AR-40 AR-39 MINERAL AGE RECORD IN NE GREENLAND - IMPLICATIONS FOR TECTONIC EVOLUTION OF THE NORTH-ATLANTIC CALEDONIDES

Ar-40/Ar-39 mineral ages have been determined from basement gneisses a nd cover sequences exposed in the Caledonides of NE Greenland. These r ange between c .438 Ma and c .370 Ma (early Silurian-mid-Devonian). Th ey are interpreted as dating cooling following polyphase Caledonian me tamorphism which completely reset argon systems within the (early Prot erozoic) basement gneisses. The Ar-40/Ar-39 results indicate that thru st-related regional deformation and metamorphism in NE Greenland conti nued into the early Devonian. This contrasts with Scandinavia, where n appe stacking was largely completed by the early-mid-Silurian and was followed by regional extension and tectonic unroofing in the late Silu rian-mid-Devonian. Tectonic models developed for the late-orogenic evo lution of the northern Atlantic Caledonides therefore need to incorpor ate considerable along- and across-strike variation in both the style and timing of major tectonothermal events.