Paleomagnetism of the Haliburton Intrusions: Superimposed magnetizations, metamorphism, and tectonics in the Late Precambrian

Abstract

Three components of remanent magnetization have been isolated by af and thermal demagnetization of highly metamorphosed dioritic and gabbro-anorthositic rocks from Haliburton County, Ontario, in the Grenville structural province of the Canadian Precambrian Shield. The predominant A component, a normal magnetization, defines a paleoinagnetic pole at 142.5°E, 35.6°S (α95 = 6.3°, k = 37.6), near southeast Australia, in agreement with published poles from widely separated areas in the Grenville Province. Paleopoles for the normal B and reversed C magnetizations fall much farther north, at 172.3°E, 24.5°N (α95 = 15.7°, k = 9.4) and 167.4°E, 3.0°S (α95 = 6.6°, k = 53.5), respectively. The C pole is close to secondary poles of other studies, but the B pole has not been detected elsewhere within the Grenville Province. It lies among poles for late Keweenawan rocks of the Lake Superior area. The A and B components are frequently superimposed in individual samples. The three components have distinctive coercivity and blocking temperature spectra, which we interpret as recording mineralogy and physical conditions (temperature and duration of heating) at different stages of slow cooling following high-grade regional metamorphism of the Grenville Province. They constitute a record, from a single locality, of apparent polar wander between about 1200 and 950 m.y. The A magnetization was acquired at high temperatures, the B at much lower temperatures. Although both B and C magnetizations nay have been set during subsidiary thermal pulses, a single-stage heating-cooling model of Grenville metamorphism is not ruled out. The proposed age sequence is A-C-B, a sequence most naturally explained by convergence of separate Grenville and North American plates. However, one-plate models which would incorporate the A-C-B sequence into a ‘Grenville Loop’ in the North American polar wander path are also possible.