THE ROLE OF MAGMATIC PROCESSES IN CONCENTRATING REE ACCESSORY PHASES; EVIDENCE FROM THE MT WALDO PLUTON, COASTAL MAINE

The 371±2 Ma Mt. Waldo pluton (MWP) is a coarse grained, porphyritic, bt + hb + sp granite, which displays definite evidence that magma mixing played an important role in the evolution of this magma chamber. Plagioclase mantled k-feldspars (rapakivi textures), disequilibrium textures (quartz and sphene ocelli), abundant magmatic enclaves of intermediate composition, and linear trends on whole-rock Harker diagrams all point to the contemporaneous invasion of this crystallizing felsic magma chamber by mafic magma. This resulted in the formation of a dynamic magma chamber in which the conditions were conducive for convection and the formation of mineral layering or schlieren, which are common in the MWP.

The schlieren observed within the MWP are mineralogically similar to the granite itself but greatly enriched in accessory minerals, up to 10% modally. Sphene, zircon, apatite and opaques are common, but the REE accessory phase allanite is distinctly abundant. The schlieren are observed in a variety of dispositions (rings, trough structures and fragmented layers), which mimic “sedimentary structures” and are similar to those observed in alkaline plutons in SW Greenland and the Tavares pluton in Brazil. Therefore these schlieren are heavily enriched in the REEs and other HFSEs relative to the main granite itself and other intrusions of similar age and composition in coastal Maine.

In this presentation we will review the evidence for a dynamic magma chamber that produced the MWP and present whole-rock and mineral data both for the schlieren and the accessory phases found within them. This will allow us to test the hypothesis that the high concentration of the REEs in the schlieren was produced predominantly by magmatic processes.