Post-tectonic magnetizations from the Cordova gabbro, Ontario and Palaeozoic reactivation in the Grenville Province

Abstract

Summary. The Cordova gabbro of southern Ontario intrudes 1300 Myr old volcanic rocks of the Hastings Lowlands in the Grenville Structural Province. Three distinct vector magnetizations (A, B and C) have been isolated, using a combination of stable endpoints, subtracted vectors from orthogonal vector plots and converging remagnetization circles. The A magnetization, with mean direction D = 294 I = - 55.5 (k = 42, ag5 = 5.5, N= 18 sites), is a high coercivity, high blocking temperature remanence recorded by 49 samples. The B magnetization was isolated in 33 samples and has a mean direction D = 305.51= - 1 .5 (k = 24, ag5 = 9.5, N= 11 sites). B has lower coercivities and blocking temperatures than A where the two are superimposed. The A and B palaeopoles, 151E, 10.5S (dp = 6, dm = 8) and 165.5 E, 24N (dp = So, dm = 9.5), fall on the Grenville Track around 900 and 820 Ma respectively. The A and B magnetizations thus date from uplift and cooling following the Grenvillian orogeny. The third magnetization, the C component, has been isolated in 23 samples. Its mean direction is D = 180 I= 27.5 (k= 18, ag5= 10.5, N= 12 sites). The C is a low coercivity, low blocking temperature overprint of A and B. Its palaeopole, 102E, 31N (dp = 6.5, dm = 12), is unlike post-1300 Precambrian poles for cratonic North America but matches Silurian and late Ordovician poles. 40Ar/39Ar plateau ages of 446 and 447 Ma determined by Lopez-Martinez and York for plagioclases from one of the Cordova samples confirm this age assignment. The C magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in Palaeozoic time, long after the Grenvillian orogeny.