THE ROLE OF GRANITE IN ASSEMBLING THE CRUST OF EASTERN COASTAL MAINE

Geochemical and isotopic investigation of Siluro-Devonian granitoids along a transect crossing Medial New England Terrane (MNET) and Composite Avalon Terrane (CAT), are used to 1) characterize source regions of granitoids with respect to proposed terrane boundaries; 2) determine temporal changes in source materials during terrane accretion; and 3) constrain the mode of terrane accretion. CAT granites are compositionally diverse and include peralkaline, metaluminous, and peraluminous granites. MNET granites are weakly to strongly peraluminous. Both suites form stocks to small discrete batholiths, and are dominantly granite rather than tonalite. Compositional diversity of CAT compared to MNET granites implies a wider variety of source materials for the former and limited sources for the later. Silurian and Devonian CAT and MNET granitoids show decreasing concentrations and overlapping fields of Fe₂O₃, MgO, TiO₂, K₂O, Al₂O₃, Zr, Sr, Ba and Ga with increasing SiO₂. Granitoids from both terranes plot on the ferroan field of Frost et al (2001). This markedly contrasts with the magnesian granite field associated with a subduction environment. Trace element discrimination diagrams (Pearce et al., 1984) indicate mixed tectonic settings for these granites and in isolation indicate a subduction component is present. MNET-Silurian and Devonian granites plot in the volcanic arc field. CAT Silurian and Devonian granites overlap between within plate, syn-collision and volcanic arc fields. Nd-Sr isotopic characteristics of Devonian and Silurian granites also indicate their derivation from heterogeneous crustal sources, with variable mantle contributions, and an absence(?) of the Silurian granite source regions during the Devonian melting event (see Mataragio et al., this session). Characteristics of these granites are atypical of felsic rocks associated with subduction zone magmatism, suggesting that trace element signatures are inherited from the source materials of the granites.