Poikiloblastic index minerals in pelitic rocks from the O m Island-Harpswell Neck area of coastal Maine contain inclusion textures that indicate sequential growth of progressively higher grade metamorphic minerals during development of a near-vertical crenulation foliation. The sequence of zones in the field is garnet, staurolite, staurolite-andalusite, staurolite-sillimanite and sillimanite. Inclusion fabrics characteristic of different stages in crenulation cleavage development indicate that index minerals nucleated and grew sequentially: biotite began to grow before deformation, garnet began to grow during early stages of crenulation cleavage development, staurolite grew during intermediate stages, and andalusite grew relatively late, when transposition of the foliation was nearly complete. Muscovite pseudomorphs and sillimanite were mainly post-kinematic. ?he fact that metamorphic index minerals grew sequentially in individual rocks in the Same order in which they appear across the field area indicates that the high temperature part of the pressure-temperature path was similar to the metamorphic field gradient. Metamorphism in the O m Island-Harpswell Neck area is consistent with the magmatic heating model that has been proposed for western Maine. Sequential development of index minerals in pelitic rocks in the Om Island-Harpswell Neck area apparently resulted from sequential nucleation after substantial overstepping of mineral-forming reactions. Once nucleation of an index mineral had taken place, initial growth was rapid and poikiloblasts preserved inclusion trails characteristic of the prevailing stage of crenulation cleavage development. Because nucleation of sillimanite may have required more overstep ping of the andalusite-sillimanite reaction than nucleation at dehydration reactions, determination of metamorphic conditions for rapidly heated rocks such as these by comparison with a petrogenetic grid is problematic. Garnet zoning patterns in these rocks should reflect the fact that growth of garnet interiors occurred early during metamorphism in equilibrium with a low-grade assemblage. Only garnet rims would be expected to record the subsequent pressure-temperature path.

The Orrs Island-Harpswell Neck area on the southern Maine coast experienced one major low-pressure metamorphic event, which was synchronous with D2 deformation. Metamorphic grade increases from southwest to northeast toward plutons outside the area. Isograds in pelitic rocks divide the area into garnet, staurolite, staurolite-andalusite, staurolite-sillimanite, and sillimanite zones. At the sillimanite isograd, andalusite began to be replaced by coarse-grained muscovite to form prograde pseudomorphs, while sillimanite formed in the matrix.At slightly higher grade, staurolite was replaced by muscovite pseudomorphs. and additional sillimanite formed.Isograd reactions deduced from changes in AFM topology were located in pressure-temperature space on the basis of experimental and thermodynamic data. The resulting petrogenetic grid constrains the pressure during metamorphism to between 2.3 and 3.0 kbars (230 and 300 MPa). Estimated temperature ranges for each of the metamorphic zones are as follows: garnet zone, <500°C; staurolite zone, 500 to 530°C; staurolite-andalusite zone, 530 to 570°C; staurolite-sillimanite zone, 570 to 590°C; and sillimanite zone, >590°C. Metamorphism in the Orrs Island-Harpswell Neck area is similar to M3 metamorphism in west-central Maine, because both events occurred at pressures where staurolite gave way to sillimanite at high grade, and both were related to plutons. Metamorphism in the study area differs from M3 in west-central Maine in that it produced andalusite at intermediate grade.Sur la cote meridionale du Maine, la region d'Orrs Island-Harpswell Neck subit un episode majeur de metamorphisme a basse pression synchrone de la deformation Dr Le degre de metamorphisme augmente du sud-ouest au nord-est en direction des plutons hors de la region. Les isogrades des pelites definissent dans cette region des zones a grenat, staurotide, staurotideandalousite, staurotide-sillimanite, et a sillimanite. A l 'isograde de la sillimanite, l'andalousite commenga a etre remplacee par une muscovite grossiere pour former des pseudomorphes progrades, alors que la sillimanite se forma dans la matrice. A un degre un peu plus eleve, des pseudomorphes de muscovite remplacerent la staurotide et d 'autre sillimanite apparut.On situa dans 1'espace pression-temperature les reactions d'isogrades deduites des changements dans la topologie AFM a partir de donnees experimentales et thermodynamiques. Le champ petrogenetique resultant ceme la pression durant le metamorphisme entre 2.3 et 3.0 kbars (230 et 300 MPa). Void l'etendue estimee des temperatures pour chaque zone metamorphique: zone a grenat <500°C, zone a staurotide 500 a 530°C, zone a staurotide-andalousite 530 a 570°C, zone a staurotide-sillimanite 570 a590°C et zone a sillimanite 590°C. Le metamorphisme dans la region d 'Orrs Island-Harpswell Neck rappelle le metamorphisme M3 du centre et de l'ouest du Maine en ce que l 'un et l 'autre episodes prirent place a des pressions auxquelles la sillimanite se substitua a la staurotide a un haut degre et que tou...