A SILURIAN DEPOSITIONAL AGE AND MIXED EASTERN AND WESTERN PROVENANCE FOR THE STRAITS SCHIST CONFIRMED BY DETRITAL ZIRCON GEOCHRONOLOGY

We have tested the interpretation that the Straits Schist (SS) in the eastern parts of Connecticut’s western highlands is a metamorphosed Silurian-Devonian sediment that unconformably overlies earlier Paleozoic (C-O?) sediments and igneous rocks. To this end we have analyzed the textures and U-Pb isotope systematics of detrital zircons in three of the earlier C-O? metasediments between Woodbury and Riverside, and in three samples of SS from Long Hill, Riverside, and Bristol. All rocks mapped as C-O metasediments contain grains with strong oscillatory zoning with Laurentian ages between 1000 and 1400 Ma, but the southeastern samples contain progressively more grains with ages of ~1700 and ~2600 Ma. A sample of the SS from Riverside contains a similar suite of Laurentian zircons, but also contains several grains ~600 Ma probably derived from Gondwanan sources, and 8 Ordovician grains the youngest of which is 449 ± 5 Ma. A sample near Bristol contains grains with similar Laurentian ages, but also four grains ~625 Ma, four Late Ordovician grains, and one grain 414 ± 4 Ma. In contrast, a sample from the lower 40 cm of the exposed basal contact of the SS near Long Hill contains an unusual suite of grains (8 of 82) with ages between 800 and 900 Ma, and no grains with concordant ages younger than these. Finally a sample of quartzite newly exposed in Westport contains three ~580 Ma (Gondwanan) grains and three ~830 Ma grains in addition to the Laurentian suite. These results support the interpretation that (1) the Straits Schist derives from eastern sources as well as from Laurentia, (2) that the lower portions of the unit contain Gondwanan grains (~600 Ma) without Paleozoic grains, and (3) that the age of deposition must be younger than Late Ordovician (<450 Ma), and (4) that the higher parts of the unit may be as young as latest Silurian (<~414 Ma). Clearly, the Straits Schist represents sediments deposited in deepening Silurian basin.