RAPAKIVI FELDSPARS: MARKERS OF MAGMA CHAMBER DYNAMICS

Irregular to ovoid alkali feldspar phenocrysts mantled by plagioclase “rapakivi” are a conspicuous texture of the coarse-grained edenite-biotite Oak Point and Stonington granites of the Deer Isle pluton. Rapakivi feldspars consist of salmon-colored alkali feldspars cores, with irregular embayed margins, overgrown by a creamy white plagioclase mantle of An₂₆ or less. Quartz commonly occurs along the core-mantle interface. Plagioclase mantles comprised of discrete grains, a single continuous rim, or variations thereof, have been observed. The modal abundance of rapakivi feldspars (crystals per 0.46m²) decreases from 129 +/-8 at Flye Point in the NE portion of the pluton (Oak Point granite) to 15+/-3 at Crotch Island in the SW portion of the pluton (Stonington granite). Rapakivi feldspar numbers gradually decrease from NE to SW within the Oak Point and Stonington granites but exhibit a rapid decrease over a narrow region corresponding roughly to the gradational contact between these two granites. Amphibole-plagioclase geothermometry record a subsolidus temperature of ~516^oC throughout the pluton consistent with mineralogical evidence for subsolidus alteration. Amphibole geobarometry, calculated at an assumed near-solidus temperature of 720^oC record pressures <0-2.6 kb. Low to negative pressure values correspond to portions of amphiboles with abundant cleavage or fractures. Amphibole crystals in the Oak Point granite yield an average pressure of 2.1 kb and in the Stonington granite yield an average pressure of 1.4 kb (excluding negative values).

The asymmetric distribution of rapakivi feldspar crystals reflects variation in magma chamber dynamics of the Deer Isle pluton. Early formed rapakivi feldspar and amphibole crystals accumulated rapidly at the base of the magma chamber (i.e., the Oak Point granite). Rapakivi and amphibole are less common in the upper part of the chamber (i.e., the Stonington granite) and are dipersed among minerals that crystallized in situ. The abrupt decrease in rapakivi feldspars marks the transition between the stagnant cumulate pile and an overlying more dynamic portion of the magma chamber. Segregation of early formed material as well as its dispersal by convective currents are important processes that operated within this felsic magma chamber.