Abstract

Certain granitic intrusions exhibit a gradational compositional zoning from relatively mafic margins to more leucocratic and potassic cores. Previous explanations of this zoning, including that of assimilation, are discussed and rejected. The proposed hypothesis involves: (1) crystallization from the margins inward, sealing in the released volatile phase which is forced downward with further crystallization; (2) migration downward of alkalis and silica, with the volatiles, and their enrichment in the magma at depth, leaving behind a more mafic residuum.

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