PETROGRAPHY AND GEOCHEMISTRY OF THE ZONED ONAWA PLUTON, CENTRAL MAINE

The Onawa pluton is an elliptically shaped intrusion ~ 60km2 in size, located in southern Piscataquis county, central Maine. It is one of several post-tectonic, Devonian-aged plutons associated with the Acadian orogeny. It was intruded into the Carrabassett Formation and produced a textbook contact aureole. Many earlier maps of central Maine show the Onawa as concentrically zoned, with an outer margin of gabbro surrounding a central granodiorite. Age dates on these two rock types are 400.1±3.7 Ma and 405± 2.9 Ma respectively (Bradley et al., 2000). However, recent field and petrographic investigations have revealed more complex and variable field relations within the pluton identifying three distinct phases. The most mafic rocks of the Onawa pluton, gabbro-diorites, are observed only along the southwest margin. They have a high color index and are variable in grain size. The central area of the pluton is composed mainly of coarse-grained biotite ± hornblende granodiorite and granite. Titanite and zircon are the dominant accessory phases. Aplite dikes cut the felsic rocks of this central zone. The latter also contains abundant microgranular mafic enclaves (MMEs). They are elliptical in shape and up to 10 cm in long dimension. In the northeast portion of the pluton, a fine-grained equigranular leuco-granite crops out. This is petrographically similar to the aplites that cross-cut the granodiorite outcrops in the northeast. In general, color index and plagioclase abundance decrease toward the center of the pluton with a concomitant increase in quartz. Therefore at the present level of erosion the field relations suggest the Onawa is asymmetrically zoned. Although the presence of MMEs, especially in the northeast of the intrusion, does not preclude the presence of more mafic rocks at depth. Contacts between the three phases are unclear. There are three possible models that could explain the field relations and petrographic variations observed within the Onawa pluton. 1) The three phases represent a single comagmatic suite that has crystallized largely in situ, 2) they represent magmatic pulses from a zoned magma chamber, or 3) they are separate intrusive events. Detailed geochemistry will be used to critically assess the comagmatic nature of the three phases and elucidate the crystallization history of the Onawa pluton.