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A newly recognized suite of trondhjemite-tonalite and dacitic gneiss forms a 10 krn wide belt of rocks within the Mount Holly Complex in the central part of the Green Mountain massif of Vermont. Field relationships and chemistry indicate that these gneisses are calc-alkaline, volcanic, and hypabyssal plutonic rocks older than the Middle Proterozoic regional deformation that affected the Mount Holly Complex. U-Pb zircon dates indicate ages as great as 1.35 Ga for crystallization of the volcanic protoliths and for intrusion of crosscutting trondhjemite. Tonalitic plutonism continued until 1.31 Ga.Map-scale contacts between the trondhjemitic-tonalitic4acitic gneisses and the paragneiss sequence of the Mount Holly Complex are sharp, suggesting that the volcanic rocks of the trondhjemite-tonalite suite underlie the paragneiss units and do not intrude them. These relationships suggest that the trondhjemite-tonalite suite is either considerably older than, and unconformable beneath, the paragneiss cover rocks or represents a volcanic edifice slightly older than the deposition of the sedimentary precursor to the paragneiss units. The paragneiss and tonalite-trondhjemite gneisses are both intruded by younger granitoids that were intruded at about 1.25 Ga during strong dynamothermal metamorphism.The trondhjemitic gneisses of the Mount Holly Complex of Vermont have high A1,0, and low Yb contents and light rareearth element enrichment patterns that are more characteristic of continental than oceanic volcanic arcs. The Mount Holly intrusives and volcanics may have formed during 1.35-1.31 Ga ensialic volcanic-arc activity, contemporaneous with ensimatic arc activity during the early part of the Elzevirian phase of the Grenville orogeny. In Vermont, later deformation and granite intrusion at about 1.25 Ga coincide with the major pulse of the Elzevirian orogeny and associated trondhjemitic plutonism in the Central Metasedimentary Belt of eastern Canada.Une skquence nouvellement indentifiCe de gneiss trondhjkmitique-tonalitique et dacitique forme une bande lithologique large de 10 km dans le complexe du Mount Holly, occupant la partie centrale du massif de Green Mountain, Vermont. Les recoupements sur le terrain et la gkochimie rkvelent que ce gneiss sont des roches calco-alcalines, volcaniques, plutoniques de semi-profondeur, plus vieilles que la dkformation rkgional qui a eu lieu au Protkrozolque moyen et qui a dkrangk le complexe du Mount Holly. L'Bge maximum de la cristallisation des protolithes volcaniques et de l'intrusion de la trondhjCmite recoupant le socle a Ctk ktabli par le chronomktre U-Pb sur zircon B 1,35 Ga. Le plutonisme tonalitique a eu lieu jusqu'h 1,31 Ga.Les contacts entre gneiss trondhjemitique-tonalitique-dacicitique et la sequence de paragneiss du complexe du Mount Holly, vus h 1'Cchelle de la cartographie gkologique, sont nets et ils indiquent que les roches volcaniques de la sequence de trondhjemite-tonalite sont sous-jacentes aux unitCs de paragneiss et ne les recoupent pas. Ces relations suggerent que ...

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