NATURE OF THE CRUST NEAR THE GRENVILLE BORDER IN NORTHERN NEW ENGLAND: CONSTRAINTS FROM NEW PB ISOTOPE DATA FROM THE MOOSELOOKMEGUNTIC IGNEOUS COMPLEX, MAINE

In the northern Appalachians, the Mooselookmeguntic Igneous Complex (MIC) straddles the tectonite zone that separates material with principally North American (Grenville) sources (Bronson Hill Belt and units to the west) from the Central Maine Belt, wherein granite plutons commonly derive largely from non-North American (Avalon-like or Ganderian) source materials. The MIC comprises meter-scale blocks of biotite tonalite/granodiorite (ca. 389 Ma, U-Pb zircon, Solar et al., 1998, Geology, 26, 711-714), an areally subordinate quartz diorite/biotite tonalite (ca. 377 Ma, U-Pb zircon, unpublished data), and an areally dominant leucogranite (ca. 370 Ma, U-Pb monazite, Solar et al., 1998). Radiogenic isotope data (initial Nd and Pb compositions) and elemental concentrations are presented for major components of the MIC in order to evaluate magma petrogenesis and crustal variations across this mid-Paleozoic (Acadian) suture zone.

With one exception, initial Nd isotopic compositions for the less evolved rocks show a limited range (e_Nd(t)=-2.7 to -0.7), whereas the leucogranites are very heterogeneous (e_Nd(t)=-6.9 to -0.5). Given the compositional ranges for potential source materials, the Nd data do not permit the nature of the source materials to be unequivocally constrained. However, there is considerable Pb isotope differentiation between Grenvillian and non-Grenvillian source materials, either of which are probable source components for MIC magmas. Combined isotope and elemental data are used to assess mantle contributions, of particular relevance among the less evolved rocks. The Pb isotope composition of leached alkali feldspar from one sample of leucogranite is identical within error (²⁰⁷Pb/²⁰⁴Pb ~ 15.61) to the literature datum (Ayuso and Bevier, 1991; Tectonics, 10, 191-212), which indicates a contribution to these rocks from non-North American crust. We will present initial Pb isotopic compositions of the remaining MIC samples, which span the full geographic extent of the complex, in order to more clearly understand the relative contributions of the differing basement sources, thereby more clearly defining the nature of the crust in this part of northern New England.