TEMPORAL CONSTRAINTS ON PALEOZOIC PLUTONISM IN SOUTHWESTERN MAINE AND SOUTHEASTERN NEW HAMPSHIRE: REVISIONS AND IMPLICATIONS

As part of the compilation of the Kittery 1:100,000 sheet (Hussey et al., 2008) high precision U-Pb analysis of zircons was obtained from a suite intrusive rocks that crop out within and across the Central Maine and Merrimack troughs and the Rye Complex in the seacoast region of New Hampshire and southwestern Maine. New and refined 206Pb/238U dates allow evaluation of previous correlations, shed light on the magmatic history of individual plutons and constrain the plutonic and deformational history. Samples were obtained from the Lyman, Lebanon, Webhannet, and Biddeford plutons in southwestern Maine and the Barrington, Exeter, and granitoid gneiss (Breakfast Hill pluton?) and biotite diorite (Appledore metadiorite?) from southeastern New Hampshire.

Peraluminous two-mica granite from the Lyman pluton (ca 290 Ma) provides the first geochemical support for postulated correlation with the Sebago batholith and extends the geographc and temporal range of Carboniferous magmatism. The Lebanon (Milton, ME) granite, also peraluminous, and both granite and granodiorite from the Webhannet pluton yield ages of ca 382 Ma. The latter represents a significant revision from a ca 403 Ma Pb-Pb determination (Gaudette et al., 1982) that suggested a magmatic connection with the Exeter pluton in NH. We report new 206Pb/238U dates from the Exeter. The dioritic and gabbroic phases yeild ages of 407.7 and 407.4 Ma, respectfully, which confirm a short history for this complex body and a Devonian age. Importantly, and perhaps a little surprisingly, are the 382 Ma ages from variably tectonized quartzo-feldspathic and metadioritic rocks in the Rye Complex.

Within the map area it is clear that middle and late Paleozoic plutonic activity was widespread spatially and temporally. At least two plutons (Barrington and Lyman) stitch early fault structures and are cut by later ones, and those within the Rye Complex may reflect emplacement under near synchronous deformational conditions. Finally, the 407.7 date for the Exeter pluton that cuts the deformed Silurian rocks of the Merrimack trough supports an early start of the Acadian.