INITIAL ND-SR ISOTOPIC COMPOSITION OF GRANITE PLUTONS FROM EASTERN AND CENTRAL MAINE: IMPLICATIONS FOR TERRANE ACCRETION

Initial Nd and Sr isotopic composition of Paleozoic granites from the Medial New England Terrane (MNET) and the Composite Avalon Terrane (CAT) in Maine indicate the basement beneath these terranes changed dramatically after the Silurian. Two major episodes of Paleozoic magmatism are documented in this area. “Silurian” magmatism is bimodal, and characterized by distinctive epizonal plutonic and volcanic igneous complexes (e.g., Vinalhaven, South Penobscot). Silurian magmatism is known southeast of the Norumbega Fault Zone (NFZ). Silurian granites exhibit a large variation in ε_Nd values (2.5 to -8) and ⁸⁷Sr/⁸⁶Sr)_i ratios (0.703 to 0.713). “Devonian to Carboniferous” magmatism is dominantly granitic and includes stocks of (1) peraluminous granite (e.g., Waldoboro); 2) metaluminous granite (e.g., Deer Isle, Mt. Waldo); and (3) Large batholiths of weakly peraluminous to metaluminous granite with megacrystic alkali feldspar (e.g., Lucerne). Devonian magmatism is wide spread throughout the area and is found on both sides of the NFZ. Devonian granites exhibit a narrow range in ε_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 the fields for Silurian and Devonian granites partially overlap, however, they exhibit distinctly different slopes. Silurian granites define the field with the steeper slope. The most radiogenic Silurian granite yields a T_DM age of 1.8 Ma. Devonian granites define the field with a shallow slope. The most radiogenic Devonian granite yields a T_DM age of 1.4 Ma. This is compatible with Silurian and Devonian granites being derived from partial melting of distinct basement source regions. Silurian granites and Devonian granites intrude both MNET and CAT. This suggests 1) “terrane-bounding” faults in coastal Maine juxtapose distinct upper crustal terranes but not distinct basement terranes and that the upper crustal terranes were assembled prior to the Silurian melting event. 2) The source region for Silurian granites was 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 the MNET basement source region prior to the Devonian melting event.