Paleomagnetism of the Morin Complex

Abstract

Metamorphosed (high amphibolite to granulite facies) anorthosites and leucogabbros of the Morin complex in the province of Quebec have two magnetizations often coexisting in the same specimens. The magnetization with high remanent coercive force has a mean direction of 266.2°, −76.8° (24 sites, 117 samples, α95 = 5.3°) with respect to the present horizontal and a corresponding pole at 42.4°S and 139.3°E. It has blocking temperatures between 550° and 650°C, remanent coercivities chiefly between 1000 and 3000 Oe, and is due to hematite with a few percent of ilmenite in solid solution. It is suggested that this high coercive force (hcf) magnetization was acquired during cooling immediately following high-grade regional metamorphism during the Grenvillian orogeny at about −1124±27 m.y. The other magnetization has remanent coercivities mostly between 100 and 1000 Oe and is referred to as the medium remanent coercive force (mcf) magnetization. This magnetization has a mean direction of 114°, + 38° (13 sites, 45 samples, α95 = 10°) and a corresponding pole at 0°, 164°E. It has blocking temperatures mostly in the range 100°–400°C, and we suggest that it was acquired later during the last stages of regional uplift at about −1000 m.y. The poles for the hcf and mcf components of magnetization define the limits of the Grenville polar track and provide for the first time tentative estimates of its age. There is no systematic deflection of the magnetization due to anisotropy. The close agreement between the pole for the hcf magnetization and similar poles derived from the Allard Lake anorthosite and basic gneisses near Haliburton, localities spaced over 1200 km apart, indicates that the present horizontal is the proper reference plane for the analysis of these magnetizations.