Abstract
A study of pelitic metamorphism in the western 70 percent of the Augusta quadrangle, Maine, has shown that this metamorphic terrane consists of products of a series of overlapping thermal events. Prograde isograds have been delineated on the basis of discontinuous reactions. Continuous reactions are also described, based on Fe–Mg–Mn phase relations and garnet zoning patterns.
Two approaches have been used to estimate the pressure and temperature of the lower sillimanite zone. Fe–Mg KD's from garnet–biotite pairs were used to estimate the temperature of metamorphism. Reactions involving staurolite were also used after adjusting their experimentally determined reaction curves for variations in mineral compositions and the composition of the fluid phase. Assuming the presence of an independent fluid phase, calculations show that X(H2O) was on the order of 0.75±0.10. Results indicate that the average temperature and pressure of the lower sillimanite zone were 570±40°C, 3.8±1 kbar.
Field relations indicate that the metamorphism, both prograde and retrograde, was closely associated with the emplacement of felsic plutonic bodies. The sequence of events postulated for the Augusta area is as follows: (1) a widespread andalusite–biotite–staurolite-producing event of uncertain cause, was followed by (2) a sillimanite-producing event whose heat source was southeast of the area. This event produced widespread slight retrogressive metamorphism to the north. (3) The final metamoprhic event was also sillimanite-producing, with a heat source supplied by a group of plutons west of the Augusta quadrangle. This event caused localized intense retrogressive metamorphism along a NNE-trending zone in the western part of the Augusta quadrangle.
