Age relationships in eastern Vermont
(excerpted from Walsh et al, 2009, NEIGC Fieldtrip Guidebook)
New mapping in the Montpelier and Barre West quadrangles shows that the rocks consist of Silurian to Devonian metasedimentary rocks of the Connecticut Valley – Gaspé synclinorium (CVGS) and metamorphic rocks of the Cambrian to Ordovician Moretown and Cram Hill Formations.
The pre-Silurian rocks to the west are separated from the (younger) rocks of the CVGS to the east by the informally named “Richardson Memorial contact” (RMC), historically interpreted as either an unconformity (Cady, 1956; Doll and others, 1961) or fault (Westerman, 1987; Hatch, 1988). Rocks east of the RMC occur in the CVGS and include the metasedimentary Shaw Mountain, Northfield, Waits River, and Gile Mountain Formations.
The metamorphic rocks are cut by Devonian granite dikes and quartz veins and Cretaceous diabase or lamprophyre dikes.
The pre-Silurian Moretown and Cram Hill Formations occur west of the Richardson Memorial Contact. In southeastern Vermont, metamorphic rocks of the Moretown Formation are cut by calc-alkaline arc-related tonalite and trondhjemite gneisses that yield Cambrian to Ordovician U-Pb zircon ages of ca. 496 – 462 Ma (Ratcliffe and others, 1997). The Late Cambrian to Early Ordovician ages from the intrusive rocks provide an upper age limit for deposition of the metasedimentary and metavolcanic rocks of the Moretown Formation, and we assign a Cambrian to Ordovician age to these rocks, in accordance with the time scale of Gradstein and others (2004).
In Springfield, Vermont, the Cram Hill Formation contains felsic volcanic rocks that yield an Early Ordovician U-Pb zircon age of 484 ± 4 Ma (Ratcliffe and others, 1997), thus we assign an Ordovician age to these rocks. Mafic rocks in the Wrightsville area include greenstone and metadiabase. The mafic rocks occur as layers, interpreted as metabasalts or metamorphosed volcaniclastic rocks, and as dikes. The dikes may be correlative in age with similar mafic dikes found in the Comerford Intrusive Suite (CIS) on the east side of the CVGS where gabbro-diorite rocks are dated at 419 ± 1 Ma (Rankin and others, 2007). Biotite-hornblende monzodiorite of the Braintree pluton in central Vermont has the same age (419.26 ± 0.39 Ma, Black and others, 2004) and appears to have the same relative age of the dikes in the Montpelier area, but differs petrographically and chemically from the greenstones and mafic dikes in the Moretown Formation (Ratcliffe and Aleinikoff, 2000; Ratcliffe, 2006). The mafic dikes and the rocks of the CIS and the Braintree pluton do not cross the RMC into the rocks of the CVGS suggesting a Pridolian age limit for the base of the CVGS in some places.
The age of CVGS metasedimentary rocks is based on limited fossil and isotopic data and regional correlations. The depositional age of sediments in the CVGS is considered to be Silurian to Devonian (Doll and others, 1961; Boucot and Drapeau, 1968; Hatch, 1988; Lyons and others, 1997). Correlative rocks in southern Québec, where the rocks are lower grade and less deformed, have yielded better fossil control. Lavoie and Asselin (2004) report Late Silurian (Pridolian) to early Devonian (Lochkovian) fossil ages. Early Devonian (Emsian) plant fossils from the Compton Formation in Québec, the northern correlative of the Gile Mountain Formation, provide age control for the upper part of the CVGS (Hueber and others, 1990). Aleinikoff and Karabinos (1990) and Hueber and others (1990) reported a U-Pb zircon age of 423 ± 4 Ma from a felsic rock from the Waits River Formation in Springfield, Vermont. The sample is from a 50-cm-thick, light-gray, fine-grained epidote-chlorite-albite-quartz granofels layer within a coarser grained sequence of phenocrystic feldspathic schist and granofels. Aleinikoff and Karabinos (1990) interpreted the layer as a dike but left open the possibility that it was a volcanic layer. Walsh and others (1996a) and Armstrong and others (1997) interpreted the layer as a bed because of the lack of unequivocal cross-cutting relationships and the presence of many similar, yet thinner, layers within a felsic volcaniclastic map unit. The Late Silurian date provides either the age of deposition of the felsic unit at that locality, and therefore the age of the Waits River there, or an upper age limit on deposition.