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

• Published in: Lithos
• Main Authors: Borst, Anouk Margaretha, Waight, Tod Earle, Finch, Adrian, Storey, Michael, Le Roux, Petrus
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2019
• Subjects: Greenland
• Online Access: https://curis.ku.dk/portal/da/publications/dating-agpaitic-rocks(6e12958a-e8ed-43df-9f3c-c9f47b978472).html; https://doi.org/10.1016/j.lithos.2018.10.037; https://curis.ku.dk/ws/files/248643072/Borst_Lithos_Agpaiticrocks_AAM.pdf

The Ilímaussaq 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±5Ma 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. Herewepresent newU-Pb, Sm-Nd and Rb-Sr isotopic data from whole rocks and mineral separates and a 40Ar/39Ar 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-systemevolution for the hydrothermal fluids (i.e. isotopically similar to the melts) and cooling within a relatively short time-frame (b0.8 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. Single- /39Ar step-heating experiments yield an amphibole plateau age of 1156.6±1.4Ma (MSWD=1.5, external error± 7.7 Ma), which we put forward as the most precise crystallization age for the agpaitic units to date. Kakortokite whole rock and mineral separates (amphibole, eudialyte, feldspar) yield a 206Pb-207Pb isochron age of 1159 ± 17 Ma (MSWD = 0.96) and a 235U-207Pb isochron age of 1168.5 ± 8.8 Ma (MSWD = 0.82). These are within error of the baddeleyite and zircon U-Pb ages from the augite syenite and alkali granite, as well as the ...