METAMORPHIC STUDIES^ 



CONVERGENCE TO MINERAL TYPE IN DYNAMIC 

 METAMORPHISM 



C. K. LEITH AND W. J. MEAD 



University of Wisconsin 



The thesis of this paper is that the formation of slates, schists, 

 and some gneisses, by rock flowage, requires both mineralogical 

 and chemical changes, and that there is convergence, both chemi- 

 cally and mineralogically, toward a comparatively few platy or 

 columnar minerals, tending to give these rocks their characteristic 

 lamellar structure. If such convergence can be demonstrated, 

 it may be used as a guiding principle in the study and interpre- 

 tation of this phase of metamorphism. In this paper it is proposed 

 to discuss anamorphism from this point of view. 



Mineral changes in anamorphism by rock flowage. — It is gener- 

 ally recognized, and need not here be demonstrated, that ana- 

 morphism by rock flowage tends to produce platy and columnar 

 minerals, like mica, chlorite, hornblende; that these increase at 

 the expense of other minerals during the process, and that in the 

 resulting slates, schists, or gneisses, they are the characteristic 

 minerals which determine the lamination and cleavage of the rock 

 and give the rock its name. Quartz and feldspar are also important 

 minerals in the slates, schists, and gneisses, but these minerals are 

 less important than the first-named group in producing the essential 

 characteristics of the slates, schists, and gneisses, and they tend 

 to diminish relatively in quantity in proportion as platy and 

 columnar minerals increase. Mica, chlorite, hornblende, feldspar, 

 and quartz make up all but an insignificant part of the mass of 

 schists, slates, and gneisses. There are of course other minerals 

 developed, such as garnet, staurolite, chloritoid, sillimanite, anda- 

 lusite, etc., which in certain rocks may be conspicuous and 



^ See also C. K. Leith, "The Metamorphic Cycle," Jour. Geol., XV (1907), 303-13; 

 C. K. Leith and W. J. Mead, "Metamorphic Studies," ibid., XX (1912), 353-61; 

 C. K. Leith andW. J. Mead, Textbook of Metamorphism (in press, Henry Holt & Co.). 



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