Timing of the Acadian orogeny in northern New Hampshire

New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one t o six grains each of zircon or monazite have been analyzed from six samples : (1) an early syntectonic diorite that records the onset of the Acadian; ( 2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Ac adian. Zircon from the syntectonic Wamsutta Diorite gives a Pb-207/Pb-206 a ge of circa 408 Ma, the time at which the boundary between the deforming or ogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migr ating Acadian orogenic front and records the beginning of the Acadian in th is part of the Central Maine Terrane. We propose a possible Acadian tectoni c model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives Pb-207/U-206 ages, suggesting the peak of Acadian metam orphism and intrusion of two-mica granites occurred at circa 402-405 Ma, th e main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terra ne of New Hampshire and western Maine range from circa 406-384 Ma, with you nger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range h ad ended by circa 355 Ma, the Pb-207/U-235 age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synk inematic metamorphism, seen in central and southern New Hampshire, Massachu setts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, t he orogenesis was typical of the style in parts of Maine dominated by postk inematic metamorphism.