TEMPORAL EVOLUTION OF CRUSTAL SOURCES FOR GRANITIC MAGMATISM IN MAINE

The geology of central to coastal Maine is best represented as a series of fault-bounded terranes with disparate stratigraphic, structural, and metamorphic histories. From Silurian to Mississippian times, these terranes were ubiquitously punctured by igneous intrusions. High precision U-Pb dating (e.g., Tucker et al., 2001) has resolved two, possibly three, episodes of magmatism. “Silurian” magmatism, known southeast of the Norumbega Fault Zone, is dominantly bimodal and represented by layered mafic complexes (Pocomoonshine), epizonal mafic to felsic plutonic-volcanic igneous complexes (Vinalhaven), and isolated granite plutons (Youngtown). Silurian granites exhibit a wide range in epsilon Nd values (2.5 to -8) and 87Sr/86Sr)i ratios (0.703 to 0.713). “Devonian to Mississippian” magmatism is wide spread, known on both sides of the NFZ, dominantly felsic, and characterized by large batholiths (Lucerne) to small stocks (Long Island) that are peraluminous (Blue Hill), metaluminous (Deer Isle), or peralkaline (Tunk Lake) in composition. Younger intrusions exhibit evidence of mantle involvement (Deer Isle, Tunk Lake). “Devonian” granites exhibit a narrow range in epsilon Nd (0.8 to -2) and a large range in 87Sr/86Sr)i ratios (0.701-0.710). On a Nd-Sr isotopic correlation diagram Silurian granites define a field with a distinctly steeper slope than Devonian granites (regardless of which terrane they intrude). The most radiogenic Silurian granite yields a TDM age of 1.8 Ma. whereas the most radiogenic Devonian granite yields a TDM age of 1.4 Ma. Silurian and Devonian granites were derived from partial melting of isotopically distinct source regions. This suggests 1) “terrane-bounding” faults in coastal Maine juxtapose distinct upper crustal terranes but not distinct basement terranes, 2) upper crustal terranes were assembled prior to the Silurian melting event, 3) source regions for Silurian granites were not “tapped” during the Devonian melting event, either because of its refractory nature, or because the Silurian granites and their host terranes are both allochthonous and were displaced over a new basement prior to the Devonian melting event. Further investigation of temporal changes in the source regions of these distinct melting events can provide deeper insights into this complex orogeny.