GEOCHRONOLOGY AND GEOCHEMISTRY OF GRANITOID PLUTONS, WESTERN MAINE

Devonian granitoid magmatism in western and central Maine resulted in a suite of igneous rocks that is temporally, petrographically, and compositionally diverse. Apparently spanning the full age range of the Devonian period, they vary in composition from quartz diorite through granodiorite to highly evolved, two-mica peraluminous granite. They intruded in a variety of styles including zoned and composite plutons, with the juxtaposition of both felsic and felsic/mafic field relations. Many of these plutons either lack reliable age dates or were dated by older methods, and hence an uneven distribution of high quality ages precludes reliable correlations either (i) among timing, tectonic setting, and possible sources (as reflected by their geochemical signature) or (ii) along the strike of the orogen.

To resolve these problems, we have undertaken a review of the ages of the plutons in western and central Maine, and are obtaining new U-Pb LA-ICP-MS ages for representative samples of this diverse suite. Plutons included in this geochronological study are: the Chain of Ponds (COP) granodiorite, which outcrops in NW Maine, the Songo granodiorite (SGd) located in western Maine, the main phases of the Flagstaff Lake complex (FLC), and selected samples from a group of plutons that outcrop in west-central Maine. The COP and SGd are both I-type, hb+bt+titanite granodiorites; the FLC is highly diverse petrographically and includes porphyritic granite, garnet tonalite, and a mingled mafic/felsic assemblage. In contrast, the plutons of west-central Maine are S-type two-mica leucogranites with the exception of the northern phase of the Rome-Norridgewock pluton, which has an I-type mineralogy.

The new dates for this heterogeneous assemblage of plutons, combined with their geochemistry, will enable correlations between the timing of magmatism and possible sources to be examined, including the role of mantle-derived magmas. In addition, comparisons with plutons along strike in Maine and New Brunswick may further enhance our understanding of the tectono-magmatic environment in the broader Acadian context.