THE EMSIAN PISCATAQUIS MAGMATIC BELT IN THE UPPER CONNECTICUT VALLEY, NEW HAMPSHIRE AND VERMONT

Rhyolite, dated from four localities in the upper Connecticut Valley on both sides of the Monroe fault, yields Early Devonian ages: one in the Meetinghouse Slate Member (Dgm) of the Gile Mountain Formation west of the fault and three in the Littleton Formation (Dl) east of the fault. Dl contains Emsian fossils in the type area and Pragian fossils in the Moosilauke septum to the east. Fossils have not been found in the Gile Mountain, but the Compton Formation, on-strike to the north in Quebec, contains Emsian plant fossils. Previously reported ID-TIMS Pb-U ages from zircon are as follows: 1) porphyritic rhyolite, Dgm, 407.0 ± 3.3 Ma (Stony Creek, Stone Mountain quad, VT); 2) phenocrysts-poor welded tuff , Dl, 407.5 ± 3.9 Ma (I-93 north of Lake Morey, VT); graded bedded tuff, Dl, 407 ± 2 Ma (Gale River, NH). A new 206Pb/238U CA-TIMS age, 407.72 ± 0.13/0.24/0.60, is from a 4.5 x 0.3 km belt of phenocryst-poor rhyolite (Dl) south of Newbury, VT, whose stratigraphic position was previously uncertain.

Correlation of Dgm with the on-strike Ironbound Mountain Formation to the north suggests that Dgm south of Maidstone, VT is the eastern base of the Eastern Vermont sequence, and that the older Frontenac Formation (possible correlative of the Waits River formation) is truncated by Monroe fault.

The rhyolite and accompanying mafic volcanic rock in the Littleton and Gile Mountain Formations are an extension of the Piscataquis magmatic belt (PMB), characterized by volcanic and coeval intrusive rocks within strata (Matagamon Sandstone, and Littleton, Seboomook and Tarratine Formations) for which the paleontologic age matches the isotopic age of the igneous rocks. Emsian strata outside the PMB, for example Dl of central and eastern New Hampshire, do not contain volcanic rock. The PMB extends from northeastern Maine down the Connecticut Valley to Massachusetts. Roughly coeval granitic plutons (some dated by others) intrude their volcanic and sedimentary cover and include the Fairlee pluton (410 ± 5 Ma), the Mt. Clough pluton (410 ± 5 Ma), and the Katahdin pluton of north central Maine (407.8 ± 1.2 Ma), which intrudes the Traveler Rhyolite (upper and lower members are 406.2 ± 0.69 and 407.3 ± 0.41 Ma, respectively). Resolution of apparently older ages for some plutons and interpretation of geochemical data are in progress.