The Geochronology of the Blind River-Bruce Mines Area Ontario, Canada

Abstract

Formations of the "original Huronian" occur in a 200-km.-long belt along the north shore of Lake Huron. The Huronian formations lie unconformably upon an Archean basement complex of granite, gneiss, and metasedimentary and metavolcanic rocks. Previous age determinations on this basement complex indicate that it is about 2,500 m.y. old. Several igneous formations are intrusive into the Huronian formations. The oldest of these is the Nipissing diabase, which forms a complex of sills and dikes throughout the area studied; granophyric differentiates occur at several localities. In the southeastern part of the area two granitic intrusive bodies occur, the Cutler granite, previously dated at 1,750 m.y. by Wetherill, Davis, and Tilton, and the Eagle granite. An olivine diabase dike which cross-cuts the Cutler granite and adjacent Huronian formations is the youngest intrusive rock in the area. Age determinations were carried out by the Rb-Sr method on mineral and whole-rock samples from the crystalline rocks of the area. A plot of whole-rock analyses from granophyre and diabase of the Nipissing diabase on an isochron diagram yields an age of . A similar plot of mineral separates from one locality yields an isochron age of These results are interpreted as showing that the Nipissing diabase was intruded about 2,155 m.y. ago and that it was metamorphosed about 1,700 m.y. ago. This age of intrusion of the Nipissing diabase limits the age of the Proterozoic Huronian formations to greater than 2,100 m.y. and younger than the 2,500-m.y.-old Archean basement. The other intrusive units were dated at for the Eagle granite and for the olivine diabase dike. Several age determinations from the Archean basement indicate that, for the most part, the Archean rocks in the area studied have undergone alteration during some later event. As a result of this work, the Precambrian history of this area may be related to the regional Precambrian history of the northern Great Lakes area. The age of the Nipissing diabase indicates that it is unrelated to any other major event in the area. However, the metamorphism of the Nipissing diabase, the alteration of the basement minerals as shown by younger measured ages, and the age of the Cutler granite show that the 1,700-1,800-m.y.-old Penokean orogeny extended eastward from Minnesota, Wisconsin, and Michigan. The Eagle granite appears to be unrelated to any major event in the area, but the olivine diabase may be considered to represent Keweenawan igneous activity.