Palaeomagnetism and the Grenville orogeny: New Rb-Sr ages from dolerites in Canada and Greenland

Rb-Sr mineral data are presented for the Sudbury and Mackenzie dolerites of Canada and for dolerites and lamprophyres from southwest Greenland. Five mineral isochrons or errorchrons, based chiefly upon feldspar separates, agree with twelve biotites from dolerites in defining best-fit ages between 1260 and 1190 m.y. Much older K-Ar and Rb-Sr mineral ages from country rocks demonstrate that the dolerite results are not due to regional heating, and basaltic magmatism in both regions between 1260 and 1190 m.y. is indicated (λ(⁸⁷Rb) = 1.42 × 10^?11 yr^?1).

This activity can be correlated with extensive dolerite sills in Sweden and Finland, the subject of parallel studies, which yield six mineral isochrons or errorchrons, ten Rb-Sr biotite ages from dolerites, and determinations by other methods falling between 1290 and 1155 m.y., although the Rb-Sr biotite ages are all grouped in the 1250-1210 m.y. range.

Palaeomagnetic poles from the dolerites define tight clusters for each of Canada/Greenland and Fennoscandia. Superimposition of the two groups of poles leads to a non-unique reconstruction in which Fennoscandia may have adjoined Canada/Greenland at 1260-1190 m.y., but in an orientation quite different from that established for 1000-850 m.y. The necessary net rotation of Fennoscandia through 90° may have been achieved by splitting, separation and final reunion of the continents into a different relative disposition between 1190 and 1000 m.y. This favours the involvement of plate tectonic processes in the Grenville-Sveconorwegian orogeny. The 1260-1190 m.y. dolerites may represent the beginning of a continental separation which preceded the Grenville orogeny, although other causes for the magmatism cannot be ruled out.