Carboniferous U-Pb age of the Sebago batholith, southwestern Maine: Metamorphic and tectonic implications

Abstract

Two phases (pink and white granite) of the Sebago batholith of southwestern Maine have been dated by the U-Pb zircon method. Identical upper concordia intercepts of both rocks indicate an intrusive age of 325 ± 3 m.y. for the batholith. The lower intercept of the pink-phase sample, 114 ± 13 m.y., is inferred to represent episodic lead loss due to the intrusion of the nearby Cretaceous Pleasant Mountain stock. The lower intercept of the white-phase sample, 18 ± 21 m.y., suggests only modern dilatancy lead loss. Monazites have ages of 272 m.y. (pink) and 282 m.y. (white) which are thought to be cooling ages. Rb-Sr whole-rock data have low initial 87 Sr/ 86 Sr ratios of 0.7031 (pink) and 0.7053 (white). These data, in conjunction with published 40 Ar/ 39 Ar, Rb-Sr, K-Ar, and fission-track ages, suggest that little or no uplift occurred in this part of New England until the Permian and that the uplift rate from 275 m.y. to 225 m.y. was ∼3 times as rapid as was the rate for 225 m.y. to the present. The Carboniferous age of the Sebago batholith suggests that currently accepted metamorphic and tectonic interpretations for southwestern Maine and for east-central New Hampshire require revision.