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

Plagioclase mantled alkali feldspars (PMAF), commonly known as “rapakivi” feldspars, are found in all three facies (cumulate pile, “active chamber”, and felsic cap) of the Deer Isle Pluton in Maine. These rapakivi feldspars 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. Using a 213 nanometer laser, sample spots, 100 microns in diameter were analyzed using Inductively Coupled Plasma – Mass Spectrometry. Samples were selected from points in a top to bottom transect of the tilted magma chamber. In each sample, spots from the core and margin of the alkali feldspar as well as the plagioclase rim were compared to spots in unmantled alkali feldspar and plagioclase crystals from the matrix. If the rapakivi feldpars formed as a result of decompression during the ascent of the magma through the crust, then we would expect that the abundance of trace and REE elements measured in the rimmed alkali feldspar would resemble those measured in the alkali feldspars of the matrix. The same holds true for the abundances in the plagioclase rims compared to the plagioclase crystals in the matrix. If the rapakivi feldpsars formed as a result of an influx of a higher temperature primitive magma during crystallization, then we would expect that the abundance of trace and REE elements measured from the alkali feldspar cores would differ from those measured from the alkali feldspars in the matrix. Ba contents of Rapakivi k-spars range from 70 - 225 ppm, and matrix feldspars show a similar range from 80 - 180 ppm. Ba concentrations in plagioclase rims more closely reflect Ba concentrations of the adjacent alkali feldspar core than the matrix plagioclase. These feldspars have a relatively low and limited range in Ba content across the pluton. We interpret this to reflect conditions present during crystallization rather than sub-solidus re equilibration, as core to rim zoning is preserved in k-spars.