SILURIAN OROGENESIS IN THE WESTERN PENOBSCOT BAY-REGION, MAINE

New Ar-40/Ar-39 mineral ages from rocks collected west of Penobscot Ba y, Maine, indicate this region was regionally deformed, metamorphosed to amphibolite facies conditions, and intruded by plutons in Silurian times rather than in the Devonian as previously assumed. Disturbed hor nblende age spectra, along with the presence of some Devonian felsic p lutons and extensive retrograde metamorphic textures do suggest, howev er, that these rocks were subsequently affected by low-grade Devonian thermal events. In sharp contrast, rocks west of the Sennebec Pond thr ust fault, a major tectono-stratigraphic boundary in this region, lack a significant Silurian tectono-thermal signature, and instead record the effects of intense Devonian deformation and high-grade regional me tamorphism. The data suggest the two regions experienced very differen t pre-Devonian histories and were most likely juxtaposed by the Senneb ec Pond thrust fault in latest Silurian to Early Devonian time. Rocks now exposed east of the Sennebec Pond fault probably occupied much hig her structural levels during Devonian orogenesis and were not subjecte d to the same intense Devonian deformation and metamorphism as those r ocks now found to the west of this structure. The Silurian tectonism n ow recognized in this region bears striking resemblance to events of s imilar age recorded along the northwest margin of the Avalon composite terrane throughout much of Atlantic Canada. This greatly extends the zone of Silurian orogenesis in the northern Appalachians and requires that previous models of New England middle Paleozoic tectonism be sign ificantly revised.