P-T PATHS FROM NORTHWESTERN NEW-HAMPSHIRE - METAMORPHIC EVIDENCE FOR STACKING IN A THRUST NAPPE COMPLEX/

Petrologic study of polydeformed metasediments on the western flank of the Bronson Hill anticlinorium in northwestern New Hampshire demonstr ates that two adjacent structures were metamorphosed under different b aric conditions during the Acadian orogeny. The Salmon Hole Brook sync line, part of a belt of metasediments that lies immediately west of th e Bronson Hill anticlinorium, experienced loading with limited heating during metamorphism. P-T path calculations and coexisting aluminum si licates indicate that garnets nucleated at pressures between 3 and 4 k b and grew during a subsequent pressure increase of 1.5 to 2 kb. Growt h of staurolite porphyroblasts that transect foliation suggests that h eating continued after deformation associated with garnet growth. In c ontrast, rocks of the proposed Piermont allochthon (Moench and others, 1987; Moench and others, 1992) located southwest of the Salmon Hole B rook syncline experienced syntectonic metamorphism at pressures betwee n 5 and 7 kb, culminating in a trend of heating with decreasing pressu re of approx 0.5 kb. Predictive thermodynamic models indicate that gar net consumption during staurolite growth and chemical diffusion have n ot significantly modified these path trajectories. The different P-T p ath trajectories obtained from the Salmon Hole Brook syncline and the Piermont allochthon suggest they occupied different structure levels d uring the Acadian orogeny. The initial steep portion of P-T paths in t he Salmon Hole Brook syncline is viewed as evidence that these rocks a re allochthonous and experienced tectonic loading at about the same ti me as they were emplaced over a cooler lower plate. The heating with u nloading recognized in the proposed Piermont allochthon conforms to th e evolution of a parauthochthonous lower plate in a convergent setting predicted by England and Thompson (1984) in which tectonically pertur bed geotherms relax during exhumation. In our interpretation, the rock s of these two regions were juxtaposed during the westward transport o f a thrust/nappe complex, such as has been described for western and s outhwestern New Hampshire and north-central Massachusetts (Thompson an d others, 1968). This interpretation is supported by the correlation o f P-T paths from northwestern New Hampshire with paths along strike to the south in the Hardscrabble synclinorium (Kohn and others, 1992) an d in the region of Fall Mountain (Spear, Hickmott, and Selverstone, 19 90).