OPEN SYSTEM DEVELOPMENT OF FABRICS IN THE PARTRIDGE FORMATION, ALSTEAD, NH

Textural relationships in schists collected from the Ordovician Partridge Formation near Alstead, NH, preserve evidence for open system behavior during deformation at upper and lower amphibolite facies conditions. Upper amphibolite facies deformation, D2, is preserved in domains containing fibrolitic sillimanite. Fibrolite commonly occurs in nearly monomineralic discontinuous layers parallel to S2, which is comprised of biotite + plagioclase ± k-feldspar ± garnet ± quartz. Sillimanite needles in these folia exhibit a strong preferred orientation and can be found embaying biotite and plagioclase, as inclusions in garnet, and forming with or embaying garnet. Additionally, the destruction by dissolution of plagioclase and biotite in these layers requires the local removal of alkalis, Mg⁺⁺, and Fe⁺⁺, which suggests ionic reactions such as 2Albite + 2H⁺ à Sillimanite + 5 SiO₂ (aq) + 2Na⁺ and 3Biotite + 14H⁺ à Sillimanite + 5SiO₂ (aq) + 2K⁺ + 6(Fe⁺⁺, Mg⁺⁺) + 9H₂O. The paucity of quartz in the sillimanite-rich zones suggests that the SiO₂ product was removed on at least the scale of the thin section, i.e. open system behavior during D2. Fibrolite is also found as randomly oriented splays along grain boundaries of plagioclase, showing that these reactions were driven by H⁺ metasomatism, at least locally.

Overprinting the high-grade assemblages is a muscovite > biotite foliation, S3, of various character in the studied samples. Locally, late micas form spectacular phyllonitic D3 shear zones up to 2 cm thick. These folia contain muscovite (>85%), biotite (10%), euhedral garnet (5%), rutile, and a conspicuous decrease of quartz and plagioclase. Biotite in the phyllonite is commonly embayed and truncated by muscovite, suggesting the replacement of biotite by garnet and rutile. Similar to the sillimanite folia, the concentration and alignment of muscovite requires both open-system syntectonic crystallization, and the dissolution and removal of Na⁺ and Ca⁺⁺. Thus, all of these fabrics require H⁺ metsomatism and the dissolution and removal of quartz and plagioclase, and indicate that parts of the Bronson Hill anticlinorium remained chemically open systems during two separate deformation events.