U-Pb zircon age of metafelsite from the Pinney Hollow Formation; implications for the development of the Vermont Appalachians

The Pinney Hollow Formation of central Vermont is part of a rift-clastic to drift-stage sequence of cover rocks deposited on the Laurentian margin during the development of the Iapetan passive margin in Late Protero- zoic to Cambrian time. Conventional U-Pb zircon data indicate an age of 571 5 Ma for a metafelsite from the Pinney Hollow Formation. Geochemical data indicate that the protolith for the metafelsite, now a quartz-albite gneiss or granofels, was rhyolite from a source that was transitional between a within-plate granite and ocean-ridge granite setting and probably came through partially distended continental crust. The transitional setting is consistent with previous data from metabasalts in the Pinney Hollow Formation and supports the idea that the source magma came through continental crust on the rifted margin of the Laurentian craton. The 571 5 Ma age provides the first geochronologic age from the rift-clastic cover sequence in New England and establishes a Late Proterozoic age for the Pinney Hollow Formation. The Late Proterozoic age of the Pinney Hollow confirms the presence of a significant mapped thrust fault between the autochthonous and para-autochthonous rocks of the cover sequence. These findings support the interpretation that the Taconic root zone is located in the hinterland of the Vermont Appalachians on the eastern side of the Green Mountain massif. INTRODUCTION The Pinney Hollow Formation of central Vermont is part of a para-autochthonous eugeoclinal rift-clastic to drift-stage sequence deposited on the Laurentian margin during the development of the Iapetan passive margin in Late Proterozoic to Cambrian time. The Pinney Hollow Formation consists of metamorphosed pelitic and psammitic sedimentary rocks and lesser amounts of predominantly mafic volcanic rocks exposed east of Middle Proterozoic basement gneiss and schist in the Green Mountains of Vermont (fig. 1). Prior to plate tectonic models, the cover rocks were considered to be an eastward-dipping and -younging homoclinal sequence of coherent stratigraphy (Doll and others, 1961). In the past 15 years, many faults have been identified in the ''eastern cover sequence'' throughout Vermont. As a result, the Pinney Hollow Formation is now considered part of a fault-bounded lithotectonic belt with its own internal stratigraphy (Walsh and Ratcliffe, 1994a, b; Walsh and Falta, 1996a, b, and in press; and Ratcliffe, Walsh, and Aleinikoff, 1997). Similarities between the metasedimentary rocks of the eastern cover sequence and metasedimentary rocks in the lower parts of the Taconic allochthons have long been recognized. These lithic similarities led early workers to suggest that the source of the Taconic slices was in the eastern cover sequence on the east side of the Green Mountain massif in Vermont and the Berkshire massif in Massachu- setts (Keith, 1932; Prindle and Knopf, 1932; Osberg, 1952). More recently, Stanley and Ratcliffe (1985) postulated that the Taconic root zone was exposed at the western edge of the Pinney Hollow, Underhill, and Hazens Notch Formations at the latitude of the northern end of the Taconic allochthons. Stanley and Ratcliffe (1985) assigned these three formations to lithotectonic belts called the Pinney Hollow and Underhill slices and interpreted the western limit of these slices as the root zone for the Taconic allochthons. The geochemistry of the mafic volcanic rocks in the eastern cover sequence, from alkalic to transitional within-plate basalts in the west to MORB-like basalts in the east, has been linked to the relative original geometry of the ancient margin. In this model, west to east