ENRICHMENT OF REE BEARING ACCESSORY PHASES BY MAGMATIC PROCESSES; EVIDENCE FROM THE MT WALDO PLUTON, COASTAL MAINE

The Rare Earth Elements (REE) are part of the high field strength group of cations that have small ionic radii but higher charge (3⁺ to 6⁺). Therefore they act incompatibly in magmatic situations, i.e. they are concentrated preferentially in the melt, most commonly in accessory phases, such as allanite. Walters et al.’s (2013) study of the Loch Loyal syenite complex, northern Scotland demonstrated that the REE can be concentrated during both magmatic and post-magmatic, hydrothermal situations. The Mt. Waldo pluton (MWP) coastal Maine, is a 371±2Ma, coarse-grained porphyritic bt+hb+sp granite, which displays abundant evidence of its evolution via magma mixing in a dynamic magma chamber. This resulted in rapakivi textured plagioclase, quartz and sphene-centered ocelli, magmatic enclaves and linear trends on Harker diagrams. The influx of hotter, more mafic magma into this crystallizing felsic chamber created conditions conducive for the formation of mineral layering (schlieren) and to concentrate high abundances of REE bearing accessory phases.

The schlieren/mineral layering observed in the MWP takes on a variety of forms - rings, trough structures, ladder dikes, and fragmented layers, and they often resemble sedimentary features. They are of similar mineralogy to the granite itself but with much higher concentrations of the accessory phases, sometimes up to 10% modally. Sphene, zircon, apatite and opaques are common but the REE bearing accessory phase allanite is distinctly abundant.

Not surprisingly this accessory phase abundance is reflected in the geochemistry of these schlieren. While their major and minor element chemistry is quite comparable to the granite, the REE are 5 to 10 times more abundant in the schlieren than the average MWP granite itself. However, compared to average crust the REE are 10 to 20 times more abundant in the MWP schlieren, with Th, Ce, Nd, and Sm displaying the highest concentrations (X12 to X20 more abundant). The lack of any evidence of alteration by hydrothermal processes and the plethora of evidence that supports the crystallization of the MWP in a dynamic, convecting magma chamber strongly suggest that the high concentration of REE in its schlieren was produced by magmatic processes.