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Mineral chemistry, petrology and geochemistry of the Sebago granite-pegmatite system, southern Maine, USA

Author(s): Wise M | Brown C D

Journal: Journal of Geosciences
ISSN 1802-6222

Volume: 55;
Issue: 1;
Start page: 3;
Date: 2010;
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Sebago granites are strongly peraluminous and show rare-element enrichment typical of evolved fertile granites (K/Rb = 87-257), Rb/Tl = 10.6-71.3, Ba/Rb = 0.18-5.04, Al/Ga = 1419-1749, Zr/Sn = 1.53-43.9). The SPG shows high levels of Be, Nb > Ta, P, Li and B with subordinate enrichment in Rb and Cs. Moderate to high levels of rare-element fractionation is encountered in pegmatitic K-feldspar (K/Rb ≈ 17, K/Cs ≈ 90, Rb/Tl ≈ 75); muscovite (K/Rb ≈ 6.6, K/Cs ≈ 14.8, Rb/Tl ≈ 127); beryl (Na/Li ≈ 1.77, Cs2O ≈ 3.15 wt. %), garnet [Mn/(Mn + Fe) ≈ 0.60] and manganotantalite and ixiolite/wodginite [Mn/Mn + Fe) ≈ 0.98, Ta/(Ta + Nb) ≈ 0.80-0.93]. Evidence that supports the pegmatitic leucogranites as the likely parent to the SPG includes the close spatial distribution of the pegmatites to the leucogranite bodies, texturally and mineralogically similar units observed within the leucogranite and the neighboring pegmatites plus gradual, yet overlapping, rare-element fractionation from the leucogranites to the associated pegmatites. A few pegmatites (e.g., the Lord Hill pegmatite and amethyst-bearing pegmatites) show NYF tendencies unlike any other pegmatites of the SPG. Differences in fractionation degree, evolution and/or relation to another fertile granite-pegmatite system may be responsible for this apparently anomalous group of pegmatite dikes. ]]>
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