The age of the synorogenic Fongen-Hyllingen complex, Trondheim region, Norway

Abstract

Syenitic differentiates from the Fongen-Hyllingen intrusion, which grade downwards into layered gabbroic rocks, give a Rb-Sr whole-rock isochron age of 405 ± 9 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7047 ± 0.0002. The age is interpreted as dating the time of magmatic crystallisation of these final differentiates. Emplacement at 500–600 MPa, followed by amphibolite facies metamorphism, occurred during the Middle-Upper Silurian (Scandinavian) orogenic event and supports a date of around 400 Ma for the Silurian-Devonian boundary.

Key Words:

• Rb-Sr
• whole-rock
• radiometric age
• isochron
• Trondheim region
• Caledonian
• Silurian-Devonian boundary
• syenites
• layered gabbro
• synorogenic
• Nonvegian Caledonides
• amphibolite facies
• initial ratio
• N6255 N6312 E1145 E1125