Mapping, petrological and geochemical explorations of the Massabesic Gneiss Complex in New Hampshire

The Massabesic Gneiss Complex (MGC) is a northeast trending belt of rocks that exposes metasedimentary rocks, migmatites and granites. Early geochronological work indicates that the MGC is Late-Proterozoic, but recent mapping (∼112 mi2, Kerwin, 2000 and ∼130 mil2, this study) of the MGC and surrounding areas has found rock textures indicating that the migmatites are possibly younger.; The MGC lies within an area of low relief that is mostly covered by glacial overburden and is structurally defined as a regional anticlinorium in which early fold axes indicate refolding during progression of the structural arching. Foliation of the metasedimentary and migmatite rocks indicates they are related either lithologically or by tectonics or both. Faulting along the two major faults in the area occurred during ductile and late brittle tectonic phases culminating in horst-like uplifting of the MGC, most likely in Permian times.; Petrographically the rocks show evidence of compression and decompression melting episodes in which muscovite and biotite dehydration reactions contribute to magmas that the mobilize and accumulate to produce the migmatite and rock textures observed during the mapping phase of this project. Geochemically the metasedimentary rocks, the migmatites and granites have two distinct Rare Earth Element patterns that are interpreted to show that some of the rocks are related to the Barrington Granite and the rest to the Milford Granite. Discrimination diagrams indicate that the granites (with the exception of the Barrington Granite) are all most likely fractionated from a Milford-like source and are produced during continent-continent collision along an active continental margin. Partial melt modeling of the metasedimentary rock into migmatites indicates that the migmatites Types 1-3 (as outlined in Kerwin, 2000) are melted Silurian rocks with one of the textures (Type 1) being restitic.; The results of these explorations are interpreted to show that metasedimentary rocks were buried and melted producing migmatites of the MGC and evidence to support at least three and possibly four melt episodes before uplift in Permian time.