GEOCHEMICAL AND ISOTOPIC (ND, O, AND PB) CONSTRAINTS ON GRANITE SOURCES IN THE HUMBER AND DUNNAGE ZONES, GASPESIE, QUEBEC, AND NEW-BRUNSWICK - IMPLICATIONS FOR TECTONICS AND CRUSTAL STRUCTURE

Siluro-Devonian granitoids span a wide compositional range (approximat ely 50-76 % SiO2) and can be subdivided into two groups: (i) monzoniti c or incompatible element enriched with affinities to within-plate mag matism (WPG); and (ii) calc-alkalic or incompatible element depleted w ith supra-subduction zone affinities (VAG). Granitoid epsilon(Nd)(T = 0.4 Ga) values range from -1 to +5.5; most lie between +3 and +5.5. Pb -207/Pb-204 isotopic compositions range from 15.52 to 15.61; most fall between approximately 15.55 and 15.59. Most deltaO-18 values lie betw een +5.5 and + 8 parts per thousand. No well-established trends exist between SiO2 and isotopic composition, and isotopic compositions do no t differ between the two trace element defined granitoid groups. Thoug h Pb isotopic data are consistent with a major contribution to the gra nitoids from Proterozoic-aged Laurentian plate rocks (i.e., Grenville basement), Nd and O isotopic data are not. These isotopic data are con sistent with major source components derived from early Paleozoic depl eted or supra-subduction zone affected mantle and (or) crustal rocks d erived from the early Paleozoic mantle(s). These protoliths would not have seen significant interaction with time-integrated old crustal mat erial or surficial processes. Granitoid Pb isotopic data can be reconc iled with an early Paleozoic mantle-crust origin, but it may also be t hat the Pb isotopes are decoupled from other isotopic systems. In eith er case, Nd and O isotopic data clearly prohibit the involvement of si gnificant amounts of Grenville crust and suggest that seismic-reflecti on data do not define crustal blocks, or at least not blocks having a tectonic and geologic history easily related to the surface geology.