Abstract:

Spatially associated carbonatite and syenite complexes at Breivikbotn on the island of Sørøy, in the Caledonian nappes of northern Norway, have been dated by isotope dilution thermal ionization mass spectrometry U–Pb analysis on zircon and titanite. Zircon crystals in malignite and silico-carbonatite contain many inclusions of Th- and U-rich minerals and have been locally affected by dissolution–reprecipitation processes, all features that tend to cause disturbed U–Pb systems. Some alkalic gneisses also exhibit evidence of new zircon growth during the Caledonian metamorphism. The carbonatite was emplaced into the Kalak Nappe Complex at 574 ± 5 Ma, and various alkaline gneisses and nepheline syenite dykes intruded the neighbouring Breivikbotn Gabbro and its metasedimentary host between 580 and 560 Ma. The alkaline rocks were coeval with, and probably genetically related to major mafic and ultramafic magmatism throughout the Seiland Igneous Province, preceding the emplacement of nepheline syenite pegmatites at 530–520 Ma. The ages of the Seiland Igneous Province indicate formation in an extensional setting, the multistage history of the host Kalak Nappe Complex pointing to a potential origin as an Avalonian-type peri-Gondwanan terrane.

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