Tectonic and regional metamorphic implications of the discovery of Middle Ordovician conodonts in cover rocks east of the Green Mountain massif, Vermont

Middle Ordovician (late Arenigian - early Caradocian) conodonts were recove red from a dolostone lens in carbonaceous schist 30 m below the base of the Pinney Hollow Formation in the Eastern Cover sequence near West Bridgewate r, Vermont. These are the first reported fossils from the metamorphic cover sequence rocks east of the Green Mountain, Berkshire, and Housatonic massi fs of western New England. The conodonts are recrystallized, coated with gr aphitic matter, thermally altered to a color alteration index (CAI) of at l east 5, and tectonically deformed. The faunule is nearly monospecific, cons isting of abundant Periodon aculeatus Hadding? and rare Protopanderodus. Th e preponderance of Periodon and the absence of warm, shallow-water species characteristic of the North American Midcontinent Conodont Province suggest a slope or basin depositional setting. The conodont-bearing carbonaceous s chist is traceable 3 km southeast to the Plymouth area, where it had been d esignated the uppermost member of the Plymouth Formation, previously regard ed as Early Cambrian in age. The age and structural position of the carbona ceous schist above dolostones of the Plymouth Formation but below the Pinne y Hollow Formation (upper Proterozoic and Lower Cambrian?) suggest that thi s unit may be correlative or time transgressive with the Ira Formation, whi ch underlies the Taconic allochthons in the Vermont Valley. Such a correlat ion supports the concept of placing the western limit of the root zone of t he Taconic allochthons beneath the Pinney Hollow Formation. An approximate absolute age assignment for the conodont-bearing rock is between 470 and 45 4 Ma. This suggests that dynamothermal metamorphism during the Taconian oro geny on the east flank of the Green Mountains was younger than early Carado cian, which is in accord with the middle Caradocian age of the Ira Formatio n west of the Green Mountain massif.