The Tunk Lake Pluton is located in the Downeast Bays-of-Maine Igneous Complex. It is a ~180 square kilometer granitic body with distinct concentric zones (Karner, 1968). Tunk Lake is one of several plutons in Downeast Maine with high uranium, and uranium analyses of these plutons is important because uranium is known to pose serious health risks. The pluton is divided into four main zones, defined by mineral assemblage and composition. From each zone, zircon grains were separated, handpicked, and mounted in epoxy to be imaged with SEM and cathodoluminescence. The grains were sorted based on textures, namely zoning, cracks, inclusions, embayments, and “wormy” textures.
This study focuses on the hornblende granite zone located in the exterior of the pluton. This zone is composed of coarse-grained, amphibole-rich rocks. Zircon textures were studied using BSE imaging on a SEM to reveal the elemental and textural variations within each crystal. Most of the hornblende zircons were found to be included within the hornblende itself and defined by oscillatory-zoning (68%), inclusions (64%), and pervasive cracking (50%). However, the hornblende granite has more zircons with no cracks (47%) than the inner zones. These zircons have relatively large aspect ratios of approximately 2.64, compared to 1.95 and 1.72 in the inner zones. 30 zircons from each zone were sent to Arizona Laserchron Laboratory to be dated with U-Pb via laser ablation ICP-MS. The resulting data from the hornblende granite zone show the zircon age to be 376.4 ± 4.08 Ma, which suggests that the pluton may have crystallized during the Acadian Orogeny. Zircons from all of the studied zones produced ages within the same uncertainty. The gravity settling hypothesis does not appear to be supported by these data because distinct textural differences between the concentric zones disagree with the model which would require homogeneous magma convecting throughout the pluton. Several petrological hypotheses, including variation in concentration of zirconium by diffusion or varying physicochemical conditions within the pluton, could explain the zircon variation. Analysis methods with greater accuracy, like ID-TIMS, would have higher resolution and could reveal a history of multiple magmatic events, explaining the discrepancies between the literature and these observations.