Potassic Feldspar Enrichment in Magma: Origin of Syenite in Deboullie District, Northern Maine

A small composite pluton, the Deboullie stock, comprises a post-orogenic, alkali-calc rock series intrusive into lower Devonian slate. The stock is a partly concordant, partly discordant pluton, and the wall rock structure shows that the magma was forcibly injected. The main intrusives are, from oldest to youngest, pyroxene-biotite syenodiorite and monzonite, pyroxene-hornblende-biotite calc-alkali syenite, and hornblende-biotite granodiorite. Dikes are the fine-grained equivalents of the salic plutonic rocks. Differentiation by fractional crystallization and by upward transfer of H₂O and components of potassic feldspar in the melt is proposed as the only satisfactory hypothesis to account for the contrasts in mineralogic evolution. Accordingly, syenitic rocks were derived from the parent magma enriched in potassic feldspar at the top of the magma column; the syenitic magma graded into granodioritic differentiate at deeper levels. The granodioritic magma, richer in plagioclase and quartz, completed the emplacement of the stock by intruding the earlier emplaced calc-alkali syenite.

Compositional differences across oscillatory zones in orthoclase and plagioclase feldspars of the main intrusive rocks apparently reflect changes in the partial pressure of volatiles in the melt. Intermittent fracturing of the roof of the magma chamber is appealed to as a cause for the pressure changes and as a mechanism for maintaining a steady-state gradient for H₂O and K-bearing components.

Neither assimilation, hydrothermal enrichment, nor liquid-crystal reactions involving the mafic minerals adequately explains the abundance of potassic feldspar in the syenite.