The Tunk Lake Pluton is a concentrically zoned pluton located in the Bays-of-Maine Igneous Complex in Downeast Maine. A pluton age of 366 ± 1 Ma is reported in the literature, but no supporting documentation was provided. Our work focuses on the textural classification and U-Pb dating of zircons found in the rapakivi granite zone of the pluton, located between the quartz-monzonite and hornblende granite zones.
The rapakivi zone can be characterized by its distinct subsolvus appearance, orthoclase surrounded by a ring of plagioclase. Mineral separation techniques were done to isolate zircons in samples collected from this zone. The 76 grains collected were then classified based on their zoning, cracking, aspect ratios, and inclusions visible in BSE (back-scattered electron) images obtained using a scanning electron microscope. The average aspect ratio of the grains was 1.97, 74% of the grains had oscillatory zoning, 90% had either pervasive or radial cracking, and 92% had inclusions.
A group of 30 zircons were chosen to be analyzed by LA ICP-MS at the University of Arizona Laserchron Center based on which contained a spot approximately 30 microns in diameter without cracks or inclusions. Out of this group, 25 grains yielded concordant ages, with a weighted mean age of 375.7 ± 3.6 Ma. This age is nearly 10 Ma years older than the previously reported age of the granite and is consistent with dates obtained for other zones of the pluton. The five grains not included in the average age calculation typically had cracks and uranium-rich inclusions throughout the crystal. The uranium concentrations and U-Th ratios in the samples ranged from 160 ppm to 4464 ppm and 0.9-3.1, respectively. There appears to be an even distribution of uranium concentration between 160 ppm to 1989 ppm, with the exception of two grains with concentrations of 3954 ppm and 4464 ppm, which is most likely a result of a high uranium concentration in their cores.
This research, when combined with data collected from other zones of the granite, provides a new, more reliable age of 375.7 ± 3.6 Ma for the Tunk Lake Pluton that correlates with the beginning of the Acadian orogeny.