STRATIGRAPHY OF THE CONNECTICUT VALLEY-GASPE TROUGH IN MASSACHUSETTS AND VERMONT: CONSTRAINTS FROM LA-ICPMS DATES FROM DETRITAL ZIRCON AND CA-IDTIMS DATES FROM VOLCANIC ROCKS

The Connecticut Valley-Gaspe trough (CVGT) extends from CT to QC, and is one of several post-Taconic basins in the northern Appalachians filled by Silurian and Devonian rocks. In VT and MA, the western contact between CVGT and Ordovician rocks of the Shelburne Falls arc is mapped as an unconformity or a fault zone. The Silurian Shaw Mountain (VT) and Russell Mountain (MA) Fms (SM/RM) form discontinuous lenses of quartzite at the contact. The Northfield (VT) and Goshen (MA) (N/G) Fms structurally overlie the quartzite lenses or arc rocks. The Waits River and Gile Mountain Fms are exposed east of the N/G Fms. Poorly preserved fossil evidence suggests an Early Devonian age for the Waits River and Gile Mountain Fms (Hueber et al., 1990), but does not resolve the relative ages of the units. Recent absolute age constraints include dates from metarhyolites in the Meetinghouse Slate Mbr of the Gile Mountain Fm (407 ± 3 Ma; Rankin and Tucker, 2009) and the Goshen Fm. (405 ± 4 Ma; Karabinos and Aleinikoff, 2011).

New U-Pb LA-ICPMS and CA-IDTIMS zircon dates constrain the provenance and age of these units. Detrital zircon age spectra from the RM/SM Fms have significant Silurian, Ordovician, Neoproterozoic and Mesoproterozoic peaks, and show a mixed Laurentian and Gondwanan provenance. Detrital zircon age spectra from the Waits River, Gile Mountain, and N/G Fms have significant Early Devonian, Silurian and Ordovician, and Proterozoic peaks, and show a mixed Laurentian and Gondwanan provenance. Airfall tephras are preserved in the Goshen Fm and one newly identified felsic bed gave a CA-IDTIMS date of 408.6 ± 0.1 Ma. The youngest grain dated from dark gray schist of the Goshen FM in MA, just south of the VT border and on strike with the Northfield Fm is 414.1 ± 0.3 Ma.

We suggest that 1) some rocks mapped as SM/RM Fm are highly sheared Ordovian arc rocks, 2) there is a significant age gap between SM/RM Fm rocks and other CVGT units, which are Early Devonian, 3) the Waits River Fm is older than the N/G and Gile Mountain Fms, which are facies equivalents of each other, and 4) a signicant topographic or drainage barrier separated the CVGT trough from the Silurian-Devonian Catskill basin, which did not receive abundant Ordovician and Silurian zircon grains until the Middle Devonian and did not receive Gondwanan detritus until the Upper Devonian (Selleck et al., 2016).