Rb-Sr isotopic evidence for the age and origin of the Ivigtut Granite and associated cryolite body, South Greenland

A Rb-Sr whole-rock isochron age of 1,248 + or - 25 m.y. (errors quoted at 2Sigma ) for the granite intrusion at Ivigtut shows that it is significantly younger than the nearby Gronnedal-Ika nepheline syenite-carbonatite complex but older than most of the major Gardar intrusions to the south and east. The initial 87 Sr/ 86 Sr ratio (0.7125 + or - 0.0048) determined for the granite is the highest reported in the Gardar Province and is largely due to the introduction of radiogenic 87 Sr in an interstitial cryolite phase ( approximately 0.720) after the bulk of the granite was already solid. The main cryolite orebody itself had an initial 87 Sr/ 86 Sr ratio ( approximately 0.709) slightly lower than the granite and was thus not directly responsible for the 87 Sr enrichment of the granite.