The Massabesic Gneiss Complex, New Hampshire: a study of a portion of the Avalon Terrane

Geochemical data from the 625 Ma Massabesic Gneiss Complex of southern New Hampshire show strong affinities for other Avalonian rocks of southern New England and suggest continental rifting in the Late Proterozoic. Migmatized paragneiss, the dominant rock type in the complex, has major and trace element compositions that are compatible with graywackes from continental arcs. The para- gneiss also has strong lithologic, metamorphic, and isotopic similarities to the rocks of the Hope Valley zone of Connecticut Avalon, suggesting a possible Hope Valley— Massabesic correlation. At 625 Ma, the paragneiss eNd values are similar to Avalonian crust in other locations of the orogen. Two types of amphibolite are present in minor amounts in the paragneiss of the Massabesic Gneiss Complex. The first type is a paramphibolite and consists of calc-silicate layers in the Massabesic paragneiss, the second type is metaigneous. Major and trace element abundances reveal that the protoliths of the orthoamphibolites range from continental rift alkaline basalts and tholeiites to N-type MORBs. Orthoam- phibolite eNd (625 Ma) values range from 2.4 to 4 as expected of rift-related magmas derived from partial melting of a depleted mantle source and have the same values as Iapetus ocean floor rocks of similar age. Orthoamphibolite major and trace element geochemical characteristics overlap those of the Middlesex Fells amphibolites of the Esmond-Dedham zone of eastern Massachusetts Avalon, which range from alkaline to transitional basalts erupted in a continental rift setting. The compositions of orthoam- phibolites define a potential magmatic continuum produced by batch partial melting of the mantle initiated during continental rifting and proceeded to ocean basin forma- tion. The inferred continuity of mafic magmatism from the Esmond-Dedham (Mid- dlesex Fells Formation) to the Massabesic Gneiss Complex (and Hope Valley zone) suggests that these zones are not distinct lithotectonic zones but are parts of a single landmass. Massachusetts Avalon (Esmond-Dedham) represents the continental section of Avalon where the alkaline to transitional magmas of the early rifting stages are preserved. According to our tectonic reconstruction, the Massabesic Gneiss Complex is the oceanward, continental margin represented by volcanoclastic sediments with the MORBs representing the initiation of ocean basin development. The leading edge of this landmass, of which the Massabesic Gneiss Complex is the only observable remnant, collided with Laurentia during the Acadian Orogeny. The inboard, thicker, more continental trailing-edge, that is, platform Avalon (Esmond-Dedham) collided later during the Alleghanian Orogeny.