ORIGIN OF GARNETS FROM THE PERALUMINOUS NORTH JAY PLUTON, WEST-CENTRAL MAINE. MAGMATIC OR XENOCRYSTIC?

The North Jay Pluton (NJP) is located in west-central Maine and crops out over an area of approximately 30km². The preliminary bedrock map of the Farmington 7.5 minute quadrangle (Pankiwskyj, 1978) suggests that the NJP is post-tectonic as it crosscuts the local Silurian country rocks (Sangerville formation). Most likely it is similar in age to the nearby Togus, Rome and Hartland plutons which have recently been age dated by U-Pb systematics at around 380 Ma (Bradley, 2000). Garnet occurs as a minor phase in many of these two-mica granitoids, including the North Jay, the focus of this study.

The NJP, a garnet bearing two-mica peraluminous granite, is cut by several late pegmatites and contains metasedimentary xenoliths of unknown origin. The granite is medium grained with an equigranular texture and contains abundant microcline along with K feldspar and plagioclase. Muscovite is present forming large euhedral crystals and is more abundant than biotite. Quartz displays undulatory extinction and sutured contacts are also common. Garnet in the granite is typically sub- to anhedral, 1 – 3mm in diameter and is often embayed with many quartz inclusions. The pegmatites are up to 2 m across trending N – S and are Kf + Plag + Qtz assemblages with garnet glomerocrysts and minor beryl. The xenoliths are usually angular, range in size from 5 – 25 cm, quartzo-feldspathic gneisses and schists. Garnets in the latter are euhedral and lack inclusions.

The garnets in the NJP could have originated from a number of possible sources. They could be metamorphic formed during a later event as the granite does appear to have been subjected to some post-crystallization strain. Alternatively, they may be xenocrysts inherited from either the adjacent country rocks or from xenoliths representing deeper crustal levels, proximal to the melting zone where the perlauminous melt formed, and therefore restite in nature. However magmatic garnets are not uncommon in peraluminous granites, e.g. the Cardigan pluton (Plank, 1987), and this would have implications for melt composition and origins. Electron microprobe data, including compositional maps and profiles for garnets from the NJP, its pegmatites and xenoliths along with data for the country rocks and geothermometry calculations should help us constrain their origin.