TECTONIC IMPLICATIONS OF DEFORMED METAGABBROIC TO GABBROIC ANORTHOSITES, SHELVING ROCK QUADRANGLE, EASTERN ADIRONDACK MOUNTAINS, NY

Deformed gabbroic and metagabbroic anorthosites are found in the Buck Mountain area of the Shelving Rock quadrangle in the eastern Adirondack Mountains, NY. These mafic rocks are part of the AMCG intrusive suite present throughout much of the Adirondack Highlands. This suite of igneous rocks is postulated to have formed as a result of lithospheric delamination following the Shawinigan Orogeny and was emplaced approximately 1.155 Ga.

Textural relationships in the mafic rocks suggest that these rocks were sheared and dynamically recrystallized. The protolith for these rocks was a coarsely crystalline, orthopyroxene-bearing gabbro. Crystals of plagioclase and orthopyroxene are now polycrystalline mosaics of smaller crystals, aligned in a planar foliation. The orthopyroxene mosaics are enveloped by hornblende, and then by garnet, which is in contact with matrix plagioclase feldspar. This corona texture is consistent with a reaction such as: Opx + Plag + H₂O= Grt + Hbl. This reaction, along with calculated P-T conditions using analyzed mineral compositions, is consistent with metamorphism under upper amphibolite to lower granulite facies conditions.

We suggest that the deformation and metamorphism recorded by these rocks is related to either latest Shawinigan tectonism or the Ottawan Orogeny. In-situ microprobe monazite ages from nearby paragneisses have both a Shawinigan and Ottawan signature. Our preferred model is that the strain is associated with Ottawan deformation. In either case the deformation was accompanied by an influx of a small amount of H₂O-bearing fluid that facilitated the formation of the corona textures.