AGE DINSTINCTIONS AMONG GRANITES AND MIGMATITES IN SOUTHWESTERN MAINE

New radiometric age determinations from rocks in the Sebago Migmatite-Granite Complex (SMGC) of southern Maine indicate Devonian regional high grade metamorphism and long-lived Permian granite magmatism. Together with previous data, the results demonstrate the construction of the Sebago pluton over a period of some 15 Ma (c. 308 to 293 Ma). The results do not permit the Sebago pluton to have been generated as part of the melting event that generated the widespread migmatites of the SMGC. Some granitic rocks exposed in this area, outside the mapped pluton boundary, have ages equivalent age to the SMGC migmatites (and are hence probably products of local segregation of melts from the event that generated the migmatites); other granites in the area have Sebago pluton ages. Considering that these granites may be petrologically indistinguishable from Sebago pluton rocks, it does not permit the simple assignment of ages based on field criteria.

Uranium-lead age measurements of zircon primarily used LA-MC-ICP-MS, with some single-grain TIMS analysis. Spatially-resolved U-Pb analyses of zircon grains from SMGC migmatite samples taken from two widely separated sites yield a mean age of 376 ± 15 Ma. This age is consistent with that of a fine-grained granite from Freeport (378 ± 2 Ma), near the eastern border of the SMGC. Zircon U-Pb ages of five spatially dispersed samples of granite from the c. 400 km² Sebago pluton cluster between 308 and 285 Ma, bracketing the previously-published U-Pb monazite age (293 ± 2 Ma; Tomascak et al., 1996a). A two-mica granite from New Glouchester, northeast of the Sebago pluton, crystallized at 288 ± 6 Ma. Although the age of this sample is equivalent to the Sebago pluton, its geochemistry is not so simply connected. Whereas the Sebago pluton has homogeneous Nd isotope initial ratios (ε = -3.7 to -1.6) and parallel lanthanide REE distributions, this sample has a slightly more negative ε (-4.1) and a steeper REE pattern. The geochemistry of this sample is akin to more texturally heterogeneous samples within the SMGC (“type 2” granites of Tomascak et al., 1996b). The geochemical distinction among granites of equivalent age is interpreted to reflect intrusion of small batches of late stage Sebago pluton magmas into surrounding rocks with subsequent contamination by interaction with surrounding country rocks.