A TEST OF THE FELDSPAR METHOD FOR THE 



DETERMINATION OF THE ORIGIN OF 



METAMORPHIC ROCKS 



CHARLES GORDON CARLSON 



The University of Wisconsin 



I. Purpose of paper. — ^That feldspars may serve as indicators of 

 the original character of gneisses and schists is dependent upon 

 the narrow range of composition possessed by the plagioclase feld- 

 spars of igneous rocks. Thus more than one kind of plagioclase 

 feldspar is rarely found in an igneous rock except in certain zonal 

 intergrowths or in some porphyries where the feldspars forming 

 the phenocrysts may be of slightly different composition from 

 those of the groundmass. 



In sediments, except in rare cases where they are derived from 

 rocks having feldspars with a narrow range of composition, the 

 limited feldspar composition found in igneous rocks is not to be 

 expected. Usually sediments are derived from many sources and 

 consequently mixtures of all kinds of feldspars are possible. It 

 would seem reasonable then to believe that gneisses and schists 

 with a narrow range of feldspar composition are probably igneous 

 in origin, whereas metamorphic rocks with several varieties of 

 feldspar are very likely of sedimentary origin. This belief, how- 

 ever, rests on the fundamental assumption that the feldspar range 

 typical of sediments does not radically change during anamorphism 

 of these sediments. It is readily seen that if such a change does 

 take place it vitiates any conclusions which might be reached. 

 Similarly, if in the anamorphism of igneous rocks a radical change 

 in the original feldspar composition results, this also would miHtate 

 against the efficacy of the feldspar method. 



That feldspars undergo alteration in various stages of the meta- 

 morphic cycle is generally recognized. It is not known that this 



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