Mafic dike swarms as absolute time markers in high-grade terranes: Ar-40/Ar-39 geochronological constraints on the Kangamiut dikes, West Greenland

The Paleoproterozoic Kangamiut dikes are valuable time markers in the compl ex high-grade terranes of West Greenland. They are used to separate the Arc hean basement from areas affected by the Proterozoic Nagssugtoqidian orogen . It is shown here that both unmetamorphosed and metamorphosed Kangamiut di kes exposed in the foreland of the orogen preserve pre-Nagssugtoqidian Ar-4 0/Ar-39 ages. Two suites of Kangamiut dikes have been identified (ca. 2530 Ma and ca, 2040 Ma). Metamorphism of the dikes in the foreland occurred sho rtly after their emplacement, within the resolution of the argon system. We suggest that crustal heating during dike emplacement or heat derived from the individual dikes caused the development of metamorphic minerals and tex tures. Internal shearing of the dikes in the foreland occurred as late as 1 740 Ma, contemporaneous with closure of the argon system in hornblende insi de the orogen, following peak Nagssugtoqidian metamorphism ca, 1850 Ma. Ins ide the orogen reworked equivalents of the Kangamiut dikes record parts of the post-Nagssugtoqidian cooling path. Thermotectonic activity during the N agssugtoqidian orogeny is bracketed between intrusion of the youngest Kanga miut dikes ca. 2040 Ma and post-peak metamorphic cooling of the orogen to < similar to 500 degrees C at 1740 Ma.