Zircon U-Pb geochronology in the Bergen arc eclogites and their Proterozoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway

Abstract

The Proterozoic crystalline Lindas nappe of the Bergen arc, Caledonides of western Norway, displays local Caledonian eclogite facies overprint along fractures and shear zones. The overprint results from fluid infiltration in a dry granulite facies protolith under eclogite facies conditions. Formation of a low Th/U zircon overgrowth in eclogites is interpreted to be coeval with crystallization of the eclogite facies assemblage. A lower intercept age of 456 ± 7 Ma obtained from combined zircon U-Pb data for an eclogite and an associated granulite are believed to reflect the eclogite facies fluid infiltration event. An alternative minimum age of 419 ± 4 Ma can be extracted from the data. A charnockite and a mafic granulite yield Proterozoic intrusion ages of 1237 +43/–35 and 951 ± 2 Ma, and a garnet- bearing granulite provides an age of 929 ± 1 Ma for the late Sveconorwegian granulite facies metamorphism. The Proterozoic ages are compatible with a correlation between the Lindas and Jotun nappes and an origin of the Lindas nappe in the Sveconorwegian province of Baltica. We propose a possible model featuring subduction of the Lindas nappe at 456 ± 7 Ma attached to the Baltica plate during a Late Ordovician (Caradoc) collision between Baltica and an outboard island arc in the Iapetus Ocean. Accretion of some of the ophiolite-bearing terranes preserved in the Caledonides of western Norway may have taken place during this event.