LATE DEVONIAN PERALUMINOUS GRANITIC MAGMATISM OF WEST-CENTRAL MAINE

A compositionally diverse suite of igneous rocks of Devonian age traverse central Maine from Houlton in the NE to Bethel in the SW in an orogen-parallel belt often referred to as the Piscataquis Magmatic Belt (PMB). Plutons vary in composition from gabbroic to highly evolved two mica granites, are zoned and composite bodies with ages ranging from Early to Late Devonian. The main focus of this presentation is a group of four plutons which outcrop to the south of the main trend of the PMB and whose temporal and geochemical relationships to the main belt is not well understood.

These plutons, the North Jay, Chesterville, and Cape Cod Hill are peraluminous in composition as is the majority of the Rome-Norridgewock pluton with the exception of its northern phase, which contains hornblende and sphene. The peraluminous rocks of these plutons are quite similar and are leucocratic, medium grained, light grey colored granites. In addition to biotite and muscovite they contain abundant microcline typical of S-type mineralogies. Garnet occurs both as discrete grains and as glomerocrysts and surmiceous enclaves are also common. The granites often display a weak foliation at the outcrop scale, however deformational textures, such as warped cleavage planes on micas and plagioclase twins plus undulatory extinction in quartz, are clearly evident in thin section.

Geochemically they have high SiO₂ contents and Zr levels along with A/CNK > 1.1 confirming their peraluminous nature. They plot in the syn-collisional field on tectonic discrimination diagrams. Available age dates, including new LA ICPMS data, confirm these plutons as Late Devonian, ranging from 380Ma for the Cape Cod Hill and Rome- Norridgewock plutons to 371Ma for its northern metaluminous phase. This would suggest a phase of upper crustal melting followed by a deformational event in the latter/waning stages of the Acadian.