CLOSELY SPACED LOW-PRESSURE ISOGRADS ALONG THE EDGE OF THE ACADIAN METAMORPHIC HIGH IN NORTHERN NEW ENGLAND

Silurian-Devonian (Acadian) amphibolite facies regional metamorphism in northern New England terminates in four northeast-trending lobes. The easternmost of these high-grade metamorphic lobes, located in mid-coastal Maine, is perhaps the most complex, with evidence of multiple metamorphic events, syn-tectonic metamorphic mineral growth, and the complicating effects of post-metamorphic shearing and faulting. This study focuses on a narrow northeast-trending belt of relatively low-grade pelitic rocks within the eastern high-grade metamorphic lobe. To the southeast, the low-grade rocks are in fault contact with retrograded andalusite-staurolite grade schists. To the northwest, however, metamorphic grade increases dramatically from chlorite zone to sillimanite zone over a distance of approximately one kilometer.

Five metamorphic zones separated by four northeast-trending isograds can be mapped within the narrow transition zone. Muscovite-chlorite phyllites (chlorite zone) represent the lowest grade rocks and the first isograd marks the initial appearance of garnet +/- chloritoid (garnet zone). Moving further to the northwest, biotite joins this assemblage (biotite zone) followed closely by the near simultaneous appearance of staurolite + andalusite or cordierite + andalusite (andalusite zone), depending on bulk rock composition. The final isograd marks the appearance of fibrolitic sillimanite (sillimanite zone).

Garnet porphyroblasts from all zones are typically euhedral and 2 mm or less in diameter. Garnet chemical zoning profiles are smooth and continuous with little to no evidence for garnet resorption. Biotite, staurolite and andalusite porphyroblasts from the andalusite and sillimanite zones are significantly coarser grained (> 1 cm) and show textural evidence for both growth during foliation development as well as post-growth deformation. Comparative mineral chemistry from phases across the transition zone suggests near equilibrium conditions during a single episode of low-pressure metamorphism. Although no large plutonic rock body is exposed in the area, the extremely rapid increase in metamorphic grade suggests near vertical isograds in a contact metamorphic environment. Post-metamorphic shortening associated with dextral shearing may have served to further compress the sequence of isograds.