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 Hu ronian Supergroup (McKim, Pecors, Gowganda, Gordon Lake) range from 3.00 to 2.55 Ga and indicate a provenance dominated by the Late Archean Superior P rovince to the north and west. The stratigraphically highest unit (Gordon L ake Formation) has a distinctive Nd-isotopic composition, with T-DM being 1 00-400 Ma younger than underlying mudstones. This suggests that the provena nce changed, consistent with a previously documented change towards more ne gative Eu-anomalies, in the Gordon Lake. Lead isotopes are consistent with a Superior Province provenance and in addition provide evidence for two epi sodes of regional post-depositional disturbance of the U-Pb system. Lower H uronian (McKim. Pecors) samples align along (207)pb/(204)pb-(206)pb/Pb-204 slopes equivalent to 2170 +/- 58 Ma (MSWD = 92, n = 9) and 2212 +/- 92 Ma ( MSWD = 9.1, n = 5), respectively. These ages are at the minimum age limit o n sedimentation and within uncertainty of the Nipissing Diabase (2219 +/- 4 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 e arly phase of Nipissing intrusion. (206)pb/(204)pb varies mostly from 19 to 34 (up to 59), whereas implied kappa (Th-232/U-238) are mostly between 2 a nd 4, only slightly below the upper crustal value of 4. Changes in (206)pb/ (204)pb imply changes of mu (U-238/(204)pb) by factors of <1-5 and thus res etting of the U-Pb system likely involved Pb loss, with or without U gain. The upper Huronian displays mon complex Pb-isotope systematics. Data align along Pb-207/(PbPb)-Pb-204-Pb-206/Pb-204 slopes of ca.1700 Ma, with regiona l variation in (207)pb/(204)pb. For the Gowganda and Gordon Lake formations , (206)pb/(204)pb ratios are 23-43 and 30-115, respectively, and imply chan ges in mu by factors commonly >2 (and up to 12.5). Values for kappa are als o in the range 2-4 and, accordingly, Pb loss appears to dominate this distu rbance. Elevated (206)pb/Pb-204 correlates with post-depositional addition of potassium in the Gordon Lake and possibly the Gowganda formations. K-met asomatism has been demonstrated previously in the underlying Serpent Format ion and in paleosols developed at the unconformity beneath the Huronian Sup ergroup, the latter being dated at 1690-1730 Ma. Widespread metasomatism, r esulting in K-addition and Pb-loss, may have been related to northerly dire cted basin wide fluid movement in response to post-tectonic intrusions and erosion of the ca. 1.85 Ga Penokean Orogen to the south. Since Ph isotope s ystematics in the lower Huronian mudstones appear unaffected, apart from na tural conduits provided by unconformity surfaces and possibly lower Huronia n sandstone aquifers (that also may have been affected by K-metasomatism), it appears that fluid movement was more pervasive in the upper levels of th is sedimentary sequence. (C) 2000 Elsevier Science B.V. AU rights reserved.