Low pressure metamorphism in the Orrs Island - Harpswell Neck area, Maine

The Orrs Island-Harpswell Neck area on the southern Maine coast experienced one major low-pressure metamorphic event, which was synchronous wilh 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, 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 diat it produced andalusite at intermediate grade. RESUME 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 D2. 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, staurotide-andalousite, staurotide-sillimanite, et a sillimanite. A l'isograde de la sillimanite, l'andalousite commenca, a etre remplacee par une muscovite grossiere pour former des pseudomorphes pro grades, alors que lasillimanite se forma dans la malrice. A un degre un peu plus elevee des pseudomorphes de muscovite remplacerent la staurotide et d'autre sillimanite apparut. On situa dans l'espace pression-temperature les reactions d'isogrades deduiles des changements dans la topologie AFM a partir de donnees experimentales et thermodynamiques. Le champ petrogeneique resultant ceme la pression durant le metamorphisme entre 2.3 et 3.0 kbars (230 et 300 MPa). Voici l’e'tendue estimee des temperatures pour chaque zone metamorphique: zone a grenat <500°C, zone a staurotide 500 a 530°C, zone a staurolide-andalousite 530 a 570°C, zone a staurt'tide-sillimanite 570 a 590°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 el l'autre episodes prirent place a des press ions auxquelles la sillimanite se substitua a la staurotide a un haut degre et que tous deux sont lies a des plutons. Le metamorphisme dans la region etudiee differede M3 du centre etde l'ouestdu Maine par la production d'andalousite a un degre intermediate. [Traduit par le journal]