FIELD RELATIONS AND GEOCHEMISTRY OF GREENSTONES IN THE MORETOWN AND CRAM HILL FORMATIONS, CENTRAL VERMONT

Greenstones and other metavolcanic rocks are abundant within parts of the metasedimentary Moretown and Cram Hill Formations, Ordovician-age rocks which are part of a north-south trending Taconian thrust slice running the length of Vermont and into adjacent parts of New England. This research focuses on greenstone samples taken throughout the Montpelier Quadrangle in Central Vermont, and in particular on two strip maps through Cram Hill and Moretown rocks just east of the Richardson Memorial Contact (RMC), a boundary separating pre-Silurian from Silurian rocks.

Metasedimentary rocks in the area of the RMC dip steeply to the northwest. Across strike along a 200 – 350 m traverse, lithology varies considerably with alternating beds of rusty black phyllite, gray granofels, and interbanded green phyllite and coticule. Distinctly quartzose and granular units generally concentrate towards the RMC, grading into coticule banded green phyllite followed by rusty gray granofels and pinstriped granofels associated with the main body of the Moretown Formation. Greenstones are found throughout, but are concentrated within the coticule-rich unit and to the west within the gray granofels dominated units.

Greenstones are found as beds or sills along foliation, ranging from 10cm to 7m thick and averaging about 1m in thickness. They are often discontinuous, and may pinch and swell. None in the immediate mapping area appear to cut the foliation of the host rocks, though elsewhere in the quad they sometimes do. Tectonic fabric varies widely, from strongly foliated and lineated to unfoliated and massive. Mineralogy consists of epidote, chlorite, plagioclase, and quartz, and is variable to some extent between individual greenstones. Some samples display plagioclase phenocrysts.

Preliminary geochemistry indicates the greenstones originated as mostly tholeiitic basalts. The data so far show two geochemical groups distinguished by variation in Ti, Zr, and Y contents. The lower Ti and Zr/Y group has affinities with oceanic spreading zones, whereas samples with higher concentrations are more like within-plate basalts. More detailed geochemistry will help refine our ideas on the tectonic environment of origin of the metabasalts.