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

• Published in: Geology (Boulder) 1999-09, Vol.27 (9), p.775-778
• Main Authors: Willigers, B J A, Mengel, F C, Bridgwater, D, Wijbrans, J R, Van Gool, J A M
• Format: Article
• Language: English
• ISSN: 0091-7613
• DOI: 10.1130/0091-7613(1999)0272.3.CO;2

The Paleoproterozoic Kangamiut dikes are valuable time markers in the complex high-grade terranes of West Greenland. They are used to separate the Archean basement from areas affected by the Proterozoic Nagssugtoqidian orogen. It is shown here that both unmetamorphosed and metamorphosed Kangamiut dikes exposed in the foreland of the orogen preserve pre-Nagssugtoqidian 40Ar/39Ar ages. Two suites of Kangamiut dikes have been identified (ca. 2530 Ma and ca. 2040 Ma). Metamorphism of the dikes in the foreland occurred shortly 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 textures. Internal shearing of the dikes in the foreland occurred as late as 1740 Ma, contemporaneous with closure of the argon system in hornblende inside the orogen, following peak Nagssugtoqidian metamorphism ca. 1850 Ma. Inside the orogen reworked equivalents of the Kangamiut dikes record parts of the post-Nagssugtoqidian cooling path. Thermotectonic activity during the Nagssugtoqidian orogeny is bracketed between intrusion of the youngest Kangamiut dikes ca. 2040 Ma and post-peak metamorphic cooling of the orogen to < ~500 {degrees}C at 1740 Ma.