Section IV, 1920 [31) Trans. R.S.C. 



A Local Occurrence of Differentiation in Granite on the Churchill River, 

 Northern Manitoba, Canada 



By Frederick J. Alcock, B.A., Ph.D. 



Presented by D. B. Dowling, B.Sc, F.R.S.C. 



(Read May Meeting, 1920) 



In the great areas underlain by granite in the Laurentian plateau 

 region of Northern Canada, one of the problems that confronts the 

 observer is whether the granites are to be referred to only one period 

 of intrusion or whether they belong to two or more periods. In 

 certain areas in the Pre-Cambrian shield it is very common to find 

 conglomerates containing boulders of granite which are in turn 

 intruded by granite and pegmatite, but whether the older granite is 

 represented in place to-day is most difficult to prove in most cases. 

 The difficulty is accentuated where there are no intruded sediments 

 or volcanics to help date the intrusion. In the granite areas, some 

 of the facts to be accounted for are the following: granites are found 

 of different degrees of coarseness and of slightly different composition; 

 banded gneisses of apparently igneous origin occur in close proximity 

 to rock with a typical granitic texture ; certain types are porphyritic and 

 massive in decided contrast to many of the granite-masses. A small 

 outcrop studied by the writer on the Churchill river affords some 

 light on this problem. 



The outcrop is found on the north bank of the Churchill river, 

 25 miles above the mouth of the Little Churchill, at a rapid where a 

 wide exposure of rock is afforded. This part of the river is bordered 

 on either side by banks of boulder clay in places 125 feet in height, 

 and it is only at the rapids that exposures of bed rock are found. 

 Farther up the river the mantle of drift is thinner and outcrops are 

 more or less continuous. The dominant rock type wherever an 

 exposure is seen is a red biotite-granite or biotite-hornblende granite, 

 in places gneissoid, in places typically granitic, varying in texture 

 from coarse to fine grained and locally porphyritic with phenocrysts 

 of pink orthoclase up to 2 inches in length. The relationships of the 

 rock types in the outcrop referred to above are illustrated in the 

 accompanying diagram. 



The regional rock is a fine-grained red granite which passes by 

 loss of quartz into a syenite. The rock is more or less gneissoid and 

 is traversed by numerous though small pegmatite dykes. In thin 



