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The petrogenesis and emplacement of the New Hampshire plutonic suite

Department of Geology, Brigham Young University, Provo, Utah 84602; dorais{at}byu.edu

A longstanding problem associated with the Acadian orogeny in New England is whether or not the orogeny involved a redistrubution of mass and energy within the midcrust without the addition of magmas and/or anomalous heat from the mantle and/or lower crust. 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 (CMT) at midcrustal levels. Crustal thickening models, shear heating along the decollement, and the presence of U-enriched metasedimentary rocks have been suggested as heat sources for anatexis. While applicable to numerous NHPS plutons, there are several difficulties with these models as explanations for the NHPS as a whole: 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 source rocks but not for the metaluminous Spaulding Tonalite and other more mafic NHPS rocks; 2) some NHPS ¹⁸O, Nd, and Sr isotopic values are outside the range of CMT and Bronson Hill terrane metasedimentary rocks, requiring a more primitive mantle-like component; 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°C; 4) magmatic epidote and hornblende geobarometry for the Winnipesaukee pluton of the Spaulding Tonalite suggest intratelluric crystallization, initially at pressures 6 Kbars, deeper than the inferred decollement between the allochthonous CMT terrane metasedimentary rocks and underlying basement. These data suggest that while the NHPS contains a significant component from the CMT 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 heat and mass beyond that associated with closed, midcrustal system processes.