REVISED MIDDLE ORDOVICIAN U-PB AGE FOR THE VOLCANIC CLEAR STREAM MEMBER, NE NEW HAMPSHIRE: REGIONAL IMPACT

In 1985 we reported a lower intercept U-Pb zircon age of 434±8 Ma for felsic metatuff at the top of the basalt-dominated Clear Stream Member of the Dixville Formation (Green, 1964), exposed ~10 km NW of Errol N.H. On the basis of this apparent Silurian age, in 1990 RHM mapped an underlying siliciclastic sequence that partly resembles units of the Silurian Rangeley sequence of western Maine, and on published USGS maps the Clear Stream and Rangeley sequence are shown within the Piermont-Frontenac allochthon. We recently redated the metatuff by SHRIMP, which gave an igneous age of 465±6 Ma, and a single detrital zircon age of ~1.3 Ga. The igneous age confirms Green's correlation of the Clear Stream to the Middle Ordovician Ammonoosuc Volcanics, and relegates the entire sub-Clear Stream sequence to facies of the mainly Lower Ordovician Dead River Formation, thereby removing the allochthon from the Errol region.

These and other recent U-Pb age data and intensive remapping yield a much-narrowed allochthon along the Connecticut Valley, with a straighter eastern boundary that aligns the Foster Hill fault, south of 44.75 degrees N lat., and the Thrasher Peaks fault to the north, forming the Foster Hill-Thrasher Peaks line. The southern portion of the allochthon contains units of the Rangeley sequence, and extensive overlying calc-psammite and gray pelite of the Silurian and Lower Devonian Frontenac and Ironbound Mountain Formations, both strongly resembling parts of the Gile Mountain Formation of Vermont. The Foster Hill-Thrasher Peaks line extends >225 km from the truncated NW side of the Chain Lakes massif, Maine, at least to Fairlee, Vt. It is interpreted as a west-dipping, basin-margin normal fault of Silurian to earliest-Devonian age that was reactivated by Acadian thrust and strike-slip faulting in the north and was deformed by folding in the south, forming the antiformal Coppermine Road window.