Plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the Sierra Nevada Batholith, California

Abstract

Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc‐alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.