Nd and Pb isotopic evidence for provenance and post-depositional alteration of the Paleoproterozoic Huronian Supergroup, Canada

Neodymium model ages for fine-grained formations of the Paleoproterozoic Huronian Supergroup (McKim, Pecors, Gowganda, Gordon Lake) range from 3.00 to 2.55 Ga and indicate a provenance dominated by the Late Archean Superior Province to the north and west. The stratigraphically highest unit (Gordon Lake Formation) has a distinctive Nd-isotopic composition, with TDM being 100-400 Ma younger than underlying mudstones. This suggests that the provenance changed, consistent with a previously documented change towards more negative Eu-anomalies, in the Gordon Lake. Lead isotopes are consistent with a Superior Province provenance and in addition provide evidence for two episodes of regional post-depositional disturbance of the U-Pb system. Lower Huronian (McKim, Pecors) samples align along Pb/Pb–Pb/Pb slopes equivalent to 2170958 Ma (MSWD=92, n=9) and 2212992 Ma (MSWD=9.1, n=5), respectively. These ages are at the minimum age limit on sedimentation and within uncertainty of the Nipissing Diabase (221994 Ma), a ubiquitous regional feature, parts of which may have intruded while much of the Huronian was unconsolidated. These Pb–Pb ages are interpreted to represent widespread diagenetic processes, possibly associated with an early phase of Nipissing intrusion. Pb/Pb varies mostly from 19 to 34 (up to 59), whereas implied k (Th/U) are mostly between 2 and 4, only slightly below the upper crustal value of 4. Changes in Pb/Pb imply changes of m (U/Pb) by factors of B1–5 and thus resetting of the U–Pb system likely involved Pb loss, with or without U gain. The upper Huronian displays more complex Pb-isotope systematics. Data align alongPb/Pb–Pb/Pb slopes of ca.1700 Ma, with regional variation in Pb/Pb. For the Gowganda and Gordon Lake formations, Pb/Pb ratios are 23–43 and 30–115, respectively, and imply changes in m by factors commonly \2 (and up to 12.5). Values for k are also in the range 2–4 and, accordingly, Pb loss appears to dominate this disturbance. Elevated Pb/Pb correlates with post-depositional addition of www.elsevier.com/locate/precamres * Corresponding author. E-mail addresses: [email protected] (S.M. McLennan), [email protected] (A. Simonetti), [email protected] (S.L. Goldstein) 1 Present address: GEOTOP, Université du Québec à Montréal, Montréal, PQ, Canada H3C 3P8. 2 Present address: Lamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, Palisades, NY 10964, USA. 0301-9268/00/$ see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0301 -9268 (00 )00070 -X S.M. McLennan et al. / Precambrian Research 102 (2000) 263–278 264 potassium in the Gordon Lake and possibly the Gowganda formations. K-metasomatism has been demonstrated previously in the underlying Serpent Formation and in paleosols developed at the unconformity beneath the Huronian Supergroup, the latter being dated at 1690–1730 Ma. Widespread metasomatism, resulting in K-addition and Pb-loss, may have been related to northerly directed basin wide fluid movement in response to post-tectonic intrusions and erosion of the ca. 1.85 Ga Penokean Orogen to the south. Since Pb isotope systematics in the lower Huronian mudstones appear unaffected, apart from natural conduits provided by unconformity surfaces and possibly lower Huronian sandstone aquifers (that also may have been affected by K-metasomatism), it appears that fluid movement was more pervasive in the upper levels of this sedimentary sequence. © 2000 Elsevier Science B.V. All rights reserved.