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Depth profile techniques for U--Pb geochronology and rare earth element (REE) geochemistry were conducted on unpolished, Archean zircon with metasomatic rims from metasedimentary rocks within the Horseshoe Lake greenstone belt, western Superior Province, Canada. These zircon crystals are shown to have isotopically distinct rims (typically 100 million years younger than the cores. The [.sup.207]Pb/[.sup.206]Pb rim ages can be temporally correlated with regional magmatism and with a later, potentially Au-bearing, hydrothermal event synchronous with greenschist-facies metamorphism and regional deformation. Notably, the zircons do not record evidence of local ca. 2741-2715 Ma magmatism manifested by the emplacement of a quartz--feldspar porphyry dike swarm. Laser ablation--inductively coupled plasma--mass spectrometry (LA--ICP--MS) REE depth profile analyses on the same unpolished zircon show that the rims are commonly characterized by low Th/U ratios, elevated Hf, and variable REE concentrations in comparison with the interior of the grains. The variations in rim and core chemistry suggest that these elements, along with common Pb, were mobilized by fluids and interacted with zircon in the metasediments, resulting in the production of the metasomatic rims. In summary, this paper shows that depth profiling techniques applied to unpolished zircon can be useful to elucidate the tectonic, and potentially metallogenic, history of a complex Archean terrane. Des méthodes d'analyse des profils de concentrations en fonction de la profondeur pour la géochronologie U--Pb et la géochimie des terres rares ont été utilisées sur des zircons archéens non polis à bordures métasomatiques provenant de roches métasédimentaires de la ceinture de roches vertes de Horseshoe Lake de la partie ouest de la province du Supérieur (Canada). Il est démontré que ces cristaux de zircon ont des bordures (épaisseur typique 100 millions d'années plus jeunes que les coeurs. Les âges [.sup.207]Pb/[.sup.206]Pb de bordures peuvent être corrélés dans le temps à un magmatisme régional et un subséquent événement hydrothermal possiblement aurifère synchrone d'un métamorphisme au faciès des schistes verts et d'une déformation régionale. Les zircons ne présentent notamment aucun indice du magmatisme local vers 2741-2715 Ma dont témoigne la mise en place d'un essaim de dykes de porphyre quartzofeldspathique. Des analyses de profils de concentrations des terres rares par spectrométrie de masse avec plasma à couplage inductif jumelé à l'ablation par laser (LA--ICP--MS) sur les mêmes zircons non polis montrent que les bordures sont souvent caractérisées par des rapports Th/U faibles, des concentrations élevées de Hf et des concentrations de terres rares variables par rapport à l'intérieur des grains. Les variations de la chimie des bordures et des coeurs donnent à penser que ces éléments, ainsi que le plomb élémentaire, ont été mobilisés par des fluides et ont interagi avec les zircons dans les métasédiments, produisant ainsi les bordures métasomatiques. L'article démontre que les méthodes d'analyse des profils de concentration en fonction de la profondeur appliquées a des zircons non polis peuvent être utiles pour élucider l'évolution tectonique et possiblement métallogénique d'un terrane archéen complexe. [Traduit par la Rédaction], Introduction Despite 30 years of mineral exploration and several provincial survey publications outlining the local stratigraphy of the Horseshoe Lake greenstone belt (HLGB; Carruthers 1961; Jensen 1987, 1989; Talbot 1996), [...]

Zircon textures and composition have been used to infer magmatic processes including closed-system fractional crystallization, magma mixing or replenishment, and country-rock assimilation. Here, we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evolution. We present field, whole-rock chemical, textural, mineral chemical, and U-Pb age data from evolved, fine-to-coarse-grained granite intrusions on Melville Peninsula, Nunavut, Canada. Zircon forms two main populations in these granites, Type-1 and Type-2 zircon. Type-1 zircon is present in all samples, but predominant in fine-grained granite. Crystals are euhedral and inclusion-rich and show periodic, fine-scale oscillatory zoning, comparatively low concentrations of U ( Keywords Alteration * Fluid * Magma * Evolved granite * Zircon, Introduction Granitic magmas emplaced into the upper crust may reach volatile saturation as a result of decompression ('first boiling') or cooling ('second boiling') (Burnham 1979; Candela 1991). Moreover, degassing of [...]

An intramontane collapse basin developed within the hanging wall above the large-scale extensional Fjord Regional Detachment of NE Greenland in middle to late Devonian times. The continental elastic sediments within the basin are derived locally from Laurentian source rocks, which makes them well suited for a study of the crustal evolution of the source terrain. This is the first integrated in-situ Pb and Hf isotope study to be presented, and zircon data on a selected sandstone from the basin are combined with Sm-Nd whole-rock data on sand/siltstones. Nd whole-rock ages of two samples of sandstones and a siltstone are 2.0-2.1 Ga. Peak frequencies of zircon [sup.207]Pb/[sup.206.]Pb ages at 1,764-1,912 Ma, and [sup.176]Hf/[sup.177]Hf values at 0.28142-0.28163 ([t.sub.DM] = 2.47 to 2.06) for the sandstone suggest the generation of a considerable volume of juvenile continental crust in the ultimate zircon provenance at 1.9-2.0 Ga. The Hf isotopic compositions of Archaean zircons in the sandstone are distinct from those of the source materials of Proterozoic protocrust at 1.9-2.0 Ga, but zircons with elevated Hf-[t.sub.DM] ages of up to 2.47 Ga can be related to a component of Archaean crust or reworked Archaean material in the ultimate zircon source area. Zircon [sup.207]Pb/[sup.206]Pb ages are also recorded at 1,480-1,572, 1,318 and 1,014 Ma (Grenvillian). The Hf isotope compositions of these zircons are consistent with reworking of the Proterozoic protocrust at these times, with little or no juvenile input. The Proterozoic zircons form two distinct groups defined by [sup.176]Yb/[sup.177]Hf > c0.05055 and [sup.176]Yb/[sup.177]Hf < 0.03301, and the latter group overlaps with Yb-Hf isotope data on the Archaean zircons. The two groups may represent zircons derived from evolved granites and intermediate to mildly felsic rocks, respectively. The repeated reworking of the continental crust also comprised erosion and deposition of sediments in the Proterozoic (the Krummedal sequence and the Eleonore Bay Supergroup, EBS) and intrusion of Caledonian anatectic granites in the EBS, which both represent provenance components to the Devonian sediments. No discrete Caledonian Pb-Pb zircon ages are recorded, but Caledonian magmatism may be represented by strongly discordant zircons which form arrays with a lower intercept age at ca. 400 Ma and an upper intercept at 1,600-2,000 Ma. One undated zircon records a [sup.176]Hf/[sup.177]Hf ratio of 0.282218, higher than that of the Proterozoic protocrust in Caledonian/late-Caledonian times (380-450 Ma) which may represent a Caledonian mantle contribution.