Deformation-induced Pb isotope exchange between K-feldspar and whole rock in Neoarchean granitoids: Implications for assessing Proterozoic imprints Jaana Halla a, ⁎, Esa Heilimo b a Geological Museum, Finnish Museum of Natural History, P.O. Box 11, FIN-00014, University of Helsinki, Finland b Department of Geology, P.O. Box 64, FIN-00014, University of Helsinki, Finland abstract article info Article history: Received 5 November 2008 Received in revised form 4 April 2009 Accepted 8 April 2009 Editor: R.L. Rudnick Keywords: Pb isotopes K-feldspar Deformation Shear zone Archean Paleoproterozoic 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 208 Pb/ 204 Pb ratios were developed at high Th/U domains of the rock. Observed correlation between 208 Pb/ 204 Pb ratios and deformation microstructures of K-feldspar megacrysts suggests a later deformation-induced Pb isotope redistribution between K-feldspars and whole rock. The coupled 238 U– 206 Pb and 235 U– 207 Pb 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction One problematic issue in the study of Archean granitoids has been the widespread Paleoproterozoic overprinting observed over large areas in the vicinity of Archean–Proterozoic boundaries. It is often difficult to distinguish between Archean and Paleoproterozoic over- printing effects, because the granitoids have generally undergone polyphase metamorphism and deformation. Deformation is one of the most important factors affecting radiogenic isotope change observed in low-grade metamorphic rocks (e.g., Kirschner et al., 1998). The recognition of deformation mechanisms and conditions is essential to correctly interpret the isotope data for deformed rocks. Because of isotope exchange in deformation, Pb isotopes in feldspars can potentially be used to determine the timing of the event. Because of its strong enrichment of Pb over U and Th, K-feldspar tends to preserve the Pb isotope ratios that were formed at the time of the last isotope equilibration, i.e., during crystallization of the rock or a metamorphic event. After equilibration, the Pb/Pb isotope ratios in the whole-rock system increase but those of the K-feldspars remain practically unchanged. During a later tectonothermal event, the Pb isotope composition of K-feldspar may homogenize with the whole- rock composition and be preserved as a record of the whole-rock Pb/ Pb isotope ratios at the time of redistribution (Rosholt et al., 1973). The time of the last complete equilibration event can be calculated on the basis of measured K-feldspar and whole-rock Pb isotope ratios provided that the rock has been a closed system with respect to Th, U, and Pb. To understand the mechanisms and conditions of deformation in shear zones of the Koitere and Nilsiä plutons, we study the shapes and microstructures of the K-feldspar porphyroclasts. We approach the timing problem by examining the Pb isotope compositions of K-feldspars and whole-rock samples to see if they can be used as a timing tool for deformation. In order to set constraints on the conditions and timing of deformation, we studied the Pb isotope compositions and micro- structures of K-feldspar porphyroclasts and whole-rock samples of well-known porphyritic sanukitoid-type granodiorites with magmatic K-feldspar megacrysts (e.g., Halla, 2005). The time interval between crystallization at ~2.73 Ga and Paleoproterozoic deformation at ~1.9 Ga was large enough to produce distinctive isotope compositions for phases with varying U/Pb and Th/Pb ratios. We use the coupled 238 U– 206 Pb and 235 U– 207 Pb system to show that the Pb isotope exchange occurred at 1.9 Ga. We also calculate Th–Pb and U–Pb model ages for K-feldspar–whole rock pairs to reveal the degree of Pb isotope equilibration. Variable model ages indicate variation in the degree of Pb isotope homogenization between the whole rock and Chemical Geology 265 (2009) 303–312 ⁎ Corresponding author. Tel.: +358 9 191 23430; fax: +358 9 19122925. E-mail address: jaana.halla@helsinki.fi (J. Halla). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.04.007 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo