DISTINGUISHING GRENVILLIAN BASEMENT FROM PRE-TACONIAN COVER ROCKS IN THE NORTHERN APPALACHIANS

ABSTRACT. Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accu-rate structural interpretations. The Cavendish Formation in southeastern Ver-mont presents a classic example of this problem. Doll and others (1961) showed the Cavendish Formation as younger than the Middle Proterozoic Mount Holly Complex but older than the lithologically similar Cambrian Tyson and Hoosac Formations. More recently, the name Cavendish Formation has been informally abandoned, and its metasedimentary units have been mapped as the Tyson and Hoosac Formations of Late Proterozoic to Cambrian age. In a radical departure from these interpretations, Ratcliffe and others (1997) reassigned metasedimen-tary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. This new age assignment, if correct, requires a completely new structural interpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzite 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 all 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 zircons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga. Seven of the grains are consistent with derivation from the Bull Hill Gneiss that postdates the Grenville orogenic cycle and predates deposition of the Cavendish Formation. Thus, the metasedimentary units of the Cavendish Formation should not be assigned to the Mount Holly Complex