Feldspar compositions and amounts from a CIPW norm

Abstract

A scheme for converting the output from a CIPW norm to compositions of coexisting feldspars is described. The scheme, empirically calibrated on suites of plutonic rocks, has three modes of output: -coexisting alkali feldspar and plagioclase -two coexisting alkali feldspars -only one feldspar The scheme has been tested on a gabbro-diorite-tonalite-trondhjemite suite (Finland), a granodiorite suite (Sierra Nevada) and an adamellite-granite suite (N.S.W.). Compositions and relative amounts of the various feldspars are calculated from the norm and they show good agreement with modal ratios and feldspar compositions from the rocks, except when modal biotite exceeds 18%. The scheme is less successful when applied to rocks from an alkaline complex (Monchique) probably because of high sodium partitioning into alkali amphiboles and pyroxenes. While the scheme yields useful information about volcanics it cannot mimic the phenocrystic character of porphyritic rocks as is indicated on some basalts and talc-alkaline volcanics (Utah).