GEOCHEMISTRY OF METAVOLCANIC ROCKS IN THE SOUTHERN WORCESTER MOUNTAIN AREA, VERMONT

Previous geochemical research on Late Proterozoic-Early Paleozoic rocks in Vermont and adjacent regions indicates that many metavolcanic rocks formed during rifting of the Laurentian continent and opening of the Iapetus Ocean sometime in the Late Proterozoic-Early Cambrian. Still other units may have originated as back or fore-arc volcanics near an Ordovician magmatic arc. All metavolcanic units are interpreted to have been deformed, metamorphosed and emplaced into their current position during the Taconian orogeny in the mid to late Ordovician.

Recent 1:24,000 scale bedrock mapping by the Vermont Geological Survey shows many elongate greenstone and amphibolite bodies with tectonic and stratigraphic contacts in the Southern Worcester Mountain Area. These bodies are likely of Cambro-Ordovician age. Although they have not been geochemically analyzed, we suspect that they will correlate with other rocks from the northern Worcester Mountains, northern Vermont, and southern Quebec. Here, we report and interpret the first geochemical data on the mafic bodies from this area.

Thirty-seven samples were collected from the greenstone and amphibolite bodies of the Southern Worcester Mountain Area. Twelve thin sections were used to determine representative mineralogy and basic petrographic characteristics of each of the metavolcanic bodies. Thirty samples were analyzed for major and trace element content using ICP-MS. Preliminary analysis of the geochemical data shows that the metavolcanics were tholeiitic basalt with TiO2 content ranging from ~1.0 to 1.5 wt%, Sc from 40 to 50 ppm, Ni from 100 to 200 ppm, Cr from 200 to 500 ppm, and rare earth elements at about 10x chondrite with flat to slightly depleted LREE patterns. Furthermore, trace element variation diagrams suggest the metavolcanics may have formed as mid-ocean ridge basalt in the Iapetus ocean.

Analysis of the full suite of samples will place the newly mapped metavolcanics of the Southern Worcester Mountain Area within a larger framework of the whole plate tectonic history of Vermont. Specifically, we will address the question: were all the volcanics part of the rift and ocean spreading sequence or were some formed near a volcanic arc?