co-genetic agpaitic nepheline syenites (kakortokite) from the lowermost exposed part of the complex. Using a multi-system geochronological approach for mineral separates and whole rocks, we explore the effects of late-stage alteration for each isotopic system. Assuming a closed-system evolution for the hydrothermal fluids (i.e. isotopically similar to the melts) and cooling within a relatively short time-frame (0.8 Ma), we evaluate whether traditional mineral-whole rock isochron methods can provide useful age constraints for agpaitic rocks. We compare our data with those in the literature, corrected for the most recent decay constants.