Abstract

The garnet line is a planar fabric occurring within the footwall wall zone just beneath the core zone in many of the western Maine pegmatites. This study focuses on the mineralogical, textural, and chemical characteristics of the garnet line within the Mt. Mica pegmatite, Oxford Co., Maine and a similar, but less pronounced, garnet line feature at the Havey pegmatite, Androscoggin Co., Maine.

Within garnet lines of both pegmatites there is evidence of multiple stages of crystallization. The first stage resulted in formation of the garnet layer, which hosts an abundance of high field strength element (HFSE)-bearing micro-inclusions (uraninite, zircon, and columbite group and microlite supergroup species) and rare earth element (REE)-bearing phosphate minerals [monazite-(Ce) and xenotime-(Y)]. Field relations suggest that this is a large-scale oscillatory nucleation feature that is similar to the line rock in San Diego Co., California pegmatites described by Webber et al. (1997).

The second stage of crystallization is inferred to be post-magmatic, related to a late-stage hydrothermal fluid which metasomatically altered the garnets and produced black and blue tourmaline rims. These late-stage fluids are inferred to have originated and migrated from adjacent highly evolved miarolitic cavities. The fluids are also responsible for presence of Cs-rich micas and Fe-rich pollucite in the fractured portions of garnets in the garnet line.

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