Rb-Sr Systematics of a Gardar-Age Layered Alkaline Monzonite Suite in Southern Norway

The Morkheia Monzonite Suite (MMS) is part of a layered alkaline complex of anorogenic affinity which intruded discordantly into high-grade gneisses of the Telemark Sector. In the MMS, one complete outcrop and low Rb/Sr samples from other outcrops preserve a Rb-Sr whole-rock date of ca. 1250 Ma, interpreted as approximating an intrusive age. The MMS is thus contemporaneous with similar alkaline complexes of early to mid Gardar age in southern Greenland and Labrador. During this period felsic alkaline magmatism was apparently a widespread expression of the anorogenic-tensional regime that obtained in the North Atlantic provinces. MMS whole-rock samples from three outcrops fall below the ca. 1250 Ma primary isochron and define secondary ages of ca. 1060 Ma. At this time these outcrop-scale Rb-Sr systems approached Sr-isotope equilibration, and Rb/Sr ratios were augmented, mainly by Rb-addition. These isotopic effects relate to minor, low-grade hydrous alteration of the primary minerals. This low-grade (gree...