Paleomagnetism of the Whitestone Anorthosite and Diorite, the Grenville Polar Track, and Relative Motions of the Laurentian and Baltic Shields

Abstract

The Whitestone anorthosite and diorite are situated in the Grenville Structural Province north of Parry Sound, Ontario. They are intruded into sediments and igneous rocks, the whole being metamorphosed to amphibolite facies. Aside from soft magnetizations due to the present field four magnetizations are present, two owing to hematite, the third mainly to magnetite, and a fourth of uncertain source. It is argued that these are thermoremanent magnetizations acquired during very slow cooling following regional metamorphism in the interval 1100 to 1000 m.y. A single-stage cooling model based on Neel's single domain theory is developed, which suggests that the hematite magnetizations were acquired during slow cooling at about 240 °C and the magnetite magnetizations at about 200 °C. The poles from Whitestone rocks fall among a group of poles from elsewhere in the Grenville Province. There are serious problems in integrating these Grenville poles with those from other parts of the Canadian Shield, and three possible ways of relating them are evaluated. Poles from Grenville-type rocks from the Baltic Shield are near to the Grenville poles after correction is made for the late Phanerozoic opening of the Atlantic, showing that the relative positions of Laurentian and Baltic Shields before and after the Caledonian orogeny were very similar. There is however a small but significant difference, and this is attributed to Caledonian diastrophism.