Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

A longstanding problem associated with the Acadian orogeny in New England is whether or not the orogeny involved a redistribution of mass and energy within the crust without the addition of mass and/or anomalous heat from the mantle. Several closed system models suggest the New Hampshire Plutonic Suite (NHPS) resulted from essentially in situ partial melting of the metasedimentary rocks of the Central Maine terrane at midcrustal levels. Crustal thickening, the presence of U-enriched metasedimentary rocks, and shear heating have been suggested as heat sources for anatexis. Several difficulties exist with these models as explanations for the entire NHPS: 1) the syntectonic Bethlehem, Kinsman, and Spaulding members are too CaO- and Na2O-rich to have been derived from the exposed metapelites; deeper seated metagraywackes may account for some of the NHPS rocks, but not for the metaluminous Spaulding Tonalite and other more mafic NHPS rocks; 2) some NHPS rocks have O, Nd, and Sr isotopic values are outside the range of analyzed Central Maine terrane metasedimentary rocks, requiring more primitive sources; 3) the temperatures of the Kinsman and the Spaulding magmas were higher than those attained during regional metamorphism associated with crustal thickening, greater than 850 degress C; 4) magmatic epidote and hornblende geobarometry for the Spaulding Tonalite suggest intratelluric crystallization, initially at pressures ~6 Kbars, deeper than the inferred decollement between the allochthonous Central Maine terrane metasedimentary rocks and underlying basement. These data suggest that although the NHPS contains a significant component from the Central Maine terrane metasedimentary rocks, the metaluminous Spaulding rocks, mafic enclaves in the Kinsman, and mafic rocks in northeastern Vermont, western Maine and southeastern New Hampshire provide evidence of external mass and heat beyond that associated with closed system processes.