210 N. L. BOW EN 



them made up almost exclusively of the single mineral, plagioclase. 

 What, then, of the magmatic temperatures of anorthosites ? Have 

 we in them an exception to the rule of moderate magmatic 

 temperatures ? 



Anorthosites apparently exert no exceptional . influence upon 

 surrounding rocks. Foreign inclusions, even very susceptible ones, 

 are apparently not melted up. Inclusions of quartz-bearing rocks 

 do not have their quartz changed to tridymite or cristobalite. 

 Nothing in the field evidence gives us any reason to believe that 

 anorthosites are in any way exceptional in this respect. Neither 

 do we find any comfort in field evidence if we entertain the possi- 

 bility that, in the absence of other minerals in amounts adequate 

 to produce a great lowering of the melting temperature of plagio- 

 clase, there was present instead a sufficient amount of the much 

 more potent "mineralizers." Typical anorthosite is notably free 

 from all those minerals which, when present in rocks, constitute the 

 principal evidence of the presence of volatile components in signifi- 

 cant amounts in their magmas. 1 



An alternative possibility is that the material of anorthosites 

 actually was in solution in something else at one time and that it 

 differentiated from this solution. This alternative is more in har- 

 mony with general opinion, for few would state that beneath those 

 places where we find anorthosites, there existed some anorthosite 

 magma and that it simply was always there. It is generally 

 believed, rather, that anorthosites are differentiates of gabbroid 

 magma, the belief being based on field association. But it is also 

 commonly believed that the differentiation took place in some 

 manner in the liquid state and produced anorthosite magma. 

 Now we must realize, and face the fact squarely, that anorthosite 

 magma, however produced, is, nevertheless, anorthosite magma, 

 and must exhibit the appropriate characteristics. It could separate 

 as a liquid, by any process whatsoever, only at temperatures at 

 which it could exist as a liquid, and we are immediately presented 

 with precisely the same temperature problem. Anorthosites, as 

 we have seen, do not give evidence of ever being at a temperature 

 approaching that requisite to melt plagioclase. 



1 This matter is considered in greater detail in connection with the Morin 

 anorthosite. 



