THE PLUTONIC RECORD OF THE LATE PALEOZOIC IN MAINE

There is an exiguous record of post-Neo-Acadian plutonism in Maine, substantially less well-represented than in central and southern New England. The 293 ± 2 Ma Sebago pluton (c. 400 km²) represents the most significant single intrusion of this period. Crystallization ages of surrounding, geochemically unrelated granite bodies within the Sebago Migmatite Domain remain undetermined, but field relations suggest that some component of these are older. The other vestige of magmatism in this time period comes from the Topsham-Brunswick area. Here, granite bodies range from meter-scale dikes to ≤ 0.1 km² masses, capable of being mapped at the quadrangle scale. The largest such body (the granite of Brunswick) crystallized at 278 ± 2 Ma. Mineralized granitic pegmatites in Topsham show a range in ages that overlap this date (269 - 274 Ma).

A common feature of these magmatic rocks is relatively juvenile initial Nd isotopic compositions. The Sebago pluton proper (previously “Group 1 granites”) preserves a Nd isotope signature of ε_Nd(293 Ma) = -3.7 to -1.6. Elemental and petrographic data suggest that these were minimally affected by contributions from local crust during transport and emplacement processes, and hence that their isotopic compositions are accurate measures of the sources of the magmas. The granites in Brunswick are similarly homogeneous, but record initial ratios reflecting greater long-term Sm/Nd fractionation (ε_Nd(270 Ma) = -5.2 to -4.3). The majority of Topsham pegmatites show distinct initial ratios (-2.2 to -1.4). A subgroup of pegmatites in the south part of Topsham (Standpipe Hill series) has Nd initials that are also distinct, and overlap local granitic rocks (-3.4 to -3.3 and -3.9 to -3.7, respectively). Initial Pb isotopic compositions for all of the granitic rocks overlap, with ²⁰⁷Pb/²⁰⁴Pb in the range 15.61 - 15.68.

Were these just the tiny volumes of granite they were originally ascribed to being, their tectonic implications might be less troubling. As it is, there is no particularly satisfying mechanism for generating widespread mid-crustal melting in this area at both c. 290 Ma and c. 270 Ma. The most plausible processes will be discussed.