Petrologic Significance of Zircon Variation in the Tunk Lake Granite, Southeastern Maine

Abstract

Zircon varies systematically in the gradational rock units of the concentrically zoned Tunk Lake pluton. Average zirconium content decreases from 660 ppm in marginal, hypersolvus, aegirine augite-hornblende granites to 170 ppm in the biotite quartz monzonite core. Zircon decreases in mean length from 0.24 mm in the margins to 0.13 mm in the core. Core zircons tend to be partially metamict and less elongate and have more complex crystal form than zircons of the margins. Frequency distributions of zircon characteristics show that marginal rocks contain zircon populations similar to the average population calculated for the pluton but with additional large crystals, and that core rocks have populations similar to the average but with large crystals removed. Broken and clustered zircons support gravity settling and magma tic convection as causes of zircon variation. Zircons accumulated in the margins of the pluton where downward-moving currents of magma were deflected inward. Zircon distribution provides info...