THE CONNECTICUT VALLEY - GASPE TROUGH IN VERMONT

The Connecticut Valley – Gaspé Trough (CVT) in eastern Vermont consists of Silurian to Devonian metasedimentary and metavolcanic rocks that were deposited in an extensional basin. New mapping shows that the western limit of the CVT is an unconformity that is locally offset along Devonian faults related to Acadian deformation. The eastern limit of the CVT is locally an early Acadian fault, an Alleghanian fault, a Mesozoic fault, or an unconformity against the rocks of the Bronson Hill anticlinorium (BHA). Where the eastern limit extends into adjacent New Hampshire, limited modern mapping makes interpretations of the CVT there problematic.The age of the CVT is based on limited paleontologic and new isotopic data. Llandovery to Pridoli age fossils in the Shaw Mountain Fm., at the western base of the CVT, loosely constrain a lower age limit for the sequence. The Waits River Fm. (WR) contains a metafelsite dated by TIMS at 423 ± 4 Ma that has been interpreted as either a metavolcanic or a dike; the analysis provides either the depositional age, or a minimum age. Silurian mafic dikes and plutons, precisely dated at 419 Ma, are restricted to older rocks on either side of the CVT, suggesting a maximum Pridolian age limit for the base of the CVT locally which would make the base of the sequence time-transgressive. Early Devonian (Emsian) plant fossils from the Compton Fm. in Québec, the northern correlative of the Gile Mountain Fm. (GM), provide a minimum age for the upper part of the CVT. U-Pb SHRIMP ages of detrital zircons obtained from quartzites in the WR and GM are mostly ca. 1900-990 Ma. The youngest detrital zircons from the quartzites are 418 ± 7 and 415 ± 2 Ma (Lockhovian) in the WR and 411 ± 8 and 403 ± 7 Ma (Praghian) in the GM. The sample from the westernmost belt of GM contains a subset of numerous elongate euhedral zircons interpreted as volcanic in origin that yielded a weighted average of 409 ± 5 Ma. A similar TIMS U-Pb zircon age of 407 ± 3 Ma from a metarhyolite in the Meetinghouse Slate Member (MSM) of the GM supports the interpretation that the GM is at the top of the CVT sequence. Metarhyolite dated at ca. 407 Ma in the Littleton Fm. in the adjacent BHA suggests that the western belt of GM, the MSM, and the Littleton Fm. of New Hampshire are lateral equivalents.