PLUTONIC STYLE AND TECTONIC SETTING OF EARLY DEVONIAN PLUTONS, CENTRAL MAINE

The Piscataquis magmatic belt (PMB) is comprised of over 40 plutons, emplaced along an orogen parallel zone from southwest to northeastern Maine. They were intruded at various crustal levels, from the epizonal Kathadin pluton (with its associated volcanic rocks) to the deeper-seated Songo granodiorite of western Maine. The plutons are compositionally diverse, ranging from gabbro to highly evolved, leucocratic granites. A variety of intrusive forms are represented including elliptically zoned intrusions along with tabular sheets. This synthesis focuses on a sub-group of Early Devonian plutons (410–400 Ma) and details their petrography and plutonic style. Furthermore, their geochemical-tectonic signatures within the setting of the Acadian orogeny in the northern Appalachians are examined. The plutons of the PMB can be broadly categorized into four compositional types, all of which are included in this group of Early Devonian plutons. Type 1 are mafic in composition as in the Moxie gabbroic sheet. Type 2 are compositionally zoned plutons, elliptical in plan, with outer rims of diorite (and sometimes gabbro) grading to central areas composed of granite, e.g. the Hunt Ridge and Onawa plutons. The Lexington pluton, an example of the Type 3 composite plutons, consists of three distinct, but perhaps, comagmatic facies, although field relations are uncertain. Type 4 plutons are geochemically evolved, two mica granites such as the Phillips and Bald Mt. plutons. The diversity of pluton types along with their internal petrographic and geochemical variations suggests they formed from multiple source regions and by disparate magmatic processes. Most of the plutons studied have volcanic arc signatures on trace element discrimination diagrams for plutonic rocks (Pearce et al.; 1984, Harris, 1990), although some straddle the boundary with the syn-collisonal, upper crustal melt field. There appears to be no correlation between compositional type and age during the development of the orogen. This preliminary data would suggest that crustal melts and mantle sources were tapped and available throughout the duration of the Acadian orogeny in central Maine.