CRYSTALLINE AND DEFORMATION AGES IN THE PROSPECT GNIESS ALONG THE PERI-LAURENTIAN MARGIN

New and existing geochronology shows a long history of deformation in the Prospect Gneiss on the Peri-Laurentian margin between Bridgeport and Seymour, Connecticut. New SHRIMP analyses of magmatic zircons show that the granodioritic Pumpkin Ground (Ohp; 448 ± 3 Ma) and dioritic Beardsley (Ohb; 443 ± 2 Ma) orthogneisses are indistinguishable in age. Field work in newly exposed rocks show that the Pumpkin Ground granodiorite cuts foliations and veins in the Ohb, and contains xenoliths of the Beardsley diorite, suggesting a latest Ordovician age for both units. Some zircon grains analyzed from Ohb are rimmed by younger zircon with a mean age of 431 ± 4 Ma, indicating renewed crystallization in the Silurian. These rims are the same age as titanites (429 ± 3 Ma) recovered from Ohb (Sevigny and Hanson 1993). Our study of these rocks shows that the titanite grains are euhedral and intergrown with fabric-forming biotite and amphibole in S₁. They thus probably grew during the recrystallization of biotite and amphibole and so record the age of S₁. The zircon rims apparently record the same event. Pegmatites cut across this foliation, and are subsequently deformed as S₁ is transposed into S₂. Regional dating of monazite at 406 ± 13 Ma (Sevigny and Hanson 1993) in the related Ansonia leucogranite shows continued syntectonic recrystallization into the Devonian, consistent with garnet dated to be ~380 Ma (Lancaster et. al 2008) in the Wepawaug Group to the east of the Prospect gneiss. The ⁴⁰Ar/³⁹Ar cooling ages of amphibole (~375 Ma) and muscovite (~365 Ma) in these accreted schists (Growdon et. al 2013), as well as in those in the Prospect gneiss (Moecher et. al 1997), record the end of high-grade metamorphism in these rocks, but more Late Devonian syntectonic titanite grains crystallized in Ohp (Sevigny and Hanson 1993). Finally, ⁴⁰Ar/³⁹Ar muscovite crystallization ages (Wathen et al. 2015) document greenschist facies Alleghanian overprinting. Cast in a regional context, these data provide evidence for a high-grade fabric-forming event in the Silurian consistent with the effects of the Salinic orogeny, but strongly overprinted by Acadian metamorphic fabrics and assemblages.