Reconnaissance Rb-Sr investigation of salic, mafic and ultramafic rocks in the Øksfjord area, Seiland Province, northern Norway

A coarse-grained syenite perthosite within the Øksfjord area of the Seiland petrographic complex in northem Norway defines an apparent Rb-Sr wholerock isochron age of 625 m.y. The intrusion of the syenite perthosite is believed to postdate the intrusion and subsequent granulite-facies metamorphism of the gabbro gneiss and other foliate igneous rocks that make up the bulk of the province. The meta-gabbros, meta-syenites, and quartz-gamethypersthene gneisses that enclose the perthosite scatter about a Rb-Sr isochron of 1300 m.y., but this cannot be considered a valid event. There is a possibility that a Precambrian event occurred at about 1600 m.y. and that subsequent (Caledonian?) shearing opened some rocks to the partial gain and loss of radiogenic Sr87. It is also possible that some of the quartz-bearing gneisses are metasediments with no genetic relationship to the associated meta-syenites and meta-gabbros. Under this model, a best-fit isochron age of these gneisses (around 1034 m.y.) could be interpreted as a provenance age and would not, in itself, be proof that the rocks of the Seiland province had a Precambrian origin. A biotite-whole rock Rb-Sr age of 445 m.y. from a pegmatite within the complex suggests that Caledonian effects included either the intrusion of pegmatites or the resetting of mineral ages.