A LAURENTIAN PROVENANCE OF THE MERRIMACK BELT, NEW ENGLAND APPALACHIANS

The tectonic setting of the Merrimack belt begs serious tectonic questions in that it lies between the Central Maine terrane rocks to the northwest and peri-Gondwanan/Avalonian rocks to the southeast. Even more problematic peri-Gondwanan/Avalonian rocks of the Massabesic Gneiss Complex are immediately adjacent to the northwest in New Hampshire. Thus a peri-Gondwanan derivation of the Merrimack belt can be expected while a Laurentian provenance has been proposed in the literature. New major, trace element and isotopic data from coastal New England rocks support a derivation of the Merrimack belt from a Laurentian source. The Central Maine terrane (CMT) to the west is dominated by metapelites with subordinate metagraywackes while the Merrimack belt in contrast, is dominated by metagraywackes with geochemical characteristics indicative of an Andean-type continental margin (i.e., REE abundances, SiO2 and FeO+MgO values, K2O/Na2O, Taylor and McLennan, 1985). The Hutchins Corner Formation however, (previously known as the Vassalboro Formation) of the CMT is also dominantly metagraywacke and has major and trace element compositions that are similar to those of the Merrimack belt rocks. Additional Merrimack belt - CMT correlation is supported by Merrimack Epsilon Nd values of -6.3 to -9.1 (@ 420 Ma) that are identical to those of the CMT (central NH - Lathrop et al., 1996; western ME - Cullers et al., 1997). Identical major, trace and isotopic compositions of the Fredericton Sequence on strike to the northeast in coastal Maine also support a Merrimack - Fredericton correlation (Bothner et al., 1999; West et al., 2000). A gradation from Laurentian-derived mudstones in the west toward graywacke to the east is apparent. In spite of the increasing arc fraction to the east (Merrimack belt), the Epsilon Nd values continue to be Laurentian. Thus the magmas of the inferred arc were derived primarily from an existing Laurentian crust.