ISOTOPIC HETEROGENEITY AND POTENTIAL VARIABLE SOURCES OF GRANITIC ROCKS OF THE SEBAGO MIGMATITE DOMAIN, SOUTHERN MAINE

Initial Pb isotopic compositions of leached K-feldspar samples from granites of and spatially associated with the Sebago pluton, southern Maine, further delineate these suites of rocks and point to subtleties of basement sources. The Sebago Migmatite Domain encompasses both the homogeneous Late Pennsylvanian two-mica Sebago pluton (c. 400 km²) and numerous bodies ranging from centimeter scale synmigmatitic components to regionally extensive masses of granite. The more extensive of the granitic bodies are generally petrographically heterogeneous, characterized by assemblages of biotite, biotite + muscovite and biotite + muscovite ± garnet ± tourmaline, with abundant micaceous schlieren and indications of magmatic flow. Some more homogeneous two-mica granites crop out on the western periphery of the Sebago pluton. Whereas some of these granites have lanthanide element distributions that overlap Sebago granites, several show petrogenetic distinctions in their lanthanides, such as Eu/Eu* > 1.0 and upturned HREE patterns.

The Sebago pluton has previously been characterized in terms of initial Nd (e_Nd = -3.7 to -1.6) and Pb (²⁰⁷Pb/²⁰⁴Pb = 15.61 to 15.68) isotopes. New data extend the range in initial Pb isotopes to slightly lower values (to 15.58). Granitic rocks that crop out on the western edge of this pluton show both lower and higher initial Nd (e_Nd = -5.1 to -0.8) and extend to lower values of initial ²⁰⁷Pb/²⁰⁴Pb to 15.57. The western granites show a positive correlation between Nd and Pb isotopes, as would be expected from simple mixtures of Laurentian crust with circum-Gondwanan basement. The trend to lower initial Nd with decreasing ²⁰⁷Pb/²⁰⁴Pb is not apparent for the Sebago pluton itself, suggesting the potential for substantially different petrogenetic processes amongst the granitic rocks of the western edge of the pluton. Although the age of the other granitic rocks in the Sebago Migmatite Domain have yet to be determined, these results set up the hypothesis that they are somewhat older and accessed Laurentian crust that later Paleozoic magmatism did not (or could not) tap.