MULTIPLE SOURCES AND MULTIPLE PROCESSES: UNRAVELING ACADIAN MAGMATISM

We review the dataset for Acadian magmatism in Western Maine to investigate sources and petrogenetic processes. Since tectono-stratigraphic zones strike NE, our review traverses NW across the Central Maine Belt (CMB) to the Bronson Hill Belt (BHB). The tectonite zone separating these two belts may also separate different types of lower crust and mantle. The Phillips pluton (PP; c. 404 Ma) is composed of dominantly two-mica leucogranite (95%), with subordinate granodiorite (5%). The Lexington pluton (LP; c. 404 Ma) is composed of porphyritic and two-mica granites, some of which exhibit magma mingling features at the deepest levels exposed, representing interaction between influx of mafic magma and resident partially-crystallized granite. The Redington pluton (RP; c. 408 Ma) comprises predominantly porphyritic biotite granite. The Moosleookmeguntic pluton (MP) is composed of predominantly two-mica granite (c. 370 Ma), which locally may contain disoriented blocks of granodiorite/tonalite (c. 377 Ma), and quartz diorite (Umbagog phase, c. 377 Ma). The central part of the Flagstaff Lake Igneous complex (FLIC; c. 400 Ma) includes diorite with magma mingling features against granite host representing influx of mafic magma into partially-crystallized granite.

Initial Nd isotope compositions (epsilon Nd@400Ma) permit certain petrogenetic constraints. Plutons most distal from the CMB-BHB boundary (e.g., PP) are derived principally from CMB-like sources (-8.1 to -5.3; leucogranite), mixed to varying degrees with Avalon-like components (-1.8 to +0.1; granodiorite). A similar mix of source components characterizes granites of the younger Sebago pluton (c. 293 Ma; ~100 km south). Closer to the boundary, plutons (e.g., LP, RP, MP) have Nd isotope characteristics similar to those of the Dunnage Zone in Eastern Canada (-5.1 to -2.2). These data are consistent with derivation from sources of mixed Avalon-like and CMB affinities, but also permit substantial BHB source components, owing to the similarity between relatively juvenile BHB and Avalon-like Nd. Interpretation of Nd isotopes alone in the FLIC (+1.9 to +2.3) is complicated by evident substantive mantle source contributions.