HEAT SOURCES AND THERMAL EVOLUTION OF HIGH-GRADE CRUST IN THE ACADIAN/ALLEGHENIAN OROGEN, CENTRAL NEW ENGLAND: COMPARING NUMERIC MODELS WITH INSIGHTS FROM MONAZITE PARAGENESIS IN LPHT METAMORPHIC ROCKS

The overlapping axes of the Acadian and Alleghenian metamorphic highs contain LPHT-type migmatized pelites that are associated with km-scale felsic to intermediate concordant plutons. In such rocks, monazite may undergo 3 or 4 growth reactions in a single P-T loop, recording these segments of the T-t path. Pelitic migmatites (LM) from Gilsum, NH, spatially associated with the Kinsman Quartz Monzonite (KQM), contain monazite formed during melt crystallization (~750°C, 355 Ma), post-dating KQM intrusion (~400 Ma) by approximately 45 m.y. Thus, the KQM was not a heat source for the melting of the LM pelites. Several hundred LM monazite analyses were used to compute petrologic T-t paths. Petrologic T-t paths were then compared with T-t paths generated from 2-D numerical models for a variety of heating mechanisms. For 4 generations of monazite growth in the LM samples, our analyses indicate: 1) a period of cooling between monazite generations 1 and 2; 2) a deceleration of heating rate between monazite generations 2-3 (14.4±2.0°C/m.y., 1s) and generations 3-4 (8.2±1.4°C/m.y.). The numeric best fit to the monazite T-t path involved simultaneous intrusion of a 900°C, 2-km thick sill 1 km below the modeled rock (325°C, depth=14 km) and asthenospheric underplating (1450°C, depth=30 km). The numeric model generates a T maximum of 730-740°C 35-40 m.y. after lithospheric delamination; a time interval similar to the KQM intrusion-LM melting time interval. Production of maximum T in this range is due to the persistence of the initial thermal perturbation caused by pluton intrusion, followed by the conductive heat transfer from underplated asthenosphere (e.g., Bodorkos et al., 2002, JMG). Independent geophysical evidence for lithospheric delamination beneath New England at ~400 Ma was presented by Levin et al. (2000, Geology). Analogous monazite age-intrusion age diachronicity is also found in pluton-proximal migmatized pelites from southwest ME, southern NH, and central MA. Central MA monazites contain both late Acadian (core: 354±11 Ma, 1 s.e.) and Alleghenian (rim: 310±12 Ma) components. These findings suggest that lithospheric delamination played a major role in supplying heat for Acadian (400-350 Ma) metamorphism in central New England; the role, if any, of delamination in Alleghenian metamorphism is unclear.