AN EMSIAN AGE FOR THE GOSHEN FORMATION IN THE CONNECTICUT VALLEY- GASPE TROUGH IN MASSACHUSETTS

The Devonian Goshen Formation is the westernmost unit in the Connecticut Valley-Gaspe Trough (CVGT) in MA. Equivalent rocks in VT are called the Northfield Formation. The Goshen Formation is the only Devonian unit in MA in contact with Ordovician rocks west of the CVGT and around the Shelburne Falls, Goshen, Granville, and Woronoco domes. This contact relationship suggested to Hatch et al. (1988) that the Goshen Formation is predominantly older than the Waits River Formation. However, they noted the lithologic similarity between the Goshen Formation and the Royalton belt of the Devonian Gile Mountain Formation in VT, which is demonstrably younger than the Waits River Formation. The age range of the Waits River Formation is approximately 423 Ma (Aleinikoff and Karabinos, 1990) to at least 415 Ma (McWilliams et al., 2010).

The Goshen Formation commonly contains graded beds that delineate numerous upright folds in the central part of the outcrop belt, where a younger member (Dgq) is defined by a greater percentage of micaceous quartzite beds. We discovered several beds within the Dgq member of the Goshen Formation near Cummington, MA, that we interpret as volcanic in origin. Prismatic, colorless zircons extracted from a 1.5 m thick felsic layer yield a SHRIMP Concordia Age of 405 ± 4 Ma (11 of 15 analyses). This age is indistinguishable from ages of 407 ± 3 Ma from a metarhyolite in the Meetinghouse Slate Member of the Gile Mountain Formation (Rankin and Tucker, 2009) and 409 ± 5 Ma from volcanic zircons extracted from a quartzite bed in the Royalton belt of the Gile Mountain Formation (McWilliams et al., 2010).

Our data show that the Dgq member of the Goshen Formation is the same age as the Royalton belt and the Meetinghouse Slate Member of the Gile Mountain Formation, and that they are all younger than the Waits River Formation. Assuming that the older part of the Goshen Formation in MA and the equivalent Northfield Formation in VT are not significantly older than the volcanic bed we dated, the youngest units in the CVGT occur along both the western and eastern margins, and the older Waits River Formation occupies the central part of the trough. Also, sedimentation in the CVGT appears to have been active from ca. 423 to at least 405 Ma, which constrains tectonic models of terrane accretion in the northern Appalachians.