A CROSS-BORDER GEOCHRONOLOGICAL COMPILATION FOR LATE SILURIAN-DEVONIAN GRANITOID ROCKS IN MAINE (USA) AND NEW BRUNSWICK (CANADA): MAGMATIC PULSES OR A CONTINUUM?

Paleozoic granitoid rocks constitute a major component of the northern Appalachian orogen. Plutons inferred to represent magmatic pulses associated with different tectonic events are dispersed through Maine and New Brunswick. However, not all of these plutons have reliable ages and/or petrological data. We have undertaken a comprehensive cross-border geochronological compilation, focusing on the Late Silurian through Devonian, the main objective of which is to look for new insights on the timing of plutonism and its implications for orogen evolution.

Of the approximately 225 Paleozoic plutons assembled so far, only about 100 have reasonably reliable U-Pb (zircon, titanite, and/or monazite) crystallization ages. Of those, about 40% are Middle to Late Silurian (435-416 Ma) including, for example, the North Union (ME; ca. 422 Ma), Cadillac Mt. (ME; ca. 419 Ma), Baring (ME; ca. 421 Ma), Utopia (NB; ca. 428 Ma), and North Pole (NB; ca. 417 Ma) plutons. These rocks vary from metaluminous to peraluminous and peralkaline, and intrude south of the Norumbega Fault Zone in Maine and central New Brunswick. They are generally attributed to the Salinic orogeny. A second group, about 20% of dated Paleozoic plutons have Early to Middle Devonian ages (410-395 Ma) including, for example, the Katahdin (ME; ca. 407 Ma), Mixer Pond (ME; ca. 400 Ma), Allandale (NB; ca. 402 Ma), and Papineau Falls (NB; ca. 397 Ma) plutons. These rocks vary considerably in composition and intrude in the Piscataquis Igneous Belt, in the Miramichi Highlands, and also in southwestern New Brunswick. Another 25% of the dated plutons are of Middle to Late Devonian (385-360 Ma) including, for example, the Threemile Pond (ME; ca. 381 Ma), Rome (ME; ca. 378 Ma), and Antinouri Lake (NB; ca. 372 Ma). These plutons are scattered throughout Maine and New Brunswick, but plutons of this age are especially voluminous south of the Norumbega Fault Zone in Maine and southern New Brunswick (Deblois, ME; ca. 384 Ma, Lucerne, ME; ca. 380 Ma, and Mount Douglas, NB; ca. 367 Ma).

Late Silurian-Devonian plutons have often been termed “Acadian” and “Late Acadian” in the literature because of wide errors on many ages and complexities in tectonic models. The wide geographic separation of some of the contemporaneous plutons suggests that they are not all related to a specific or the same orogenic event.