GEOCHEMICAL CHARACTERIZATION OF A FEEDER-DIKE SYSTEM IN THE SEILAND IGNEOUS COMPLEX, NORTHERN NORWAY: IMPLICATIONS FOR GEOTECTONIC ENVIRONMENT OF EMPLACEMENT

The 575-560 Ma Seiland Igneous Province is an intrusive suite of mafic and ultramafic rocks, occasional monzonites and diorites, and, carbonatite and (nepheline)-syenite alkaline dikes on the northwest margin of Baltica that have a contested geo-tectonic origin. The carbonatite and alkaline magmas were used to argue for a rift environment, but such magmatism may occur in back-arc basins or during de-suturing of a continent-continent boundary. Another source of complexity is that many rocks are cumulates with modified geochemical signatures that makes application of compositional-based discrimination techniques challenging. A suite of mafic-ultramafic, monzodiorite and nepheline monzonite dikes that may represent a feeder system to structurally higher plutons were sampled on the island of Sørøy. They were geochemically analyzed and the data used to test compositional-based models for geo-tectonic environments of emplacement.

Geochemically, with increasing SiO₂, the rocks have increasing Al₂O₃, Na₂O, and K₂O and decreasing TiO₂, Fe₂O₃, MgO, CaO, and P₂O₅, and are enriched in Sr, Ba, and Rb relative to N-MORB. Some high-field strength elements (Th, Ta, Nb) are enriched by factors of 10 to 100 compared to N-MORB, while others (Zr to Yb) have values closer to unity or less. Chondrite normalized whole-rock REE patterns are LREE enriched and the mafic to ultramafic samples have consistently negative REE slopes, although HREE slopes flatten and turn slightly positive as silica content increases. All samples have positive Eu anomalies that increase in magnitude from ultramafic to mafic through to the nepheline monzonites. Chondrite normalized REE patterns for amphibole in the mafic to ultramafic samples have flat or slightly positive slope LREE patterns that becomes negative from MREEs to the HREEs. Trace element geochemistry reflects increasing Ce/Yb, Th/Yb and Ta/Yb ratios with increasing alkalinity and SiO₂ content. Pearce discrimination diagrams show that many of the dikes are compositionally similar to within-plate basalts.

The geochemical data, coupled with geochronology and paleogeographic reconstructions, are used to argue that emplacement of the Seiland Igneous Province occurred during de-suturing of Rodina along the Lapland-Kola Suture during the earliest stages of the opening of Iapetus.