PRECAMBRIAN TECTONIC PROVINCES 





In southern Ontario, particularly in Hastings County a younger series, 

 the Hastings, overlies the Grenville with erosional unconformity but, 

 apparently with little structural discordance. The series consists of 

 gray, blue-weathering limestone interstratified with argillite, except near 

 the base where beds of congolmerate interstratified with argillite, buff- 

 weathering dolomite, graywacke, and mica schist occur. Both Grenville 

 and Hastings rocks are intruded by a group of gabbros, anorthosites, 

 pyroxene diorites, and pyroxene syenites. Later still are dikes, sills, and 

 batholiths of granite and syenite, and their gneissic equivalents. 



The Grenville subprovince is believed to be separated from the Timis- 

 kaming subprovince by a great fault called the "Lake Mistassini-Lake 

 Huron fault" by M. E. Wilson (1956) and the Grenville front or fault 

 zone" on the Tectonic Map of Canada (1950). The fault marks a zone 

 of considerable disturbance, and in the Lake Mistassini area it seems 

 evident that the Grenville rocks have been thrust over those of the Timis- 

 kaming subprovince. The theoretical fault lies under lakes and glacial 

 deposits for most of its length, and considerable controversy centers 

 about it. 



For further discussion of the many rock units already described over 

 the vast Canadian Shield read M. E. Wilson 1956 and 1958, and Harrison, 

 1957. A recent symposium publication, "The Grenville Problem," pub- 

 lished by the University of Toronto Press, presents a fascinating picture 

 of the many problems involved. 



Tectonic Provinces 



With the advent of physiochemical age determinations (about 1931) 

 Imuch new light has been shed on the relative ages of rocks in the 

 Canadian Shield. The ages are actually for minerals occurring in igneous 

 rocks or in reconstituted rocks, metamorphosed during an orogeny; the 

 | original age of the graywacke, shale or lava is not determined but rather 

 the age of the orogeny. Therefore, with the absolute age determinations 

 has come an increased attention to orogenic belts, and certain geologists 

 have postulated a division of the Canadian Shield into tectonic provinces 

 or orogenic belts, in place of the "geological provinces." See Fig. 4.3. 



The oldest orogeny in Minnesota is called the Laurentian by Goldich 

 et al. (1961), but this he regards as an early phase of folding to the 



Fig. 4.3. Precambrian orogenic belts of North America defined by isotope oges. 



