RB/SR AND SM/ND SYSTEMATICS OF TWO MIXED MAGMA SYSTEMS FROM THE COASTAL MAINE MAGMATIC PROVINCE

The Deer Isle and Mt. Waldo granites are undeformed, post-tectonic, Acadian plutons that crop out in the Penobscot Bay area, Maine. U/Pb dating of zircons (Stewart, 1998) suggests both are 371 million years old and are therefore part of the younger phase of granitic magmatism in the coastal Maine magmatic province. Magma mixing was important in the evolution of both plutons. Evidence for this includes abundant mafic to felsic microgranular enclaves, complex zoning and dissolution textures in plagioclase, plagioclase-mantled alkali feldspars, linear relationships between major elements along with synplutonic composite and mafic dikes.

21 samples from the Deer Isle pluton and 15 from the Mt. Waldo pluton were analyzed for Nd, Sm, Rb, and Sr concentrations and isotopic compositions. As the age of each pluton is known, data were plotted on Rb/Sr isochron diagrams to test for linearity relative to a 371 Ma reference isochron. Samples that deviate significantly from the reference isochron were excluded from further consideration because of apparent open system behavior, and εNd and (⁸⁷Sr/⁸⁶Sr)_i ratios were calculated for the remaining samples. Samples from the Deer Isle pluton show a limited range in both εNd and (⁸⁷Sr/⁸⁶Sr)_i values, whereas the Mt. Waldo samples show a significant range in (⁸⁷Sr/⁸⁶Sr)_i but a limited range in εNd.

The relatively large variation in (⁸⁷Sr/⁸⁶Sr)_i but homogeneous εNd for samples of the Mt. Waldo pluton, suggest that mixing was important in the evolution of the Mt. Waldo pluton, and that end-members with variable (⁸⁷Sr/⁸⁶Sr)_i but relatively homogeneous εNd mixed incompletely. Because petrographic, chemical and textural data from the Deer Isle pluton indicate that magma mixing was important in its petrogenesis, the more homogeneous εNd and (⁸⁷Sr/⁸⁶Sr)_i data suggest that the mixing process was thorough. Low ⁸⁷Sr/⁸⁶Sr_i values and young T_DM ages are consistent with derivation from a relatively young volcanic source material.