Garnet as Indicator of Pegmatite Evolution: The Case Study of Pegmatites from the Oxford Pegmatite Field (Maine, USA)

Almandine-spessartine garnets, from the Oxford County pegmatites and Palermo No. 1 pegmatite, record significant compositional variations according to degree of evolution their hosting rock. Garnets most fractionated (Mt. Mica, Berry-Havey, Emmons) show highest Mn, Nb, Ta, Zr, Hf values, followed by those intermediate grade (Palermo 1) and, finally, garnets barren lowest values (Perham Stop-35). Iron, Ca, Mg contents follow an inverse order, with in latter pegmatites. Major element zoning shows increasing Mn core rim garnet samples, while trace is not systematic except for some crystals which a depletion these elements. Chondrite normalized HREE (Heavy Rare Earth Elements) spectra positive slopes pegmatites, both negative associated or flat highly Ion exchange mechanisms, including Fe2+−1Mn2+1, (Fe2+, Mn2+)−1Si−1Li1P1; (Y, Ho3+)2(vac)1(Fe2+, Mn2+)−3, could explain observed garnets. These are reflection composition pegmatitic magma (barren originate more ferromagnesian than pegmatites); coexisting mineral phases competing host certain chemical elements, such as biotite, schorl, plagioclase, apatite, Fe-Mn phosphates, Nb-Ta oxides, zircon, xenotime, monazite.