Plagioclase stratigraphy; a record of magmatic conditions and events in a granite stock

Abstract

Details of plagioclase zoning were correlated throughout a 3-sq mi quartz-monzonite stock on Mount Pilchuk, Wash. A typical zone sequence in all phenocrysts is: andesine cores with oscillatory zoning; core-rim boundaries marked by sharp drops of 8 to 15 percent An, and rims normally zoned from An ₂₈ to An ₁₅ . Discontinuities between rims and cores represent a period of core resorption and non-crystallization caused by pressure decrease during intrusion. Coarse oscillations within cores (3 to 7 percent An) record convection and turbulence in an inhomogeneous magma during the early stages of crystallization. Constant composition of the outermost core zones indicates homogenization of the magma prior to and during intrusion. Zones within the outermost core show low compositional correlation and indicate a high degree of crystal mixing.