Discussion

Abstract

D r R. G. P ark expressed interest in the new c. 1750 m.y. dates from the basement gneisses of the Norwegian Caledonides which provided more evidence of the essentially Svecofennian nature of that basement and added to the growing body of evidence from many orogenic belts of the relative antiquity of much of their material. The two younger ages of c. 1200 m.y. were somewhat puzzling since they lay far to the north of where Grenville-Sveconorwegian front ought to lie when projected eastwards from Canada across Scotland to Scandinavia. The only dates of this age in the Lewisian are at the southern end of the outcrop and there are none to the speaker's knowledge in Greenland. Could the authors give some more information on the rocks giving these dates and say whether any alternative explanation was considered, e.g. Caledonian resetting of the ages. The Authors said that as Heier & Compston (1969) emphasized, their determination on a whole rock sample (c. 1100 m.y.) may be a true age reflecting total loss of radiogenic strontium at that time or it may be an incomplete adjustment to a younger event. Our data suggests that the rocks collected in Junkerdal and within the Krågdal window (which are lithologically described in section 2A) suffered homogenization of Sr isotopes at around 1200 m.y., the initial 87 Sr/ 86 Sr ratio being raised to about 0.88. The linear distribution of the data points in Fig. 7B is difficult to explain by Caledonian resetting. It seems possible that the Sveconorwegian event