PALEOMAGNETISM AND AGE OF 3 CANADIAN ROCKY-MOUNTAIN DIATREMES

Paleomagnetic results, and age estimates derived from them, are presented for three diatremes, using as a basis of comparison the combined apparent polar wander (APW) path for North America and Europe of Van der Voo. The Cross diatreme of the Front Ranges of the Canadian Rocky Mountains has yielded a radiometric age of 241 Ma (earliest Triassic) and is hosted by the flat-lying Pennsylvanian Tunnel Mountain Formation. It has normal polarity magnetization and yields a paleopole correctly placed according to its radiometric age on the APW path. The Blackfoot diatreme (for which no radiometric age is available), which is located in the Main Ranges of the Rocky Mountains, is known to be post-Late Ordovician because it is hosted by rocks of that age. It also has magnetization of normal polarity and yields a paleopole that, when calculated with respect to present horizontal, is coincident with the latest Cretaceous to Paleocene paleopole for North America. The paleopole, when calculated with respect to bedding, lies on the Middle Ordovician portion of the combined APW path. A clockwise rotation of 10-degrees brings the paleopole into agreement with the latest Ordovician. Hence, from a paleomagnetic standpoint, a latest Cretaceous to Paleocene or latest Ordovician age is possible. The HP pipe (radiometric age 391 +/- 5 Ma or Early Devonian), previously studied by D. T. A. Symons and M. T. Lewchuk, is hosted in limestones of Upper Cambrian to Middle Ordovician age. It has reversed polarity and yields a paleopole that, when compared with the combined APW path, suggests an age of mid-Permian, although errors are such that it could be somewhat younger, roughly coeval with the Cross diatreme. We conclude, therefore, that the radiometric age estimated for the HP pipe could be too old by about 130 million years.