CHARACTERISTICS OF ZIRCON AND RADIOACTIVITY OF THE DEVONIAN TUNK LAKE PLUTON, MAINE

The Tunk Lake pluton is a zoned granitic intrusion intruded at 366 Ma and it is one of several late orogenic plutons in Downeast Maine. It is compositionally zoned with five primary phases (from outside inwards): hornblende-augite and hornblende granite (HNB), hornblende biotite granite, biotite granite, and quartz monzonite (QM). We measured the radioactivity on outcrops at 81 sites in the field using portable gamma spectroscopy (RS230). Total radioactivity increases systematically inward in the pluton and this trend results from an increase in uranium (3 to 9 ppm) and thorium (13 to 38 ppm), but potassium is nearly uniform in all phases. The central quartz monzonite phase is the most radioactive and the most variable, but high-uranium rocks in this unit are typically between 10 and 15 ppm. Previous studies have shown that whole rock zirconium decreases from ~660 ppm in the outer HNB phase to ~170 ppm in the inner QM phase, and mean zircon length decreases from 0.24 to 0.13 mm from rim to core (Karner, 1970), and thus uranium increases while zirconium decreases toward the center. We analyzed zircon suites from the HNB and the QM phases to better understand the inverse correlation between Zr and radioactivity. The zircons were analyzed using SEM, EMP, and CL to quantify uranium, inclusion content, and cracking. HNB zircons are typically large, with typical uranium concentrations (485 to 700 ppm, Avg: 580 ppm), euhedral, rich with inclusions (apatite, quartz, biotite), weakly oscillatory zoned, and small cracks. QM zircons are small, with high and variable uranium concentrations (340 to 12,189 ppm, Avg: 4253 ppm), uncommon inclusions (uraninite and uranothorite), strong zonation, with damaged and altered rims, and large cracks. The increased uranium and radioactivity in the center of the pluton may be related to the zircon distribution and thus helps us make predictions of the health hazard in groundwater.