Evaluation of boundary lines in the total alkali - silica diagram for the discrimination between subalkali and alkali basalts, and a new method to distinguish transitional basalts

摘要

Some boundary lines in the total alkali-silica diagram have been evaluated for their effectiveness to discriminate between alkali and subalkali basalts. The boundary line of Macdonald and Katsura has been found to be the most effective line for this purpose. Another equally effective boundary line, introduced in the present paper, is a line with constant suite index of 2.2 constructed in a modified Rittmann diagram. An assessment of available graphical means for distinguishing transitional basalts has revealed that none of these methods is satisfactory. A new method, based on a combination of CIPW norm, the two discriminants of the Macdonald and Katsura boundary line, the Poldervaart index, and Rittmann's suite index, has been introduced as a more reliable method to distinguish transitional from alkali and subalkali basalts.