MIDDLE ORDOVICIAN CONODONTS IN NORTHERN VERMONT PROVIDE A STRATIGRAPHIC LINK ACROSS THE GREEN MOUNTAIN ANTICLINORIUM AND CONSTRAIN THE TIMING OF TACONIAN METAMORPHISM

Middle Ordovician conodonts have been recovered from dolostone marbles in graphitic schists formerly included in the Underhill Formation on the west side of the Green Mountains at two localities, Buels Gore, and 25 km to the north at West Bolton. The dolostones occur below and west of Taconian thrust faults that carry Late Proterozoic to Cambrian (?) rift clastic rocks over the parautochthonous cover sequence. The structural and stratigraphic settings are similar to those of previously reported Middle Ordovician conodonts from the West Bridgewater Formation, east of the Green Mountain anticlinorium in south-central Vermont. Although the conodont elements are encrusted with apatite overgrowths, they are only slightly deformed. Distinctive morphologies allow assignment of several P elements from the West Bolton site to Cahabagnathus chazyensis or C. sweeti, and a few elements from Buels Gore to Leptochirognathus quadratus. Both sites have numerous elements most likely assignable to species of Phragmodus, Plectodina or Aphelognathus. The faunas indicate shallow, warm-water biofacies, however the present collections likely were transported from a shelf environment and deposited penecontemporaneously into a more basinal environment. The faunas differ from the West Bridgewater site in that the latter lacks shallow-water species, and indicates a less well constrained, but compatible, age range. The newly found assemblages indicate a stratigraphic range of no older than uppermost pre-Chazyan through Chazyan (upper Whiterockian Series), or an approximate absolute age range of about 463 to 460 Ma. This age range is at odds with reported metamorphic amphibole Ar/Ar ages of 470 to 463 Ma. The 470 Ma age was by total fusion from an amphibolite in the Underhill Formation structurally below and only 0.6 km west of the Buels Gore site. The 463 Ma age was by stepwise heating from greenstone in the Pinnacle Formation 25 km northwest of the West Bolton site. However, the spectrum did not maintain a plateau and the age could be as young as 452 Ma. Conodont CAI values of 7 1/2 to 8 suggest minimum post-depositional host-rock heating to more than about 500°C, presumably during Taconian metamorphism.