Intracrystalline redistribution of Pb in zircon during high-temperature contact metamorphism

Zircons in the high-T (700-900 °C) contact aureole surrounding the Makhavinekh Lake Pluton (MLP), northern Labrador, were studied using conventional thermal ionization mass spectrometry (TIMS) and sensitive high-resolution ion microprobe (SHRIMP) geochronology to test for evidence of high-T Pb mobility. Metasedimentary gneisses in the country rocks (Tasiuyak Gneiss) contain ∼1850 Ma zircons that formed during regional (M1) metamorphism that were reheated in the aureole during emplacement of the MLP at 1322 Ma (M2). M1 zircons that experienced M2 temperatures <750 °C are concordant at ∼1850 Ma and were, thus, virtually unaffected by M2 contact metamorphism. In contrast to this well-established baseline, sector-zoned M1 zircons in samples that reached T>800 °C scatter along a discordant array between M1 and M2 are locally reversely discordant and commonly return younger apparent ages for lower-U cores than higher-U rims. These data collectively require widespread intracrystalline Pb redistribution during M2. Isometric M1 overgrowths and inherited magmatic cores in the same samples were unaffected, indicating that susceptibilities to subsequent Pb redistribution may have been controlled by variations in M1 zircon growth mechanisms. The scatter of U-Pb ages in sector-zoned M1 grains is consistent with Pb expulsion from low-U domains (e.g., broad bright-CL sector boundaries) that had accumulated high lattice strains prior to M2. Lattice strain is inferred to have resulted from a combination of high intrinsic growth defects compounded by self-induced internal stresses from expanded metamict high-U sectors. Recovery of strained domains occurred while high-U sectors were partly metamict, allowing Pb to accumulate in the remaining amorphous fraction.