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Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA
Geophysical Laboratory, 2801 Upton St. N.W., Washington, DC 20008, USA
The metamorphic terrain in central New Hampshire is characterized by over 9000 km2 of primarily sillimanite — muscovite-bearing rocks. Superimposed on this large thermal high are a series of hot spots about 20 km2 in area. Thermal models suggest that the large-scale thermal structure of central New Hampshire can be explained by conductive heat transfer processes. However, the hot spots can neither be explained by conductive heat transfer nor by intrusion of plutons. Isotopic and petrological studies demonstrate that the hot spots were formed by the focusing of hot metamorphic fluids into zones that had relatively high fracture permeability.
The infiltration of hot fluids has significantly affected the P-T paths of rocks in central New Hampshire. Within the region of abundant hot spots, the P-T history of the metamorphic rocks is different from that outside the hot spot region. The P-T paths in the central portion of New Hampshire, the area with the hot spots, are characterized by isobaric-heating paths. The rocks passed near aluminium silicate triple point pressures and temperatures and later equilibrated at similar pressures, around 4.0 kbars, and at higher temperatures, approximately 650–750°C. The absence of other mechanisms to produce the isobaric-heating paths and the evidence for intense hydrothermal activity suggest that fluids played an integral role in creating the thermal spikes along the P-T trajectory.
Analytical solutions to differential equations that describe focused fluid flow are used to estimate the duration of hydrothermal activity and the amount of fluid involved. The steep thermal gradients adjacent to the hot spots and the observed temperature increase within hot spots are consistent with large fluid fluxes, > 1 x 10–9 m3 fluid per m2 s–1, over short periods of time, c. 100000 years. This indicates that P-T paths from rocks within hot spots record only small portions of the geological evolution of the metamorphic terrain.
