Deformation-induced Pb isotope exchange between K-feldspar and whole rock in Neoarchean granitoids: Implications for assessing Proterozoic imprints

Abstract

We present Pb isotope and microstructural studies of deformed K-feldspar megacrystic granodiorites in the westernmost part of the Karelian Craton near the Archean–Proterozoic boundary zone in east-central Finland. Our studies place constraints on the timing and the temperature and strain rate conditions of the deformation. The Pb isotope evidence indicates that U was decoupled from Th approximately at the time of crystallization, which generated high whole-rock Th/U ratios. With time, high 208Pb/ 204Pb ratios were developed at high Th/U domains of the rock. Observed correlation between 208Pb/ 204Pb ratios and deformation microstructures of K-feldspar megacrysts suggests a later deformation-induced Pb isotope redistribution between K-feldspars and whole rock. The coupled 238U– 206Pb and 235U– 207Pb systematics show that this Pb isotope exchange occurred at ca. 1.9 Ga concurrently with the peak of the Svecofennian orogeny. Our Th–Pb and U–Pb model age calculations for K-feldspar–whole rock pairs indicate different degrees of incomplete equilibration that correlates with the degree of deformation. The microstructures of K-feldspars indicate low- to medium-T conditions (400–500 °C) and variable strain rates for deformation. We conclude that Pb isotope compositions and microstructures of K-feldspars provide valuable constraints on assessing Proterozoic imprints in Archean rocks.