Abstracts
Abstract
The Orrs Island-Harpswell Neck area on the southern Maine coast experienced one major low-pressure meiamorphic event, which was synchronous wilh D2 deformation. Meiamorphic 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; stauroli te-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 Mj 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 m£ridionale du Maine, la region d'Orrs Island-Harpswell Neck subit un Episode majeur de mdtamorphismc a basse pression synchrone de la deformation D,. Le degr£ de metamorphisme augmente du sud-ouest au nord-est en direction des plutons hors de la region. Les isogrades des peiites deTinissent dans cette region des zones a grenat, staurotide, staurotide-andalousite, staurotide-sillimanite, et a sillimanite. A1 'isograde de la sillimanite, 1 'andalousite commenc, a a etie remplac^e par une muscovite grossiere pour former des pseudomorphes pro grades, alorsque lasillimanite sc forma dans la malrice. A un degre" un peu plus eieve\ des pseudomorphes de muscovite remplacerent la staurotide et d'autre sillimanite apparut.
On situa dans l'espace pression-tempcrature les reactions d'isogrades deduiles des changements dans la topologie AFM a partir de donn^es experimentales et thermodynamiques. Le champ p£trogen&ique resultant ceme la pression durant le metamorphisme entre 2.3 et 3.0 kbars (230 et 300 MPa). Voici 1'e'tendue estimde des temperatures pour chaquc zone m^tamorphique: 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 M} du centre et de 1'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 degr6 et que tous deux sont lies a des plutons. Le metamorphisme dans la region etudieedifferedeM du centre etdel'ouestdu Maine par la production d'andalousite a un degre intermediate.
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