CONSTRAINING THE ORIGIN OF RAPAKIVI FELDSPARS IN THE DEER ISLE PLUTON: A WINDOW INTO MAGMA CHAMBER DYNAMICS

The Devonian Deer Isle pluton Maine has been interpreted to have crystallized from an open system zoned magma chamber (Lux et al., 2007). Subsequent tilting and erosion of this pluton has revealed several distinct facies including the cumulate pile, “active” chamber, and felsic cap. Common to all facies are plagioclase mantled alkali feldspar phenocrysts (PMAF or “rapakivi”) that are thought to have formed as a result of either decompression during magma ascent or as a result of influx of higher temperature primitive magma during crystallization. Barium abundances of 14 feldspar crystals from a bottom to top transect across the pluton were determined using Laser Ablation ICPMS. Other trace element abundances as well as REE abundances are yet to be determined. The feldspars include the alkali feldspar cores and plagioclase mantles of PMAF phenocrysts as well as alkali feldspar and plagioclase crystals present in the matrix within the same thin section but not exhibiting PMAF textures. Matrix alkali feldspars from the cumulate pile (400 ppm) and the felsic cap (356 ppm) exhibit higher Ba concentrations than matrix alkali feldspars from the interior portion of the pluton (298-155 ppm). This trend is mimicked by matrix plagioclase crystals – cumulate pile (370 ppm), felsic cap (262 ppm), and interior facies (212-160 ppm). Ba concentrations of the alkali feldspar cores in three PMAF feldspars can be zoned from the core (601 ppm) to the rim (226 ppm) or show little variation (235 ppm core to 260 ppm rim). Ba concentrations of plagioclase mantles more closely reflect the Ba concentrations of the adjacent alkali feldspar core (e.g., 196 and 226 ppm respectively) than the Ba concentration of plagioclase in the adjacent matrix (370 ppm). Speculation based upon this limited data set suggests 1) The cumulate pile and felsic cap formed early and crystallization proceeded inward, 2) feldspar associated as PMAF crystallized at a different time/place relative to their corresponding feldspars in the matrix, 3) the PMAF forming event(s) happened after significant depletion in Ba content of the magma composition due to alkali feldspar crystallization, and then were dispersed throughout the chamber, 4) the plagioclase mantles crystallized in part from a boundary layer melt surrounding the dissolving alkali feldspar crystal which they mantle.