The Bays of Maine Plutonic Complex is in the Downeast interior and is an area of particular concern due to high emitting uranium concentrations from the bedrock. Our study focuses on understanding the age of the Tunk Lake pluton through uranium and thorium concentrations based on U/Pb dating of zircons by LA-ICPMS. The Tunk Lake pluton is concentrically zoned consisting of the (1) outermost hornblende-augite granite, (2) hornblende granite, (3) hornblende-biotite granite, (4) biotite Rapakivi granite, and (5) innermost biotite-quartz monzonite. Zircons from the hornblende granite (n=100), Rapakivi granite (n=76), and biotite-quartz monzonite (n=247) were analyzed for this study.
We specifically focused on the biotite-quartz zone. Zircons from this region are characterized by high oscillatory zoning and cracking – specifically 73% and 26% having pervasive (continuous through-going) and radial cracks, respectively – 33% of zircons being partially embayed, and all but four zircons contain inclusions. Compared to other zones, our zircons yield the least number of grains containing oscillatory zoning. The highest number of uranium rich rims are observed in the quartz monzonite zone; highest number of uranium rich cores are observed in the outer hornblende granite followed by quartz-monzonite zircons. Inclusions are seen in at least half of the zircons in each zone, while embayments are seen in no more than half.
Our new U/Pb data indicate the age of the quartz monzonite is 377 ± 3 Ma, which is comparable within error to the other zones in the pluton. This new date is older than previously published K-Ar and U/Pb dates of of 357 ± 10 Ma and 366 ± 1 obtained from the outer pyroxene granite with euhedral zircon crystals. Thus, this new dating of the Tunk Lake pluton suggests that it may have crystallized at the start of the Acadian Orogeny (~375 Ma).