THE SYENITE AT SOUTHFORD FALLS: ANOTHER PERMIAN INTRUSION IN WESTERN CONNECTICUT

New detailed mapping in the Southbury quadrangle of southwestern Connecticut shows that the syenite mapped as a stock by Scott (1974) is a dike with trends between ~330° for almost one km through Southford Falls State Park in Oxford. The dike intrudes local schists and metadiorite with a near vertical contact all metamorphosed in the Acadian . The interior of the dike is coarse-grained and contains euhedral amphibole crystals up to 2 mm long, euhedral biotite and titanite grains up to 0.5 mm long, with accessory apatite. These are randomly oriented within interlocking poikilitic K-feldspar grains up to 5 mm long. Chilled margins show a strong alignment of biotite flakes up to 500 µm long in a fine-grained matrix of K-feldspar and amphibole grains < 100 µm in diameter. The syenite contains many 5-15 cm xenoliths of granite and syenite, but xenoliths of the local country rock schists are rare.

New 40Ar/39Ar dating of the amphibole and biotite show that the body is Permian. An amphibole separate yields a plateau age of 284 ± 2 Ma, and a biotite separate yields a flat age spectrum with a weighted average age of 281 ± 2 Ma that agrees with a conventional K-Ar biotite age of 277 ± 4 Ma (Scott et al. 1980). The phenocrysts of amphibole and biotite yield nearly identical 40Ar/39Ar ages, despite closure temperatures that differ by ~200C. Given the fine grain size of the matrix, and the presence of chill margins on this dike body, the most likely explanation for these ages is rapid cooling in shallow country rock. Therefore, we interpret 284 Ma to closely approximate the emplacement age of the syenite. A conventional K-Ar whole rock age of 263 ±5 Ma (Scott et al. 1980) is certainly dominated by the poikilitic K-feldspar, and provides further evidence for shallow emplacement of the dike.

The NW trend of this dike roughly parallels the trend of the ~290 Ma Pinewood adamellite suite ~15 km to the south. Recognition of these NW oriented structures active in the Permian supports the idea that the Acadian metamorphic rocks of western Connecticut acted as strong foreland against the SE verging thrust sheets of eastern Connecticut during the Alleghanian orogeny. The response of this cold and thus strong Acadian terrane was to fracture and extend to the NE, allowing felsic liquids to rise into these NW oriented fractures.