ISOTOPIC SIGNATURES OF ACADIAN GRANITES FROM THE LAURENTIAN BORDER: AN EYE INTO BASEMENT SOURCES AND MANTLE CONTRIBUTIONS IN A CONVERGENT SETTING

Granite pluton complexes in western Maine and New Hampshire, along and southeast of the border between Laurentian and Central Maine belt crust, define coherent initial Nd and Pb isotopic signatures, interpreted to reflect derivation from sources dominated by accreted arc crust. The group of plutons spans a range in age between ca. 404 and 370 Ma (Solar et al., Geology, 1998), and their formation and modes of emplacement have been linked to orogen-scale transpressive deformation synchronous with high temperature metamorphism (Brown & Solar, Tectonophysics, 1999). Granites range from elementally primitive quartz diorite and monzodiorite to two-mica leucogranite, and the dataset includes samples from five separate intrusive complexes.

Two plutonic complexes have received the most intensive study: the Mooselookmeguntic igneous complex (MIC) and the Phillips pluton. Derivation of the more primitive rocks of the MIC most likely occurred by melting of Bronson Hill belt (BHB) crust of mafic composition. In contrast, the MIC granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of Nd and Pb initial isotopic compositions with data for Central Maine belt (CMB) metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this Ordovician to Siluro-Devonian stratigraphic package. However, a mass contribution from the BHB crust to the granite magmas cannot be ruled out. New Pb isotope data for the Phillips pluton permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the CMB (Pressley & Brown, Lithos, 1999), but suggest that the volumetrically minor granodiorites were extracted from sources more similar to BHB crust. Based on these data, we infer that BHB crust was more extensive beneath the CMB than previously recognized, and that mafic melts from the mantle did not make an important mass contribution to genesis of granite magma in this part of the Acadian orogen.