Cambrian Sm-Nd dates for an ultramafic intrusion and for high-grade metamorphism on the Øksfjord peninsula, Finnmark, North Norway
pp. 275-291

The Øksfjord peninsula in West Finnmark is composed of voluminous igneous and meta-igneous rocks that form a southern extension of the Seiland Igneous Complex. The igneous rocks have a large compositional range, and complex contact relations document several intrusion and deformation events. Deformation took place under high-grade metamorphic conditions, with complete reactions to granulite in high-strain zones. Both well preserved igneous rocks and thoroughly equilibrated meta-igneous rocks (granulites) were selected for dating of igneous activity and high-grade metamorphism. The ultramafic complex in the Tappeluft-Øksfjordbotn area is one of the youngest of the massive Seiland Province intrusions on the peninsula. A combined whole-rock and mineral isochron dates this intrusion at 550 ± 34 Ma (2o) (I = 0.512118 ± 34, MSWD = 1.6).

This Early Cambrian age is a minimum age for that of the major gabbroic, monzonitic and syenitic igneous activity in the Øksfjord area. Recent results have also confirmed that some of the older intrusions have Late Precambrian ages. Recrystallized, metamorphic minerals combined with whole rock from a fine-grained mafic granulite (metagabbro) yield a Sm-Nd isochron age of 502 ± 28 Ma (2o) (I = 0.511983 ± 28, MSWD = 0.008). This is interpreted as da ting the recrystallization of the rock under high-grade metamorphic conditions. Metamorphic miner als and whole rock from a coarse-grained garnet-rich granulite (monzonite) yield a Sm-Nd isochron age of 531 ± 10 Ma (2o) (I = 0.511803 ± 12, MSWD = 0.95), essentially dating garnet formation in this rock. The metamorphic conditions correspond to a high geothermal gradient situation. The intrusion of the ultramafics may record the termination of the Late Proterozoic and possibly also Early-Mid Cambrian active rifting of Barents Shelf or Greenland lithosphere, while some of the deformation at high-grade conditions may record the transport of the cooling paleorift in Late Cambrian-Early Ordovician time.