U-Pb Monazite Geochronology of Granitic Rocks from Maine: Implications for Late Paleozoic Tectonics in the Northern Appalachians

Abstract

Crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern Maine constrain the timing of final ductile motion of a major shear zone to approximately 280 Ma. U-Pb monazite ages of granite at Brunswick ($$278 \pm 2$$ Ma) and evolved granitic pegmatites in Topsham (275-269 Ma) provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation. These granitic rocks were evidently generated during the final high temperature event experienced by this portion of the Appalachian orogen. During this time, the host rocks record initial rapid cooling (c. 16°C/m.y.) followed by slower cooling (c. 4°C/m.y.) into the Mesozoic. The similarity of ages of granite in Brunswick and pegmatites in Topsham permit the rocks to be related to the same magmatic system. The spatially associated Sebago batholith, the largest exposed pluton in New England, has a concordant U-Pb monazite age of $$293 \pm 2$$ Ma. Thus the Sebago batholith cannot be the source of regional high-temperature metamorphism during the Mississippian, as previous models have suggested. These disparate ages also do not permit the Sebago batholith to be genetically related to granite magmatism in the Brunswick area, although they most likely were generated during different periods of the same tectonic event.