ELEMENTAL GEOCHEMISTRY OF MIGMATITES WITHIN THE SEBAGO MIGMATITE DOMAIN, SOUTHWESTERN MAINE

The Sebago Migmatite Domain (SMD) encompasses the homogeneous Sebago granite pluton (c. 400 km²; 293 ± 2 Ma) as well as a broad (c. 5000 km²) surrounding region of heterogeneous granitic bodies and migmatitic semipelitic to pelitic rocks. Analyses of separate migmatitic constituents (leucosome/melanosome) serve to further constrain genetic relationships among rocks in the area. Major and trace element contents of migmatite leucosomes fall into three categories: (1) compositions equivalent to the Sebago pluton two-mica granites; (2) compositions reflecting variable retention of residual plagioclase + biotite; (3) fractionated melts from which plagioclase + biotite were removed. Many migmatite samples did not have physically separable leucosomes. These rocks have compositions that are either Ca+Na-poor and potentially residual, or weakly ferromagnesian, more similar to Sebago pluton two-mica granites. Texturally heterogeneous granitic rocks within the SMD, cropping out as sheets, pods and lenses within the migmatites, are also compositionally heterogeneous, with a significant population of samples recording melt extraction from systems that left plagioclase and biotite behind during melt extraction. Leucosomes and melanosomes have chondrite-normalized (Gd/Yb)_N of 1.8 ± 0.6, similar to the Sebago pluton (2.1 ± 0.5). Granites in the SMD are much more variable in their HREE patterns (4.5 ± 3.3), suggesting source rocks with variable proportions of residual trace element enriched minerals, most likely zircon. The majority of the bulk migmatite samples have elevated Sm/Nd, in common with the granites in the SMD, as well as possessing steeper HREE patterns, with (Gd/Yb)_N ~ 3. An earlier hypothesis about the connection of the Sebago pluton granites and those in the surrounding envelope of migmatites considered that the latter granites represent mixtures of Sebago pluton magmas with melts derived more or less in situ. The current analysis does not invalidate this. The hypothesis, however, remains to be more definitively tested by geochronology, as it requires that the Sebago pluton and surrounding granites and migmatites are components of a single magmatic event in the late Carboniferous to early Permian.