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Potassic feldspar enrichment in magma; origin of syenite in Deboullie District, northern Maine

Gary M. Boone
Potassic feldspar enrichment in magma; origin of syenite in Deboullie District, northern Maine
Geological Society of America Bulletin (1962) 73 (12): 1451-1476


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 H (sub 2) O 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 H (sub 2) O 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.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 73
Serial Issue: 12
Title: Potassic feldspar enrichment in magma; origin of syenite in Deboullie District, northern Maine
Author(s): Boone, Gary M.
Pages: 1451-1476
Published: 1962
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Accession Number: 1962-012048
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus., tables
N43°00'00" - N47°30'00", W71°04'60" - W67°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
Update Code: 1962
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