EMPLACEMENT LEVELS OF DEVONIAN I-TYPE GRANITOIDS OF THE PISCATAQUIS MAGMATIC BELT, CENTRAL AND WESTERN MAINE

The Piscataquis Magmatic Belt (PMB) is a group of over 40 compositionally diverse plutons, which traverse central Maine. They vary from gabbro, to granodiorites and two-mica peraluminous granites; temporally they range form Early Devonian, such as the 405 Ma Onawa pluton, to Late Devonian, e.g. the 364 Ma Songo pluton. These intrusions were also emplaced at differing crustal levels, for example the Katahdin granite was emplaced epizonally as evidenced by its texture and associated volcanic rocks. The zoned Onawa pluton displays a classic contact aureole and must therefore have also been emplaced into cooler, higher crustal levels. In contrast, at the southwestern end of the PMB the Songo granodiorite pluton lacks a contact aureole and evidence from the surrounding metamorphic envelope suggests it was emplaced ~ 18km. The question addressed in this presentation is whether plutons were emplaced at regular intervals in the crust or if magmas ponded at specific crustal level(s)?

This study aims to examine the emplacement level of plutons in the PMB utilizing the Al in hornblende geobarometer. This necessitates that the granites have a metaluminous I-type mineralogy, that is coexisting hornblende + bitoite + sphene. The plutons examined in this study are – the Songo granodiorite, the North Lexington granite, the Chain of Ponds pluton and the metaluminous phases of the Mooselookmeguntic and Rome – Norridgewock plutons. These plutons not only represent a range of pluton ages but are located along traverses, which are both orogen parallel (NE – SW) and perpendicular to this (NW – SE). Although this selection of plutons is restricted compositionally to the metaluminous granites it may be possible to infer information regarding emplacement depths of related peraluminous rocks of the PMB. Therefore, understanding the emplacement levels of the plutons of this suite across central Maine will further elucidate the assembly of the continental crust during the Acadian orogeny.