Distinguishing Grenvillian basement from pre-Taconian cover rocks in the northern Appalachians

Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accurate struct ural interpretations. The Cavendish Formation in southeastern Vermont prese nts a classic example of this problem. Doll and others (1961) showed the Ca vendish Formation as younger than the Middle Proterozoic Mount Holly Comple x but older than the lithologically similar Cambrian Tyson and Hoosac Forma tions. More recently, the name Cavendish Formation has been informally aban doned, and its metasedimentary units have been mapped as the Tyson and Hoos ac Formations of Late Proterozoic to Cambrian age. In a radical departure f rom these interpretations, Ratcliffe and others (1997) reassigned metasedim entary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. Th is new age assignment, if correct, requires a completely new structural int erpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzi te layer from Cavendish Gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the Cavendish Formation. Grain shapes of the zircons vary from euhedral to nearly spherical. Virtually al l the grains have pitted surfaces and show at least some rounding of edges and terminations; grains exhibit oscillatory zoning typical of zircons that crystallized from a magma. Single-grain Pb evaporation analyses of ten zir cons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga Sev en of the grains are consistent with derivation from the Bull Hill Gneiss t hat postdates the Grenville orogenic cycle and predates deposition of the C avendish Formation. Thus, the metasedimentary units of the Cavendish Format ion should not be assigned to the Mount Holly Complex.