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

Abstract

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...

Sequential porphyroblast growth during deformation in a low‐pressure metamorphic terrain, Orrs Island–Harpswell Neck, Maine

“…Rocks in the Orrs Island-Harpswell Neck area have experienced only one major metamorphic episode (Dunn, 1988;Dunn & Lang, 1988). Metamorphic assemblages are consistent with a gradual increase in metamorphic grade to the northeast, and all pseudomorphs present in the rocks can be attributed to prograde reaction of andalusite or staurolite to sillimanite.…”

“…The Om Island-Harpswell Neck study area (Fig. 1) is on the southern coast of Maine east of the Norumbega fault zone, but it has a metamorphic history that is similar to the history of areas in western Maine (Dunn, 1988;Dunn & Lang, 1988).…”

“…Comparison of this sequence of zones and isograds with a petrogenetic grid for pelitic rocks (Fig. 3) provides an estimate of the range of peak metamorphic conditions reached by rocks from each zone (Dunn, 1988;Dunn & Lang, 1988). Metamorphic conditions are constrained by the assemblages to lie between intersection I and i n t e d o n I1 on Fig.…”

“…Estimated pressure during metamorphism was 0.23-0.30GPa and estimated temperatures for each zone were as follows: < 5 W C in the garnet zone (a), 500-5WC in the staurolite zone (b), 530-570" C in the staurolite-andalusite zone (c). 570-590°C in the staurolite-sillimanite zone (d), and >590"C in the sillimanite zone (e) (Dunn, 1988;Dunn & Lang, 1988). Geothermobarometry has not been applied t o these rocks.…”

“…its position in the figure was calculated using the program GEOCALC of Berman, Brown & Perkins (1987). For furlher discussion of the positions of thew reactions, see Dunn & Lang (1988). Both the m.f.g.…”