Mesonorms of granitic rock analyses

SINCE Cross, Iddings, Pirsson, and Washington 09o3) first published their system of deriving normative minerals from chemical analyses, the CIPW norm has been successfully used to identify, compare, and classify trends in igneous rocks with similar or differing mineral compositions. The norm has been used for metamorphic rocks and rocks of somewhat uncertain mode of origin (Bowes, ~967). Various minor modifications to the CIPW norm have been advocated (Eskola, I954; Barth, I959; Kelsey, I965), and the further modification presented here is considered desirable when chemical data for granitic rocks are to be compared graphically with phase diagrams. This paper represents an expansion, into the igneous field, of Barth's (I96Z) mesonorm calculation. The basis of his mesonorm calculation, unfortunately a rather neglected technique, is that certain metamorphic rocks require mica and amphibole representation in normative calculations. Recently, Barth 0966) indicated the desirability of modifying CIPW norms of granitic rocks in conjunction with the quinary system An-Ab-Or-Q-H20 . A footnote in Dunham et al. (x965, p. 394) indicates that a similar modification was used for analyses of the Weardale granite.