"The Deer Isle pluton is an edenite-biotite granite with ubiquitous rapakivi feldspars. It has been divided into four facies, the Flye Point facies, Oak Point facies, Settlement Quarry facies and the Crotch Island facies based on mineralogical and textural variation. Rapakivi feldspars, which have long been considered the result of magma mixing or decompression by magma ascent, are found in all facies and their abundance varies systematically across the pluton. Rapakivi feldspars are minimal in the Crotch Island facies, and increase dramatically towards the Flye Point facies. Petrographically, the rapakivi feldspars exhibit different mantle characteristics ranging from a continuous rim to discrete grains. The An (Anorthite) component of the plagioclase mantle is commonly ~ An20 or less, the highest An content measured is An26. Plagioclase phenocrysts also have an An content of ~ An20, with rare grains with an An content up to An40. The alkali feldspars cores are resorbed and commonly exhibit quartz at the interface with the mantle. Amphibole-plagioclase geothermometry records a subsolidus temperature of ~516°C. Amphibole geobarometry calculated at 720°C record pressures ranging from <0-2.6 kb.
The facies of the pluton are interpreted to correspond with locations within the magma chamber. The presence of rapakivi feldspars in each facies suggests the rapakivi feldspars were present when the pluton was emplaced. The Flye Point facies exhibits abundant rapakivi feldspars and represents a cumulate at the base of the chamber, and the Oak Point facies represents the transition between the cumulate and the more active magma chamber. The Settlement Quarry facies represents the active chamber; rapakivi feldspars are common, and both convection and crystallization occurred. The Crotch Island facies is the most evolved portion of the chamber; rare rapakivi feldspars are present. This interpretation of the facies requires tilting of the pluton towards the southwest. The amphiboles in the Flye Point (1.2-2.6 kilobars) and Oak Point (<0-2.5 kilobars) facies yield slightly higher pressures than the Settlement Quarry facies (<0-2.2 kilobars); this may correspond to depth assuming the pluton is tilted. Negative pressures are not geologically significant and occur primarily in highly zoned amphiboles, which suggest migration of a fluid phase along fractures and cleavage. The plot of X Mg vs. Si shows amphiboles from separate facies plot in distinct groups.
The geobarometry, varying mantle types, and rapakivi distribution do not indicate the mechanisms of formation. Magma mixing played a role in the history of the pluton, however there is no evidence supporting it is responsible for the formation of rapakivi feldspars. The role of decompression can not be determined due to re-equilibration. Despite inconclusive evidence regarding formation, rapakivi feldspars remain a useful marker of magma chamber dynamics"--Abstract, pages iii-iv.
Hogan, John Patrick
Wronkiewicz, David J.
Miller, F. Scott, 1956-
Geosciences and Geological and Petroleum Engineering
M.S. in Geology and Geophysics
University of Missouri--Rolla
xii, 160 pages
© 2003 Jessica Jean Terrien, All rights reserved.
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Formations (Geology) -- Maine
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Terrien, Jessica Jean, "Rapakivi feldspars in the Deer Isle pluton, Maine: Distribution, characterization and the role of decompression vs. magma mixing in their formation" (2003). Masters Theses. 2294.
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