Petrogenetic Significance of Some Feldspars from the Judith Mountains, Montana

Most of the alkali-rich, lime-poor porphyritic rocks of the Judith Mountains, Montana, contain phenocrysts of a single, optically homogeneous, alkali feldspar (cryptoperthite). A few rocks that are chemically similar to those contain two alkali feldspars; these feldspars are not the result of exsolution or replacement but are orthomagmatic. Textual relations suggest that in some magmas there was a reaction relation between the two feldspars and in others a eutectic relation. Vesicular and intrusive brecciated textures and alkali metasomatism of sedimentary inclusions indicate that the Judith Mountains rocks known to contain two alkali feldspars crystallized from wet magmas; a high pressure of water vapor and probably other volatiles is believed to have been the dominant factor in reducing the temperature of initial crystallization of the alkali feldspars to the reaction and eutectic points. Rocks containing plagioclase phenocrysts of two different compositions may possibly result either from fractionation...