DETRITAL ZIRCON CONSTRAINTS ON THE AGE OF THE GRANVILLE DOME MANTLING SEQUENCE

The Granville dome straddles the CT-MA border, occurs within the Connecticut Valley trough, and is considered part of the Ordovician Shelburne Falls arc. As mapped, it is cored by the Ordovician Collinsville Fm, with an inner mantle of Devonian Goshen Fm and an outer mantle of Ordovician Cobble Mountain Fm. Mapping of Devonian schist between the Ordovician core and outer mantle of the Granville dome led to complex structural models of dome formation. Assignment of the inner mantle rocks to the Devonian is critical to the mechanism of dome formation, yet outcrops of highly deformed, coarsely recrystallized, kyanite-grade schist mantling the dome are difficult to assign to specific units. Here we test the assignment of the inner mantle rocks to the Goshen Fm using detrital zircon from schist on the east flank of the dome. Preliminary U-Pb data from 81 analyses show (1) a dominant Grenville component, including a distinctive peak at 980 Ma that links the sediment to western MA and southern VT, (2) a significant population at 600 to 700 Ma consistent with a Gondwanan source, (3) a sharp Ordovician peak derived from arc-related rocks, and (4) a sharp Early Devonian peak. These results are consistent with assigning the inner mantle rocks to the Goshen Fm, which has been dated in western MA at ca. 405 Ma by U-Pb zircon analysis of an interbedded volcanic tuff. The zircon population is similar to those from the Silurian-Devonian Waits River and Gile Mountain Fms from the Connecticut Valley trough in VT (McWilliams et al., 2010). To the north, the Ordovician cored Goshen and Shelburne Falls domes are mantled only by the Devonian Goshen Fm, lacking an outer mantle of Cambrian to Ordovician Cobble Mountain Fm. Therefore, the formation of the Granville dome appears to require an episode of Devonian back-thrusting of the Cobble Mountain Fm over the structurally underlying Devonian units to the west.