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

Authors

  • Gilbert R. Dunn Department of Geology and Geography, West Virginia University, Morgantown, West Virginia 26506; Current address: Weston, 955 L'Enfant Plaza, S.W., 8th Floor, Washington, D.C. 20024
  • Helen M. Lang Department of Geology and Geography, West Virginia University, Morgantown, West Virginia 26506; Contact for reprint requests

DOI:

https://doi.org/10.4138/1655

Abstract

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, <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 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. RÉSUMÉ Sur la cote méridionale du Maine, la région d'Orrs Island-Harpswell Neck subit un épisode majeur de métamorphisme à basse pression synchrone de la déformation D2. Le degré de métamorphisme augmente du sud-ouest au nord-est en direction des plutons hors de la région. Les isogrades des pélites définissent dans cette région des zones à grenat, staurotide, staurotide-andalousite, staurotide-sillimanite, et a sillimanite. A l'isograde de la sillimanite, l'andalousite commença, à être remplacée par une muscovite grossiere pour former des pseudomorphes pro grades, alors que lasillimanite se forma dans la malrice. A un degré un peu plus éleveé des pseudomorphes de muscovite remplacerent la staurotide et d'autre sillimanite apparût. On situa dans l'espace pression-température les réactions d'isogrades déduiles des changements dans la topologie AFM à partir de données expérimentales et thermodynamiques. Le champ pétrogénéique résultant ceme la pression durant le métamorphisme entre 2.3 et 3.0 kbars (230 et 300 MPa). Voici l’e'tendue estimée des températures pour chaque zone métamorphique: zone à grenat <500°C, zone a staurotide 500 à 530°C, zone a staurolide-andalousite 530 à 570°C, zone à staurt'tide-sillimanite 570 à 590°C et zone a sillimanite 590°C. Le métamorphisme dans la région d'Orrs Island-Harpswell Neck rappelle le métamorphisme M3 du centre et de l’ouest du Maine en ce que l'un el l'autre épisodes prirent place à des press ions auxquelles la sillimanite se substitua à la staurotide a un haut degré et que tous deux sont lies à des plutons. Le métamorphisme dans la région étudiée différede M3 du centre étde l'ouestdu Maine par la production d'andalousite à un degré intermédiate. [Traduit par le journal]

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Published

1988-12-01

How to Cite

Dunn, G. R., & Lang, H. M. (1988). Low pressure metamorphism in the Orrs Island -Harpswell Neck area, Maine. Atlantic Geoscience, 24(3). https://doi.org/10.4138/1655

Issue

Vol. 24 No. 3 (1988)

Section

Articles

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