THE ORIGIN OF AUGITE ANDESITE AND OF RELATED 

 ULTRA-BASIC ROCKS 



REGINALD A. DALY 

 Massachusetts Institute of Technology, Boston 



CONTENTS 

 Introduction. 



Temperatures and Order of Crystallization in Basalt. 

 Sinking of the Phenocrysts. 



Formation of Ultra-Basic Magmas and of an Andesitic Mother-liquor. 

 Tests of the Hypothesis. 



1. Chemical relations. 



2. Observed cases of sunken and risen phenocrysts. 



3. Andesite actually observed to have been derived from basalt at an active 

 volcano. 



4. General field relations. 



5. Relation of augite andesite to other andesites. 



6. The rival hypothesis of magmatic differentiation. 

 Summary and Conclusion. 



INTRODUCTION 



Petrographers are in general agreement as to the existence of 

 many close mineralogical and chemical similarities between augite 

 andesite and basalt. It has, in fact, been found to be impossible to 

 draw any sharp line between the two species. Nevertheless, the 

 olivine basalts, volumetrically the most important class of lavas on 

 the globe, are distinctly characterized by the great abundance of the 

 basic phenocrysts, augite and olivine with which basic plagioclase 

 and much magnetite are regularly associated as minerals of early 

 generation. The list of phenocrysts in augite andesite normally 

 includes the pyroxene and an average plagioclase which is more acid 

 than that in the olivine basalts; olivine is absent and magnetite is 

 less abundant than in the basalt. 



These relations suggest the hypothesis that the andesite has been 

 differentiated from the basalt by a process of fractional crystallization. 

 This hypothesis is, in principle, nothing new, but it seems never to 

 have been applied in detail to this particular pair of rock-species. 



401 



