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Highlights • 2045.8 ± 1.0 Ma and 2042.7 ± 3.0 Ma U–Pb ages for the lower Union Island Group (UIG) • UIG is the oldest Paleoproterozoic strata in the circum-Slave region. • Stratigraphy of the East Arm basin is revised. • UIG basalt geochemistry indicates a continental rift origin for the East Arm basin. • Implications for a connected Rae–Slave craton prior to ca. 2.05 Ga. Abstract The Slave craton underwent widespread extension during the Paleoproterozoic. The East Arm basin of Great Slave Lake formed during this period and preserves a protracted record of sedimentation and volcanism in the southeastern Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate sedimentary strata. We report the first high precision U–Pb zircon and baddeleyite dates for the Union Island Group; 2045.8 ± 1.0 Ma for a volcaniclastic unit from the lower basalt formation and 2042.7 ± 3.0 Ma for a diabase intrusion within volcaniclastic horizons in the lower basalt formation. We fundamentally revise the lower basin stratigraphy and demonstrate that the Union Island Group is the oldest identified supracrustal package in the East Arm basin, ∼120 Myr older than previously thought. This revised stratigraphy is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples, which is dominated by 2.76–2.56 Ga ages, reflecting prominent input from the local Archean Slave basement. The Union Island Group contains two geochemically distinct mafic volcanic packages. The lower basalt package is predominantly alkaline, characterized by an OIB-like chemical signature with high and variable incompatible element contents (108–438 ppm Zr, 13–62 ppm Nb). In contrast, the upper basalt package is tholeiitic, contains much lower and uniform incompatible element contents (83–101 ppm Zr, 2–4 ppm Nb), and has an E-MORB-like chemical signature. Both packages display overlapping, depleted time-integrated εNd(i) values (+1.1 to +3.2). Petrological modeling suggests the lower package originated as OIB-like melts from the asthenosphere that interacted with a depleted reservoir; the upper package was produced by larger degree of partial melting of a similar mantle source, in addition to possible lithospheric input. The petrogenesis established in this study is consistent with a passive continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. We therefore propose that the 2046 Ma Union Island Group represents an incipient rift basin sequence formed during Paleoproterozoic extension that marked the initial breakup of the southern margin of the Slave craton from a pre-Laurentia supercontinent. [ABSTRACT FROM AUTHOR]

Abstract Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3–6 ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb = 13.57–13.52) and low 238U/204Pb (1.0–1.5) give single-stage model Pb dates of 2.90–2.84 Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18 ± 0.45 Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20 ± 0.54 Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu* > 1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model μ (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0 Ga addition of a sedimentary component, possibly derived from reworking of a high-μ basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work. Highlights • Solution and laser ablation Sr-Pb isotope ratios for cpx in three mantle eclogite suites • Decoupling due to multi-stage evolution (protolith formation, subduction, mantle residence) • Gabbroic (cumulate) eclogites most likely to retain least radiogenic Sr and Pb • Palaeoproterozoic to Archaean minimum ages determined for all suites • High-μ (~9) source recognised for sediment melt-contaminated Kaapvaal eclogites [ABSTRACT FROM AUTHOR]

Kimberlites are rare volatile-rich ultramafic magmas thought to erupt in short periods of time (<1 Myr) but there is a growing body of evidence that the emplacement history of a kimberlite can be significantly more protracted. In this study we report a detailed geochronology investigation of a single kimberlite pipe from the Renard cluster in north-central Québec. Ten new high precision ID-TIMS (isotope dilution - thermal ionization mass spectrometry) U-Pb groundmass perovskite dates from the main pipe-infilling kimberlites and several small hypabyssal kimberlites from the Renard 2 pipe indicate kimberlite magmatism lasted at least ~20 Myr. Two samples of the main pipe-infilling kimberlites yield identical weighted mean 206Pb/238U perovskite dates with a composite date of 643.8 ± 1.0 Myr, interpreted to be the best estimate for main pipe emplacement. In contrast, six hypabyssal kimberlite samples yielded a range of weighted mean 206Pb/238U perovskite dates between ~652-632 Myr. Multiple dates determined from these early-, syn- and late-stage small hypabyssal kimberlites in the Renard 2 pipe demonstrate this rock type (commonly used to date kimberlites) help to constrain the duration of kimberlite intrusion history within a pipe but do not necessarily reliably record the emplacement age of the main diatreme in the Renard cluster. Our results provide the first robust geochronological data on a single kimberlite that confirms the field relationships initially observed by Wagner (1914) and Clement (1982); the presence of antecedent (diatreme precursor) intrusions, contemporaneous (syn-diatreme) intrusions, and consequent (post-diatreme) cross-cutting intrusions. The results of this detailed U-Pb geochronology study indicate a single kimberlite pipe can record millions of years of magmatism, much longer than previously thought from the classical viewpoint of a rapid and short-duration emplacement history. [ABSTRACT FROM AUTHOR]

Previous studies have shown that radiometric dating of fossils can be complicated by post-fossilisation alteration. A key challenge to obtaining geologically meaningful fossilisation dates in bioapatite is identifying regions that have remained relatively undisturbed since the time of fossilisation. In order to determine whether a geochemical indicator can be used to identify least-disturbed regions, we combine in situ trace element analysis and U-Pb geochronology of fossil teeth from the Arroyo Chijuillita Member, New Mexico, and the Dinosaur Park Formation, Alberta. The reliability of these U-Pb dates is supported by good agreement between the isotope dilution thermal ionization mass spectrometry (61.5 ± 1.6 Ma) and laser ablation ICP-MS (64.2 ± 2.4 Ma) dates for a targeted tooth region. Of fourteen teeth analyzed from these units, two record dates within uncertainty of the depositional age: one has a “diffusion” concave upward cross-sectional yttrium profile, the other a concave downward profile reflective of trace element loss. It was found for teeth from both units that smaller yttrium concentration ranges are correlated with younger U-Pb dates, likely the result of post-fossilisation alteration. A yttrium screen based on this relationship is proposed, and is found to be a better indicator of post-fossilisation open system behaviour in the samples analyzed than other geochemical features thought to be produced during fossilisation. This screen provides a new tool for identifying fossil material least disturbed since fossilisation, facilitating the use of bioapatite to date the time of diagenesis. [ABSTRACT FROM AUTHOR]

We report the U–Pb age and Sr/Nd isotope composition for perovskite isolated from forty six kimberlite samples located in the newly discovered Chidliak field on Baffin Island, Canada. The minimum duration of kimberlite magmatism in this field was 17.9 m.y. from 157.0 to 139.1 Ma and represents a new Jurassic kimberlite field in NE Canada. The most prolific period of kimberlite magmatism occurred between 152 and 142 Ma (80% of dated kimberlites). Kimberlitic perovskite from these intrusions display a range in 87 Sr/ 86 Sr (0.7043 to 0.7030) and ε Nd T values (+3.9 to −0.4), overlapping the isotopic field previously defined for southern African Group I kimberlites. The ages and isotopic compositions obtained for Chidliak magmatism are identical to a number of Jurassic kimberlite fields in eastern North America and SW Greenland. Some of this Jurassic kimberlite magmatism has a link to one or more mantle plume hotspot tracks but the Chidliak kimberlites have an origin in the deep subcontinental lithospheric mantle and are part of a Jurassic magmatic province that erupted along both margins of Davis Strait; linked to upwelling asthenosphere, continental rifting, and Mesozoic–Cenozoic development of oceanic crust in the Labrador Sea basin. In contrast, the location of other eastern North American Jurassic kimberlites when plotted on a Jurassic continental reconstruction aligns closely to the northernmost projection of contours 2–4 of the African large low shearwave velocity province, consistent with a link to mantle plumes derived from the African mantle plume generating zone. [ABSTRACT FROM AUTHOR]

Establishing the emplacement ages and distribution pattern of Central Slave kimberlites has played a key role in diamond exploration within the Lac de Gras kimberlite field. Nonetheless, emplacement age information is lacking for approximately 80% of the known kimberlites in this field making the assessment of emplacement age patterns difficult. This study expands the number and geographic coverage of kimberlite emplacement ages within the Lac de Gras field to re-assess the absolute timing, duration, and possible number of pulses of kimberlite eruption. U–Pb perovskite ages for eight previously undated kimberlites and an additional thirteen kimberlites, which were previously dated by either the Rb–Sr or U–Pb methods, fall within the age range of 75–45 Ma, as previously suggested for kimberlite magmatism in this area. We report the first Carboniferous age kimberlite in the Central Lac de Gras field – the Eddie kimberlite – with a U–Pb perovskite age of 321.0 ± 3.0 Ma. A compilation of 57 kimberlite emplacement ages from the central Lac de Gras field was evaluated using probability density and mixture modeling methods. Five short-duration (4–5 Ma) periods of kimberlite magmatism are recognized at 48, 54, 61, 66 and 72 Ma; the three younger pulses have been previously recognized and remain relatively unchanged. The 54 Ma pulse represents the major kimberlite eruption event containing ~ 40% of the currently dated kimberlites in Lac de Gras field. A detailed evaluation of the temporal–spatial evolution of Lac de Gras kimberlites reveals that the oldest diamond-poor kimberlites (75–60 Ma) were emplaced in the northern and eastern parts of the field whereas the younger (55–48 Ma) economic kimberlites are concentrated in the center of the field. [ABSTRACT FROM AUTHOR]

The first precise ID-TIMS U–Pb baddeleyite/zircon dating of dykes representative of the main activity in the Florianópolis Dyke Swarm (Paraná–Etendeka Magmatic Province, south Brazil) reveals a restricted age range (134.7 ± 0.3 to 133.9 ± 0.7 Ma). This is at least 5 Ma older than indicated by previous 40 Ar/ 39 Ar stepwise heating dating, and precludes a direct relationship with syn-rifting magmatism connected with opening the south Atlantic Ocean. Instead, as indicated by their age and geochemical signature, these dykes are feeders to the nearby Urubici lavas (and coeval Khumib lavas in Etendeka). These new geochronological results also cast doubt on a large (> 10 Ma) time span for the emplacement of the Florianópolis Dyke Swarm implied by 40 Ar/ 39 Ar dates, but the determination of its upper age limit still requires accurate dating of younger, low-volume fine-grained dykes with distinct low-Ti chemistry; crystals suitable to U–Pb dating were not obtained thus far from these dykes. [ABSTRACT FROM AUTHOR]

Highlights: [•] A revised U–Pb zircon age for the Copper Cliff rhyolite is 2452.5±6.2Ma. [•] Six geochemically distinct lava series at Thessalon have been identified. [•] An origin involving mantle source heterogeneity is proposed. [•] An unusual high Th/Nb mantle reservoir is required. [ABSTRACT FROM AUTHOR]

The NNW- to NNE-trending Kaminak diabase dykes, exposed in the Hearne craton of the western Churchill Province, Canada, are components of a Neoarchean–Paleoproterozoic LIP that forms an important pin in plate tectonic reconstructions impacting models for the assembly of the Nuna supercontinent. This study provides the first detailed information on the age, geochemistry and isotopic composition of these dykes. A precise U–Pb baddeleyite date of 2497.6±1.1Ma, obtained for one dyke sample, provides the best estimate for the timing of Kaminak mafic magmatism. The dykes are tholeiitic basalts to basaltic andesites with consistent compositions over an areal extent of ∼10,000km2. Major and compatible trace element variations indicate that the dykes were not in equilibrium with anhydrous peridotitic mantle and evolved through fractionation of Pl+Cpx±Ol from parental melts in the deep crust. All dykes record remarkably similar, incompatible element enriched, closely parallel multi-element profiles with variable HFSE troughs, comparable to Phanerozoic low-TiO2 continental tholeiites. Time-corrected Nd isotopic compositions overlap, or are slightly below bulk Earth ( to −0.8), and yield T DM values (2.75–2.87Ga) comparable to those of igneous crust in the Hearne craton. Implied short residence times, along with the geological, geochemical and isotopic data, are interpreted to imply that parental magmas of the dykes were low-TiO2 tholeiites generated via contamination of primary, depleted N-MORB-like magmas by low-degree, calc-alkaline lamprophyre melts in the continental lithospheric mantle. The melts homogenized in chambers near the Moho and underwent extensive fractional crystallization (Pl+Cpx±Ol), and possibly minor assimilation of lower crust, prior to injection into the upper crust. Paucity of evidence for contemporaneous lithospheric rifting, homogeneous lithogeochemical and Nd isotopic compositions, and petrochemical variations distinct from the Matachewan and Mistassini swarms of the Superior Province suggest the Kaminak dykes are the magmatic products of a Late Archean–Early Paleoproterozoic mantle thermal anomaly that extended, but did not break apart the Hearne craton. The results of this study do not support direct correlation of the Kaminak and Matachewan or Mistassini dyke swarms. Further evaluation of the proposed linkage of the Hearne and Superior cratons prior to Nuna supercontinent assembly requires additional geochronological, lithogeochemical and radiogenic isotopic constraints. [ABSTRACT FROM AUTHOR]

U-Pb zircon geochronological investigations at Battle Harbour, in the Pinware terrane, eastern Grenville Province show that a cross-bedded psammitic rock from Battle Island was deposited after 1200 Ma and prior to Grenvillian orogenesis, as part of a package of psammitic, semi-pelitic and calc-silicate rocks. This represents a major finding as no previous data had indicated any supracrustal-rock deposition after ca. 1500 Ma in the interior eastern Grenville Province. The age is comparable to that obtained from supracrustal units of minor extent elsewhere in the interior Grenville Province, indicating that change in crustal level in these regions was modest during Grenvillian orogenesis. Grenvillian metamorphism at amphibolite facies occurred at 1030 ±4 Ma, based on evidence from a concordant mafic rock within the supracrustal assemblage that is interpreted to be a sill. Grenvillian orogenesis at Battle Harbour was accompanied by abundant pegmatite injection over an extended period, as indicated by cross-cutting relationships and various states of deformation displayed by the pegmatites. An amazonite-bearing pegmatite has an age of 1024 ±3 Ma. An even younger, cross-cutting pegmatite contains only inherited zircon grains. Des recherches géochronologiques U-Pb sur zircon à Battle Harbour, dans le terrane de Pinware, dans l'est de la Province de Grenville, montrent qu'une roche psammitique de Battle Island a été déposée plus récemment que 1200 Ma et avant l'orogène de Grenville; elle était comprise dans un ensemble de roches psammitiques, semi-pélitiques et calco-silicatées. Ces recherches représentent une découverte importante puisque aucune donnée antérieure n'avait indiqué une déposition d'une roche supracrustale plus récemment que ~1500 Ma à l'intérieur de la Province de Grenville orientale. Cet âge est comparable à ceux obtenus des unités supracrustales de moindre étendue ailleurs à l'intérieur de la Province de Grenville, indiquant que le changement dans le niveau de la croûte de ces régions était modeste durant l'orogène de Grenville. Le métamorphisme grenvillien au faciès des amphibolites a eu lieu à 1030 4 Ma, selon des évidences provenant d'une roche mafique concordante à l'intérieur de l'assemblage supracrustal, laquelle a été interprétée en tant qu'un filon couche. À Battle Harbour, l'orogène de Grenville était accompagné d'une importante injection de pegmatites au cours d'une période prolongée, tel qu'indiqué par les relations d'entrecroisement et les divers niveaux de déformation visibles dans les pegmatites. Une pegmatite contenant de l'amazonite a un âge de 1024 3 Ma. Une pegmatite encore plus jeune qui recoupe en travers ne contient que des grains de zircon hérités. [ABSTRACT FROM AUTHOR]

Abstract: We report seven high precision U–Pb age determinations for mafic dykes from a number of major Precambrian swarms located in the Dharwar craton, south India. These new age results define two previously unrecognized widespread Paleoproterozoic dyking events at 2221–2209 and 2181–2177Ma, and confirm a third at 2369–2365Ma. Three parallel E–W trending mafic dykes from the petrographically and geochemically variable Bangalore dyke swarm, the most prominent swarm in the Dharwar craton, yield indistinguishable U–Pb baddeleyite ages of 2365.4±1.0, 2365.9±1.5 and 2368.6±1.3Ma, indicating rapid emplacement in less than five million years. A compilation of Paleoproterozoic U–Pb ages for mafic magmatic events worldwide indicates that the 2369–2365Ma Bangalore dyke swarm represents a previously unrecognized pulse of mafic magmatism on Earth. A protracted episode of Pan-Dharwar mafic dyke swarm emplacement at ∼2.2Ga is the most geographically widespread Paleoproterozoic mafic magmatic event currently recognized in the Dharwar craton. U–Pb baddeleyite dating of two dykes located south of the Cuddapah basin yield ages of 2220.5±4.9Ma (N–S trending Kandlamadugu dyke) and 2209.3±2.8Ma (NW–SE trending Somala dyke), indicating emplacement of two distinct subswarms in the region. Two dykes from the northern Dharwar craton (the E–W Bandepalem and NW–SE Dandeli dykes) separated by ∼550km yield indistinguishable U–Pb ages of 2176.5±3.7 (baddeleyite and zircon) and 2180.8±0.9Ma (baddeleyite), respectively. Collectively, these age data hint at the presence of a giant 2.18Ga radiating mafic dyke swarm that spans >100,000km2 of the Dharwar craton, which has a focal point west of the present day Deccan Flood basalt province. The identification of Pan-Dharwar mafic dyke swarm emplacement at ∼2.2Ga allows for new global correlations with contemporaneous mafic magmatic events previously recognized in many other Archean cratons worldwide (e.g., within Antarctica, Australia, Finland, Greenland, and North America). Increasing evidence suggests that this global mafic magmatism at ∼2.2Ga was linked with intracontinental rifting, enhanced mantle plume activity, and the breakup of one or more Archean continents. We highlight the similarity in timing of Paleoproterozoic (2.23–2.17Ga) mafic dyke swarm emplacement recorded in the Dharwar and Slave cratons and use the correlation of high precision U–Pb ages, geometry and regional patterns of mafic dyke swarm emplacement, and matching Archean geology in these cratons as reconstruction pins to constrain the configuration of the ∼2.2Ga supercraton Sclavia. [Copyright &y& Elsevier]

A review of the ages determined for mantle material (xenoliths and xenocrysts entrained in kimberlite) derived from the Slave Province continental lithospheric mantle (CLM) indicates that a portion of the central Slave lithosphere may be ancient (3.5–3.3 Ga) harzburgite, but the majority of this lithosphere is much younger (2.9–2.0 Ga). Relying on the most robust chronometers, the majority of Slave lithosphere peridotite formed in the Neoarchean (peak at 2.75 Ga), whereas the majority of eclogite formed in the Paleoproterozoic (2.2–2.0 Ga). The northern Slave lithosphere contains evidence of peridotite xenolith ages that young with depth. The Paleoproterozoic eclogites may have multiple origins including remnants of subducted oceanic crust and mafic–ultramafic magmas that crystallized at great depth (100–200 km). Re–Os studies of sulfide inclusions in diamond indicate that some diamonds currently mined are ancient (~3.5 Ga), but many Slave diamonds could be considerably younger. Most eclogitic diamonds recovered from the Slave craton are interpreted to be related to the formation of Paleoproterozoic eclogite. There is abundant evidence for Mesoproterozoic modification of the Slave lithosphere (e.g., heating by magma emplacement at great depth and metasomatism) and possible new addition to the lithosphere at that time. The Canadian Slave and African Kaapvaal lithospheres have similar peaks in cratonic peridotite formation ages at about 2.8 Ga, indicating that a large portion of the CLM in these two cratons formed and stabilized in the Neoarchean. One difference is that the Slave peridotites are much less enriched in SiO2, possibly reflecting the more metasomatized nature of the Kaapvaal CLM. The dominance of Paleoproterozoic formation ages for Slave mantle eclogites contrasts with the dominance of Neoarchean formation ages for Kaapvaal mantle eclogites. Une révision des âges déterminés sur des matériaux du manteau provenant du manteau lithosphérique continental de la Province des Esclaves (des xénolites et des xénocristaux entraînés dans de la kimberlite) indique qu’une portion de la lithosphère centrale des Esclaves pourrait être une ancienne harzburgite (3,5–3,3 Ga) mais que la plus grande partie de cette lithosphère est beaucoup plus jeune (2,9–2,0 Ga). En se basant sur les chronomètres les plus robustes, la plus grande partie de la peridotite de la lithosphère des Esclaves s’est formée au Néoarchéen (crête à 2,75 Ga) alors que la plus grande partie de l’éclogite s’est formée au Paléoprotérozoïque (2,2–2,0 Ga). La lithosphère nord des Esclaves contient des évidences d’âges de péridotite xénolite qui rajeunissent en profondeur. Les éclogites datant du Paléoprotérozoïque peuvent avoir de multiples origines, incluant des lambeaux de croûte subductée et des magmas mafiques–ultramafiques qui se sont cristallisés à de grandes profondeurs (100–200 km). Des études Re–Os des inclusions de sulfures dans les diamants indiquent que quelques diamants actuellement exploités sont anciens (~3,5 Ga) mais que plusieurs des diamants provenant de la Province des Esclaves pourraient être beaucoup plus jeunes. La plupart des diamants de paragenèse éclogitique provenant du craton des Esclaves sont interprétés comme étant reliés à la formation de l’éclogite paléoprotérozoïque. Il existe beaucoup de preuves de modifications, au Mésoprotérozoïque, de la lithosphère des Esclaves (p. ex. un réchauffement par la mise en place de magma à grande profondeur et du métasomatisme) et l’ajout possible de nouvelle lithosphère à ce moment. Les lithosphères des Esclaves et de Kaapvaal ont des crêtes similaires dans la formation de la péridotite cratonique vers environ 2,8 Ga, indiquant qu’une grande portion du manteau lithosphérique cratonique dans ces deux cratons s’est formée et s’est stabilizée à ce moment. Une différence est que les péridotites des Esclaves sont beaucoup moins enrichies en SiO2, reflétant possiblement la nature plus métasomatisée du manteau lithosphérique cratonique Kaapvaal. La dominance des âges de formation paléoprotérozoïques pour les éclogites du manteau des Esclaves contraste avec la dominance des âges de formation néoarchéens pour les éclogites du manteau du Kaapvaal. [ABSTRACT FROM AUTHOR]

Abstract: A unique suite of diamond-rich eclogites from the Jericho kimberlite, Nunavut, Canada, has a uniform but unusual geochemical and isotopic composition that is unlike any other eclogite suite worldwide. Compared to other eclogite suites, garnets from Jericho diamond-bearing eclogites have high MgO (19.6–21.2 wt.%), Cr2O3 (0.28–0.60 wt.%), Sc (88–113 ppm), and Zr (32–36 ppm) contents and are characterized by consistent highly fractionated chondrite-normalized HREE patterns ([Lu/Gd] N =4.0–6.6). Na-poor clinopyroxene has a uniform LREE-enriched pattern and radiogenic 87Sr/86Sr values between 0.7057 and 0.7061, higher than other eclogite groups at Jericho and the Jericho kimberlite itself. The distinct geochemical and isotopic compositions of the Jericho diamond eclogites are not directly compatible with either the subducted oceanic crust or high-pressure mantle melt models commonly invoked for the origin of mantle eclogites. We propose a multi-stage origin that involves multiple metasomatic events coupled with hybridization between basaltic eclogite and mantle peridotite. Emplacement of basaltic eclogites in the diamond stability field was followed by metasomatism by a carbon-bearing, LREE-enriched metasomatic fluid or melt as recorded by clinopyroxene and diamond inclusions in garnet. Subsequent or concurrent partial melting of eclogite produced melts that facilitated diffusional elemental exchange between residual eclogite and ambient peridotite. This process enabled the eclogites to attain their distinct high-Mg and Cr composition. Finally, carbonatite-like modal metasomatism grew phlogopite, carbonate and apatite and potentially also facilitated additional diamond growth, producing the extreme diamond enrichments found in these eclogites. [Copyright &y& Elsevier]

Abstract: The timing and origin of Paleoproterozoic mafic and ultramafic magmatism in the NW Superior craton is important for constraining the tectonic evolution of this margin during formation and closure of the 2.1–1.8Ga Manikewan Ocean. An important component of this magmatism is the 1.88Ga Molson Igneous Events, which comprise the Molson dyke swarm, various ultramafic sills and mafic volcanic rocks located in both the Fox River and Thompson Nickel belts, northern Manitoba. Many Molson dykes have relatively flat primitive mantle-normalized patterns, overlapping the compositions of coeval mafic/ultramafic volcanic rocks and sills in northern Manitoba, supporting a common origin. Six new U–Pb zircon ages were obtained for mafic and ultramafic units and one monzogranite from both within and external to the TNB. Within the TNB the 1876.7±5.1Ma Paint Lake pyroxenite sill provides further evidence that at least some ultramafic magmatism associated with nickel mineralization is linked to Molson Igneous Events. A deformed monzogranite unit encountered in a drill hole located at Mystery Lake East has a U–Pb zircon age of 1874.6±3.4Ma and provides further evidence for the generation of contemporaneous felsic and ultramafic magmas along the Superior margin. External to the TNB, magmatism linked to Molson Igneous Events has been confirmed within the TNB transition zone (1885.2±2.1Ma Bear Island dyke, Sipiwesk Lake) and up to 100km east of the TNB, for example the 1900.0±5.8Ma Wakehao Lake and 1884.5±3.8Ma Molson Lake dykes. Molson Igneous Events span a period of approximately 25m.y. but the majority of magmatism is short-lived; emplaced within the narrow time span of about 8m.y. between 1885 and 1877Ma. The tectonic setting of Molson Igneous Events is intriguing as it occurs at a convergent margin during initiation of oceanic arc formation and closure of the Manikewan Ocean. This magmatism is relatively short-lived (<20m.y.), has a dominantly MORB-like chemical and neodymium isotopic composition, and can be traced into the highly deformed Thompson Nickel Belt where it forms sediment–sill complexes associated with Ni–Cu mineralization. The majority of the northern Superior craton margin, including the Thompson Nickel Belt, Fox River Belt, Cape Smith Belt and the New Québec Orogen, share a strikingly similar Paleoproterozoic evolution with two periods of mafic/ultramafic magmatism at ∼2.2–2.1 and 1.90–1.87Ga, similarity in basalt geochemistry, and the development of ∼1.88Ga fine-grained sediment–sill complexes that are host to significant nickel mineralization. The similarity in the timing and geochemistry of the 1.88Ga mafic/ultramafic magmatism and its location along the western, northern and eastern Superior Province margin, extending for more than 3000km, is interpreted to indicate Molson Igneous Events are linked to a unique and widespread Paleoproterozoic depleted mantle melting phenomena. Possible tectonic settings include development of a series of contemporaneous continental back-arcs during convergence and subduction of Manikewan oceanic crust or more likely passive upwelling of asthenosphere focused along the thinned margin of the Superior craton. [Copyright &y& Elsevier]

Abstract: A key to proving the accuracy of U–Pb age determinations by the various techniques available is routine analysis of an internal, well-characterized matrix-matched mineral standard of known age. In general, there are very few well-studied natural mineral standards available to the geological community. In this study, the nature, geochemistry and U–Pb systematics of baddeleyite, titanite and perovskite from three potential mineral standards are evaluated. A compilation of concordant to near-concordant analyses of a single Phalaborwa baddeleyite crystal yields a precise U–Pb age of 2059.60±0.35 Ma. A large variation in uranium (51–2124 ppm), thorium (0.5–73 ppm) and lead (19–782 ppm) concentrations and the presence of some U-bearing mineral inclusions (zircon, monazite) and baddeleyite/zircon intergrowths require some caution when using aliquots of this Phalaborwa baddeleyite crystal as a mineral standard. The Khan titanite crystal investigated here overall has a relatively uniform chemical composition (e.g. U=584±95 ppm; Th=473±73 ppm; Pb=57±8 ppm; Th/U=0.81±0.05) but parts of the crystal contain abundant mineral inclusions. The model 1 upper intercept age of 522.2±2.2 Ma for nine Khan titanite analyses is considered the current best estimate for its crystallization age. Some variability in the U–Pb systematics with 207Pb/206Pb ages that vary between 508 and 560 Ma was observed and could be related to the presence of tiny U-bearing mineral inclusions, such as rutile or allanite, so careful selection of material from this titanite crystal is also required. A single perovskite crystal from the Ice River alkaline intrusion has a relatively uniform chemical composition: uranium (141±25 ppm), thorium (1954±270 ppm), lead (41±5 ppm) and Th/U (14.5±1.3). The U–Pb results for eight perovskite fractions yield a weighted average 206Pb/238U age of 356.5±1.0 Ma. Although there are always some limitations to using natural mineral crystals as U–Pb standards, such as the presence of mineral inclusions, alteration, and geochemical heterogeneity, the three crystals investigated here are considered suitable to be used as internal U–Pb standards for any of the currently used U–Pb dating techniques. [Copyright &y& Elsevier]

Glacial erratics collected on Melville Island, western Canadian Arctic Archipelago, were analyzed to determine their mainland provenance, thereby constraining their long-distance transport by the Laurentide Ice Sheet. These erratics can be broadly subdivided into three main lithologies: granite (n = 15), quartzite (n = 7), and diabase–diorite (n = 3). The granite erratics are most distinctive from a provenance perspective and can be further subdivided into three geochemical groups based on their potassium content: (1) a high-K2O group (K2O > 4.0 wt.%), (2) an intermediate-K2O group (K2O between 2.0 and 4.0 wt.%), and (3) a low-K2O group (K2O < 2.0 wt.%). In situ thin section laser ablation inductively coupled plasma mass spectrometer U–Pb zircon ages obtained for eight granite erratics yielded both Archean (2575 Ma) and a range of Paleoproterozoic (2472–1778 Ma) crystallization ages. In addition, three overprint ages were identified at 1.90, 1.84, and ~1.0 Ga. The most compelling constraint for a northward regional ice flow originating on the mainland are two high-precision conventional U–Pb zircon ages of 1969.5 ± 1.0 and 2472.3 ± 0.5 Ma, indicating that these granite erratics must have been derived from the 2.0–1.9 Ga Taltson–Thelon Orogen and the nearby 2.5–2.4 Ga Queen Maud Block, respectively. These granite-dominated terranes are located 600 km due south and southeast of the collection area on Melville Island. Although it is unknown whether the final deposition of these erratics on Melville Island involved transport by one or more glaciations, it is apparent that this ice flow cannot be accommodated by the proposed north–south axis of the M’Clintock Ice Divide, the primary topographic feature of the northwestern Laurentide Ice Sheet during the last glacial maximum. The transport of erratics reported here would have required a former ice divide oriented east–west over the mainland, close to that proposed for the Ancestral Keewatin Divide. An east–west ice divide in this region is consistent with previously reported ice-flow indicators that document northward flow from the mainland and recent thermomechanically coupled ice-sheet numerical modeling that indicates former maximum ice thickness on the mainland immediately south of Melville Island. [ABSTRACT FROM AUTHOR]

Kimberlite-sourced bentonite layers have been discovered in Late Cretaceous sedimentary drill cores located within the Buffalo Head Hills kimberlite field, north-central Alberta. Two bentonites are unambiguously differentiated from “common” intermediate to felsic volcanic-derived Alberta bentonite by having similar whole-rock geochemical composition to ultramafic rocks from the Buffalo Head Hills kimberlite field and worldwide crater-facies kimberlite. The results demonstrate that the geochemical analysis of bentonite can provide a quick, cost-effective means of testing for low-volume kimberlite volcanism. The kimberlite-sourced bentonite is associated with a cluster of Buffalo Head Hills kimberlites known as the K14 complex. The K14A kimberlite occurrence has a previously reported 206Pb/238U perovskite emplacement age of 86.8 ±2.1 Ma. This age is compatible with early or mid-Cenomanian (~99–96 Ma) and mid-Coniacian to Santonian (~88–84 Ma) palynological results for mudstone underlying and bracketing the K14A and K14B kimberlites, respectively. Conversely, biostratigraphically significant palynomorphs in host rock cores bracketing the K14C kimberlite indicate emplacement during the Cenomanian or Turonian (~96–92 Ma). U–Pb detrital zircon ages from syndepositional kimberlite-sourced bentonite directly adjacent to the K14C kimberlite contains a high frequency of young (~99–92 Ma) zircon with a best maximum 206Pb/238U depositional age of 91.7 ±2.9 Ma. Thus, this innovative approach may provide evidence of multiple episodes of kimberlite emplacement within or near the K14 complex. Des couches de bentonite provenant de kimberlites ont été découvertes dans des carottes de forage provenant de roches sédimentaires du Crétacé tardif situées dans le champ de kimberlites de Buffalo Head Hills, dans le centre nord de l’Alberta. Deux bentonites ont été différenciées de manière non ambiguë de la bentonite « commune »trouvée en Alberta et qui provient de roches volcaniques intermédiaires à felsiques; leur composition géochimique de la roche entière est en effet semblable à celle des roches ultramafiques provenant du champ de kimberlites de Buffalo Head Hills et de kimberlites de faciès de cratère trouvées à travers le monde. Les résultats démontrent que l’analyse géochimique de la bentonite peut être un moyen rapide et rentable de vérifier le volcanisme kimberlitique de faible volume. La bentonite de source kimberlitique est associée à un groupe de kimberlites de Buffalo Head Hills connu sous le nom de complexe K14. Auparavant, l’occurrence de la kimberlite K14A avait signalé un âge de mise en place de 86,8 ±2,1 Ma, tel que déterminé par 206Pb/238U sur une pérovskite. Cet âge est compatible avec un âge Cénomanien précoce ou moyen (~99–96 Ma) et Coniacien moyen à Santonien (~88–84 Ma) provenant de résultats palynologiques sur des mudstones respectivement sous jacents aux kimberlites K14A et K14B et les encadrant. Réciproquement, des palynomorphes biostratigraphiquement significatifs dans les carottes de la roche encaissante encadrant la kimberlite K14C indiquent une mise en place au cours du Cénomanien ou du Turonien (~96–92 Ma). Des âges U–Pb sur des zircons détritiques dans une bentonite provenant d’une kimberlite déposée en même temps, et directement adjacente à la kimberlite K14C, indiquent la présence de nombreux jeunes zircons (~99–92 Ma) avec un meilleur âge maximum de déposition de 91,7 ±2,9, tel que déterminé par 206Pb/238U. Ainsi, cette approche innovatrice pourrait fournir des évidences de multiples mises en place de kimberlites à l’intérieur ou à proximité du complexe K14. [ABSTRACT FROM AUTHOR]

Abstract: A newly recognized remnant of a Paleoproterozoic Large Igneous Province has been identified in the southern Bastar craton and nearby Cuddapah basin from the adjacent Dharwar craton, India. High precision U–Pb dates of 1891.1±0.9Ma (baddeleyite) and 1883.0±1.4Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southern Bastar craton, and 1885.4±3.1Ma (baddeleyite) for a mafic sill from the Cuddapah basin, indicate the existence of 1891–1883Ma mafic magmatism that spans an area of at least ∼90,000km2 in the south Indian shield. This record of ∼1.9Ga mafic/ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south-eastern margin of the south Indian shield is a widespread geologic phenomenon on Earth. Similar periods of intraplate mafic/ultramafic magmatism occur along the margin of the Superior craton in North America (1.88Ga Molson large igneous province) and in southern Africa along the northern margin of the Kaapvaal craton (1.88–1.87Ga dolerite sills intruding the Waterberg Group). Existing paleomagnetic data for the Molson and Waterberg 1.88Ga large igneous provinces indicate that the Superior and Kalahari cratons were at similar paleolatitudes at 1.88Ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a Limpopo-like 2.0Ga deformation zone in the Superior Province. The widespread occurrence of 1.88Ga intraplate and plate margin mafic magmatism and basin development in numerous Archean cratons worldwide likely reflects a period of global-scale mantle upwelling or enhanced mantle plume activity at this time. [Copyright &y& Elsevier]

Abstract: Precambrian kimberlites are relatively rare in the geological record and yet those that do exist tend to form in relatively narrow windows of geologic time. We report new age information for three Proterozoic kimberlites from two distinct clusters in the Eastern Dharwar craton, India: a U–Pb perovskite age of 1124+5/−3Ma for Wajrakarur Pipe-2, a Rb–Sr phlogopite age of 1102±23Ma for Wajrakarur Pipe-6 and a Rb–Sr phlogopite–whole rock composite isochron age of 1093±4Ma based on two samples from the recently discovered SK-1 kimberlite pipe at Siddanpalli. These new age results are identical to a number of previously reported kimberlite emplacement ages (1085–1099Ma) from several clusters in the Eastern Dharwar craton, India and contribute to a growing database that documents extensive kimberlite magmatism encompassing an area of more than 30,000km2 within this province alone. Coeval ∼1.1Ga kimberlite/lampröite/carbonatite magmatism in central India (Majhgawan) about 1000km north of the Eastern Dharwar craton kimberlite clusters, and in North America, Greenland, Scandinavia, Southern Africa, Liberia, and Australia confirms a significant period of global ultrapotassic and alkaline magmatism at this time. The timing of this Mesoproterozoic kimberlite magmatism coincides with the development of several Large Igneous Provinces and attendant intracontinental rifting; such as those formed in the intervals 1109–1086Ma in Laurentia (Midcontinent Rift), 1112–1102Ma in the Kalahari craton (Umkondo), and 1079–1070Ma in west central Australia (Warakurna). The association of wide spread contemporaneous ultrapotassic, alkaline and mafic magmatism at ∼1100Ma may be linked to a global period of enhanced short-lived mantle plume activity and/or a major change and re-organization of the mantle convection regime at this time. [Copyright &y& Elsevier]

Abstract: The latest evolution of the Neoproterozoic Agudos Grandes Batholith (Apiaí domain, SE Brazil) is marked by an important change in the type of granitic magmatism. The “late-orogenic” Piedade, Roseira, Serra dos Lopes, and Pilar do Sul granites are elliptical plutons with roughly concentric zoning and a spatial arrangement suggesting a continuous southwestward migration of the magmatic focus. The main rock types are “contaminated” calc-alkaline granites that range from mafic-rich (color index>10), porphyritic biotite (±muscovite) granite-granodiorite in Piedade to pink, equigranular, muscovite–biotite leucogranite (CI<5) strongly affected by hydrothermal effects in Pilar do Sul. U–Pb monazite dating indicates that these plutons were emplaced during 600–605Ma, slightly after the main “synorogenic” magmatic stage (615–610Ma), which was dominated by high-K, calc-alkaline, metaluminous, porphyritic hornblende-biotite granites with minor peraluminous leucogranite bodies. The “postorogenic” granites are divided into two groups on the basis of pluton shapes and U–Pb dating, both with “A-type” affinities. The approximately 585Ma group (São Miguel Arcanjo and Capão Bonito granites) relates to the Itu granitic province, which developed around 10m.y. after the cessation of the main regional compressional events, and cross-cuts the reworked border of the Paranapanema plate; the younger, approximately 565Ma group is represented by two elongated plutons (Serra da Batéia and Serra da Queimada) that seem to reflect coeval orogenic events farther east in the Ribeira belt. The modal composition, magnetic susceptibility, and mafic mineral chemistry of the late-orogenic granites are consistent with an origin by contamination of metaluminous, oxidized, calc-alkaline magmas with crustal melts. [Copyright &y& Elsevier]

Abstract: We report U–Pb zircon and baddeleyite age results for two mafic units in the Kangerlussuaq area which are part of the Paleogene East Greenland igneous province; the Kangerlussuaq macrodyke, which represents a feeder to some flood basalts, and the nearby Kærven Gabbro. Baddeleyite from the macrodyke yielded a concordant age of 54.7±0.4 (2σ)Ma and this is among the most precise age determinations from the province. This age is interpreted to reflect the time of emplacement. The macrodyke belongs to a swarm of dykes emplaced perhaps in a failed arm during continental rifting, and the age predates the completion of continental breakup at the end of emplacement of the Plateau Basalts of the East Greenland eruption sequence. Geochemically the macrodyke is correlated with the Milne Land Formation lavas. The magma experienced slight crustal contamination. Based on the age and geochemistry of the macrodyke, the age of the Skaergaard intrusion and Skrænterne Formation lavas, an age span of 1–2Ma is confirmed for the extrusion of the entire Plateau Basalts. U–Pb analysis of zircon from the Kærven Gabbro yields an age of 53.0±0.3 (2σ)Ma. The Kærven Gabbro was intruded after extension had ceased in the area and we suggest that the post plateau basalt magmatism did not take place at a distinct later time, but instead that igneous activity continued for some time at a lower rate. Slightly fractionated HREEs indicate that the Kærven Gabbro magma was extracted at relatively shallow levels in the mantle. [Copyright &y& Elsevier]

Abstract: A laser ablation–multiple collector–inductively coupled plasma mass spectrometry (LA–MC–ICP-MS) analytical protocol is used to date accessory minerals (zircon, monazite, and titanite) at high spatial resolution (5–40μm) using standard petrographic thin sections. The MC–ICP-MS instrument is equipped with a modified collector array containing a combination of Faraday buckets and multiple ion counters, which produces accurate and precise geochronological data using small sample volumes (pit depth ≤2μm at 5μm and ≤15μm at 40μm spot sizes). Standardization and normalization factors for the 206Pb/238U and 207Pb/235U values are calculated based on well-characterized external mineral standards previously dated by high precision ID-TIMS analysis. During an analytical session, the 2σ relative standard deviation (i.e., external reproducibility) for the 206Pb/238U and 207Pb/235U values is ≤3%. The measured 207Pb/206Pb value is simultaneously corrected for instrumental mass bias by the aspiration of a Tl solution resulting in a 2σ relative standard deviation of between ∼0.3 and 1%. The accuracy of the analytical protocol was verified on petrographic thin sections of several samples previously dated by ID-TIMS. The capacity of this new, ‘small volume’ in situ dating technique to provide contextual, relatively rapid and accurate age information is a substantial improvement in reconnaissance-style studies of geological areas with scarce geochronological age information. [Copyright &y& Elsevier]

The Jericho kimberlites are part of a small Jurassic kimberlite cluster in the northern Slave craton, Canada. A variety of dating techniques were applied to constrain the nature and age of two Jericho kimberlites, JD-1 (170·2 ± 4·3 Ma Rb–Sr phlogopite megacrysts, 172·8 ± 0·7 Ma U–Pb eclogite rutile, 178 ± 5 Ma U–Pb eclogite zircon lower intercept) and JD-3 (173 ± 2 Ma Rb–Sr phlogopite megacryst; 176·6 ± 3·2 Ma U–Pb perovskite), and all yielded identical results within analytical uncertainty. As there is no discernible difference in the radiometric ages obtained for these two pipes, the composite Rb–Sr phlogopite megacryst date of 173·1 ± 1·3 Ma is interpreted as the best estimate for the emplacement age of both Jericho pipes. The initial Sr isotope composition of 0·7053 ± 0·0003 derived from phlogopite megacrysts overlaps the range (0·7043–0·7084) previously reported for Jericho whole-rocks. These strontium isotope data, combined with the radiogenic initial 206Pb/204Pb ratio of 18·99 ± 0·33 obtained in this study, indicate that the Jericho kimberlites are isotopically similar to Group 1 kimberlites as defined in southern Africa. The Jericho kimberlites are an important new source of mantle xenoliths that hold clues to the nature of the Slave craton subcontinental mantle. A high proportion (30%) of the Jericho mantle xenolith population consists of various eclogite types including a small number (2–3%) of apatite-, diamond-, kyanite- and zircon-bearing eclogites. The most striking aspect of the Jericho zircon-bearing eclogite xenoliths is their peculiar geochemistry. Reconstructed whole-rock compositions indicate that they were derived from protoliths with high FeO, Al2O3 and Na2O contents, reflected in the high-FeO (22·6–27·5 wt %) nature of garnet and the high-Na2O (8·47–9·44 wt %) and high-Al2O3 (13·12–14·33 wt %) character of the clinopyroxene. These eclogite whole-rock compositions are highly enriched in high field strength elements (HFSE) such as Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934 ppm) and Hf (23–64 ppm). This HFSE enrichment is linked to growth of large (up to 2 mm) zircon and niobian rutile crystals (up to 3 modal %) near the time of eclogite metamorphism. The diamond-bearing eclogites on the other hand are characterized by high-MgO (19·6–21·3 wt %) garnet and ultralow-Na2O (0·44–1·50 wt %) clinopyroxene. Paleotemperature estimates indicate that both the zircon- and diamond-bearing eclogites have similar equilibration temperatures of 950–1020°C and 990–1030°C, respectively, corresponding to mantle depths of 150–180 km. Integration of petrographic, whole-rock and mineral geochemistry, geochronology and isotope tracer techniques indicates that the Jericho zircon-bearing eclogite xenoliths have had a complex history involving Paleoproterozoic metamorphism, thermal perturbations, and two or more episodes of Precambrian mantle metasomatism. The oldest metasomatic event (Type 1) occurred near the time of Paleoproterozoic metamorphism (∼1·8 Ga) and is responsible for the extreme HFSE enrichment and growth of zircon and high-niobian rutile. A second thermal perturbation and concomitant carbonatite metasomatism (Type 2) is responsible for significant apatite growth in some xenoliths and profound light rare earth element enrichment. Type 2 metasomatism occurred in the period 1·0–1·3 Ga and is recorded by relatively consistent whole-rock eclogite model Nd ages and secondary U–Pb zircon upper intercept dates. These eclogite xenoliths were derived from a variety of protoliths, some of which could represent metasomatized pieces of oceanic crust, possibly linked to east-dipping subduction beneath the Slave craton during construction of the 1·88–1·84 Ga Great Bear continental arc. Others, including the diamond-bearing eclogites, could be cumulates from mafic or ultramafic sill complexes that intruded the Slave lithospheric mantle at depths of about 150–180 km. [ABSTRACT FROM AUTHOR]

Abstract: In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (∼2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989±67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1±0.1 and 2.3±0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date. The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (∼2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the ∼2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages. [Copyright &y& Elsevier]

Abstract: The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite ɛNd (+0.7 to +1.8) and ɛSr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (∼0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite. [Copyright &y& Elsevier]

The La Ronge Domain is a granite–greenstone belt in the Saskatchewan segment of the ca. 1.9–1.8 Ga Trans-Hudson Orogen. The La Ronge volcanic arc was accreted to the Archean Hearne craton from ca. 1.87 to 1.86 Ga. Subduction of oceanic lithosphere beneath the accreted La Ronge – Hearne margin produced a voluminous suite of continental-arc intrusions. In the Waddy Lake area, the 1852.6 ± 1.5 Ma Corner Lake stock and 1859 ± 4 Ma and 1861 ± 2 Ma feldspar porphyry dykes crystallized from magmas generated from melting of the subducted oceanic slab. During the ca. 1.83–1.80 Trans-Hudson collision of the Hearne craton with the Archean Sask and Superior cratons, a penetrative regional foliation and a steeply plunging lineation formed within the La Ronge Domain. During further contraction across the domain, the deformation became localized in dextral and oblique-slip shear zones that generally follow contacts between more competent and less competent rock units. Orogenic gold mineralization is associated with quartz veins that are surrounded by hypozonal potassic and sulfidic alteration zones. The Komis gold deposit, the only past-producing gold mine in the Waddy Lake area, formed in the strain shadow of the Round Lake stock during the development of the regional foliation and lineation. Mineralization is associated with quartz veins that cut through tonalite dykes that behaved more brittlely than the surrounding metavolcanic rocks. The Golden Heart and Corner Lake gold deposits are hosted by south-side-up oblique-slip shear zones, which belong to a regional system of structures that extend from Saskatchewan to Manitoba. [ABSTRACT FROM AUTHOR]

At present, 48 Late Cretaceous (ca. 70–88 Ma) kimberlitic pipes have been discovered in three separate areas of the northern Alberta: the Mountain Lake cluster, the Buffalo Head Hills field and the Birch Mountains field. The regions can be distinguished from one another by their non-archetypal kimberlite signature (Mountain Lake) or, in the case of kimberlite fields, primitive (Buffalo Head Hills) to evolved (Birch Mountains) magmatic signatures.The dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine, which acts as the main criteria to distinguish between primitive and evolved Group I-type kimberlite fields in the northern Alberta. This is important from the viewpoint of diamond exploration because the majority (about 80%) of the more primitive Buffalo Head Hills kimberlites are diamondiferous, whereas the more evolved Birch Mountains pipes are barren of diamonds for the most part. Petrographically, the Buffalo Head Hills samples are distinct from the Birch Mountains samples in that they contain less carbonate, have a smaller modal abundance of late-stage minerals such as phlogopite and ilmenite, and have a higher amount of fresh, coarse macrocrystal (>0.5 mm) olivine. Consequently, samples from the Buffalo Head Hills have the highest values of MgO, Cr and Ni, and have chemistries similar to those of primitive hypabyssal kimberlite in the Northwest Territories. Based on whole-rock isotopic data, the Buffalo Head Hills K6 kimberlite has 87Sr/86Sr and #x03B5;Nd values similar to those of South African Group I kimberlites, whereas the Birch Mountains Legend and Phoenix kimberlites have similar #x03B5;Nd values (between 0 and +1.9), but distinctly higher 87Sr/86Sr values (0.7051–0.7063).The lack of whole-rock geochemical overlap between kimberlite and the freshest, least contaminated Mountain Lake South pipe rocks reflects significant mineralogical differences and Mountain Lake is similar geochemically to olivine alkali basalt and/or basanite. Intra-field geochemical variations are also evident. The K4 pipe (Buffalo Head Hills), and Xena and Kendu pipes (Birch Mountains) are characterized by anomalous concentrations of incompatible elements relative to other northern Alberta kimberlite pipes, including chondrite-normalized rare-earth element distribution patterns that are less fractionated than the other kimberlite samples from the Buffalo Head Hills and Birch Mountains. The Xena pipe has similar major element chemical signatures and high-Al clinopyroxene similar to, or trending towards, the Mountain Lake pipes. In addition, K4 and Kendu have higher 87Sr/86Sr and lower #x03B5;Nd than Bulk Earth and plot in the bottom right quadrant of the Nd–Sr diagram. We suggest, therefore, that the K4 and Kendu pipes contain a contribution from old, LREE-enriched (low Sm/Nd) lithosphere that is absent from the other kimberlites, are affected by crustal contamination, or both.Based on xenocryst populations, the northern Alberta kimberlite province mantle is dominated by carbonate-saturated lherzolitic mantle. Higher levels of melt depletion characterize the Buffalo Head Hills mantle sample. Despite high diamondiferous to barren pipe ratios in the Buffalo Head Hills pipes, mineral indicators of high diamond potential, such as G10 garnet, diamond inclusion composition chrome spinels and high-sodium eclogitic garnet, are rare. [Copyright &y& Elsevier]

North American kimberlite magmatism spans a period of time in excess of 1 billion years from Mesoproterozoic kimberlites in the Lake Superior and James Bay Lowlands region of Ontario to Eocene kimberlites in the Lac de Gras field, N.W.T. Based on a compilation of more than 150 robust radiometric age determinations, several distinct kimberlite emplacement patterns are recognized. In general, the temporal pattern of kimberlite emplacement in North America can be broadly subdivided into five domains: (1) a Mesoproterozoic kimberlite province in central Ontario, (2) an Eocambrian/Cambrian Labrador Sea Province in northern Québec and Labrador, (3) an eastern Jurassic Province, (4) a central Cretaceous corridor and (5) a western mixed domain that includes two Type-3 kimberlite provinces (i.e. multiple periods of kimberlite emplacement preserved in the Slave and Wyoming cratons). For some provinces the origin of kimberlite magmatism can be linked to known mantle heat sources such as mantle plume hotspots and upwelling asthenosphere attendant with continental rifting. For example, the timing and location of Mesoproterozoic kimberlites in North America coincides with and slightly precedes the timing of 1.1 Ga intracontinental rifting that culminated in the Midcontinent Rift centered in the Lake Superior region. Many of the kimberlites in the Eocambrian/Cambrian Labrador Sea province were emplaced soon after the opening of the Iapetus Ocean at about 615 Ma and may also be linked to mantle upwelling associated with continental rifting. The eastern Jurassic kimberlites record an age progression where magmatism youngs in a southeast direction from the ∼200 Ma Rankin Inlet kimberlites to the 155–126 Ma Timiskaming kimberlites. The location of several kimberlite fields and clusters in Ontario and Québec lie along a continental extension of the Great Meteor hotspot track and represents one of the best examples in the world of kimberlite magmatism triggered by mantle plumes. The central Cretaceous (103–94 Ma) corridor extends for more than 4000 km from Somerset Island in northern Canada through the Fort à la Corne field in Saskatchewan to the kimberlites in central USA. This is the first recognized corridor of kimberlite magmatism of this magnitude. The possible westward younging of Cretaceous to Eocene corridors of kimberlite magmatism could reflect major changes in plate geometry during subduction of the Kula–Farallon plate. [Copyright &y& Elsevier]

We present the results of a U–Pb perovskite age study of kamafugites from Mata da Corda (MC) and Santo Antônio da Barra (SAB), Minas-Goiás alkaline province, Brazil. Perovskite crystals were separated from MC mafurites, ugandites, and cognate pyroxenites, as well as from SAB melilite mafurite. The range of ages of Brazilian kamafugitic samples is 15 Ma. The 206Pb/238U perovskite ages generally cluster into three age groupings: 88–90, 80–81, and 75–76 Ma. The two younger periods of kamafugitic magmatism occur in the MC area, whereas the older samples are from the SAB area. These new age results provide the first robust evidence of a progressive eastward younging of mafic alkaline magmatism, most likely related to a mantle plume hotspot track. [Copyright &y& Elsevier]

The Mallery Lake area contains precious metal-bearing quartz–chalcedony stockworks that are pristine examples of ancient low-sulfidation epithermal deposits. Fluorite extracted from these epithermal deposits define a Sm–Nd errorchron age of 1434 ± 23 Ma mean square of weighted deviates (MSWD) = 4.8. This date is interpreted to have age significance because (1) a simple linear trend does not exist between the 143Nd/144Nd ratios of the fluorite with respect to their 1/Nd concentrations as would be expected for mixing of two geochemical end members; (2) microthermometric studies indicate that the fluorite analysed in this study has an intimate association with a single high-salinity, calcic brinal fluid; and (3) the age determined from seven fluorite samples extracted from a single outcrop location yielded an identical age result (1434 ± 60 Ma; MSWD = 5.5) compared to the fluorite composite. Rhyodacites of the Pitz Formation and syenites from the Nueltin suite (intrusive equivalent to the rhyodacites) are the youngest volcanic–plutonic rocks that are observed in outcrop in the Mallery Lake area, and they were dated by U–Pb zircon analysis at 1760 ± 43 Ma and 1755.4 ± 1.8 Ma, respectively. The ~320 million year age difference between the epithermal deposits and the hosting rhyodacitic flows suggests that the epithermal stockworks may have formed by a regional hydrothermal event unrelated to this earlier Paleoproterozoic volcanic activity. Uranium deposits in the Thelon and Athabasca basins, to the northwest and south of the Baker Lake Basin, were determined to have similar ore emplacement ages with no evident heat source. [ABSTRACT FROM AUTHOR]

The Assean Lake Crustal Complex (ALCC) located at the northwest periphery of the Superior Province in northern Manitoba, Canada, hosts an ancient assembly of rocks that record a complex and prolonged crustal history spanning more than two billion years from ∼3.9 to 1.8 Ga. A supracrustal assemblage consisting of amphibolite-grade greywacke, quartz arenite, arkose, mafic to intermediate volcanics, and to a lesser extent iron formation dominates the ALCC and likely formed in a shallow marine arc/back-arc setting. The supracrustal rocks are intruded by amphibolite-grade, calc-alkaline tonalite to granite orthogneiss. Nd model ages for felsic para- and orthogneisses range from ∼3.5 to 4.2 Ga and suggest that Paleoarchean basement may be preserved at Assean Lake. SHRIMP U–Pb zircon ages and ID-TIMS conventional U–Pb zircon and monazite ages show that felsic magmatism occurred at ∼3.1–3.2 Ga whereas a record extending back to at least 3.9 Ga is preserved in detritus in paragneiss that may have a depositional age of ∼3.2 Ga. Orthogneiss was metamorphosed at ∼2.68 Ga whereas local migmatisation of paragneiss occurred at ∼2.61 Ga. Monazite growth events at ∼2.6, 2.45, and 1.8 Ga might represent episodes of accretion of Archean crustal segments including the ALCC and possibly the Sask craton (Trans-Hudson basement) along the northwest Superior Province margin. At present, matching segments of the exotic ALCC are unknown but remnants may exist in parts of the Sask craton. [Copyright &y& Elsevier]

We evaluate the potential for applying the electron microprobe (EM) chemical dating technique to the mineral baddeleyite (ZrO2), an accessory phase commonly used to determine crystallization ages of mafic rocks in conventional isotopic U–Pb studies. Baddeleyite is well suited for EM chemical dating because it has relatively high concentrations of U, negligible common Pb, and is highly resistant to Pb loss. EM chemical and isotope dilution U–Pb ages are presented for five baddeleyite reference samples, including the 780 Ma Faber Lake gabbro sheet (Great Bear Magmatic Zone, Canada), the 1109 Ma Moore Lakes gabbro (Churchill Province, Canada), the 1269 Ma Muskox intrusion (Slave Craton, Canada), the 2060 Ma Phalaborwa carbonatite (S. Africa), and the 2410 Ma Binneringie gabbro dyke (Yilgarn Craton, Australia). Overall, the EM chemical ages show good correlation with the conventional U–Pb ages. Three of the reference samples yielded EM ages within ∼30–60 Ma of the conventional ages, well within analytical uncertainty. For reasons that are not known, two of the samples yielded EM chemical ages that are systematically old by ∼140 Ma. We conclude from these data that the EM chemical dating technique provides reasonable first-order age information for typical Precambrian baddeleyite. The systematic variation of U and Pb at the micron scale in baddeleyite, coupled with the excellent spatial resolution of the EM (∼1 μm), allows for determination of chemical U–Th–total Pb isochrons from individual baddeleyite crystals in polished thin section. EM chemical dating offers a non-destructive, time efficient (a matter of hours for each sample), and cost effective way to obtain contextural age information from baddeleyite in mafic rocks with a grain size too small for isolation and conventional U–Pb dating (<20 μm). In reconnaissance geochronological investigations of Precambrian mafic dyke swarms, a limited number of high-precision U–Pb ages could be used in combination with a larger number of EM ages to obtain robust age constraints on a vast number of dykes. [Copyright &y& Elsevier]

Investigates the geochemical features of the Jericho eclogite xenoliths in the Northwest Territories, Canada. Dominance of Nb and Zr anomalies; Abundance of mantle xenoliths in the jericho kimberlite; Effect of mantle xenolith metasomatism on Na[sub 2]O.

Presents isotopic results on the nature and structure of the Archean-Proterozoic boundary in Manitoba. Geologic setting of the Northwestern Superior Province margin; Isotopic results and implications for the structure and age of superior boundary zone; Potential for nickel mineralization in the northeast extension of the Thompson Nickel Belt.

U-Pb geochronology by isotope dilution– thermal ionization mass spectrometry (ID-TIMS) has the potential to be the most precise and accurate of the deep time chronometers, especially when applied to high-U minerals such as zircon. Continued analytical improvements have made this technique capable of regularly achieving better than 0.1% precision and accuracy of dates from commonly occurring high-U minerals across a wide range of geological ages and settings. To help maximize the long-term utility of published results, we present and discuss some recommendations for reporting ID-TIMS U-Pb geochronological data and associated metadata in accordance with accepted principles of data management. Further, given that the accuracy of reported ages typically depends on the interpretation applied to a set of individual dates, we discuss strategies for data interpretation. We anticipate that this paper will serve as an instructive guide for geologists who are publishing ID-TIMS U-Pb data, for laboratories generating the data, the wider geoscience community who use such data, and also editors of journals who wish to be informed about community standards. Combined, our recommendations should increase the utility, veracity, versatility, and “half-life” of ID-TIMS U-Pb geochronological data. [ABSTRACT FROM AUTHOR]

Analyzes the ages of Paleoproterozoic flood basalts, dike swarms and layered mafic intrusions worldwide using radiometric dating. Determination of global mafic magmatism at 2.45 Ga; Elusiveness of the exact timing of the magmatism.

We present new geochemical, U Pb age, Hf O isotope compositions for more than 400 detrital zircons collected from modern sands located throughout western Fuerteventura, Canary Islands as a proxy for the timing and origin of its plutonic history. This approach provides a new perspective and more complete time-integrated record of the islands geologic evolution. Fuerteventura U Pb detrital zircon age spectra document that magmatism was initiated at ~35 Ma, was most prolific during the main shield-building stage (30–15 Ma), youngs southward, corroborates the existence of a low magma fluxing period (15–6 Ma), and provides new evidence for the existence that Pliocene and younger (at least as young as 0.24 ± 0.02 Ma) plutons with a variety of magma compositions accompanied post-shield volcanism until recent times. Most Fuerteventura detrital zircons have sub-mantle SIMS oxygen isotope compositions (δ18O < 5.0‰), and overlap compositions reported previously for gabbro/basalt xenoliths entrained in nearby Gran Canaria lavas, interpreted to be fragments of high-temperature seawater-altered Layer 3 oceanic crust. The detrital zircon Hf isotope compositions (ε Hfi : +15.0 to −0.2) display a much larger variation than recorded in previous studies of volcanic rocks from the Canary or Cape Verde islands. This extreme isotopic variation demonstrates that Fuerteventura detrital zircons provide a more sensitive record of the involvement of multiple sources in magma genesis. The high-ε Hf (>+12) zircons document the involvement of a depleted reservoir that is isotopically most similar to Middle Atlas SCLM. The low-ε Hf zircons, specifically those <+1, document the involvement of a reservoir that is more enriched than HIMU and overlaps the composition of NW African lower crust. These extreme ε Hf compositions indicate the presence of NW African lithosphere beneath Fuerteventura during the shield-building stage of island growth. The absence of a highly depleted Hf isotopic signature in young detrital zircons is interpreted to document the removal of SCLM during a magmatic shutdown initiated at ~15 Ma. Lithosphere erosion could be linked to a pulsed edge-driven convection cell responsible for SCLM delamination along a NE-trending corridor in NW Africa. The detrital zircon Hf and O isotope compositions indicate that both oceanic and continental lithosphere have existed beneath the island. This lithosphere is heterogeneous, consisting of high-T seawater altered oceanic crust, continental lower crust, and subcontinental mantle lithosphere; consistent with formation above a complex lithospheric transition zone created during Triassic-Jurassic rifting and opening of the Central Atlantic Ocean. Importantly, this detrital zircon study demonstrates that a more complete time-integrated record of a long-lived ocean island's geological history is preserved in modern sand samples, in part because a significant portion of Fuerteventura has been removed by erosion. • Fuerteventura magmatism derives from oceanic and continental lithosphere sources (81). • Fuerteventura alkaline plutonism is unusually long-lived (~35 m.y.) (67). • A low magma fluxing period on Fuerteventura is confirmed between 15 and 6 Ma (73). • A period of continental lithosphere loss occurred at about 15 Ma (65). [ABSTRACT FROM AUTHOR]

The Late Cretaceous kimberlites in northern Alberta, Canada, intruded into the Paleo-proterozoic crust and represent a nonconventional setting for the discovery of diamonds. Here, we examined the origin of kimberlite magmatism using a multidisciplinary approach. A new teleseismic survey reveals a low-velocity (−1%) corridor that connects two deep-rooted (>200 km) quasi-cylindrical anomalies underneath the Birch Mountains and Mountain Lake kimberlite fields. The radiometric data, including a new U-Pb perovskite age of 90.3 ± 2.6 Ma for the Mountain Lake intrusion, indicate a northeast-trending age progression in kimberlite magmatism, consistent with the (local) plate motion rate of North America constrained by global plate reconstructions. Taken together, these observations favor a deep stationary (relative to the lower mantle) source region for kimberlitic melt generation. Two competing models, mantle plume and slab subduction, can satisfy kinematic constraints and explain the exhumation of ultradeep diamonds. The plume hypothesis is less favorable due to the apparent age discrepancy between the oldest kimberlites (ca. 90 Ma) and the plume event (ca. 110 Ma). Alternatively, magma generation may have been facilitated by decompression of hydrous phases (e.g., wadsleyite and ringwoodite) within the mantle transition zone in response to thermal perturbations by a cold slab. The three-dimensional lithospheric structures largely controlled melt migration and intrusion processes during the Late Cretaceous kimberlite magmatism in northern Alberta. [ABSTRACT FROM AUTHOR]

The authors argue against the comments of Koenig and colleagues regarding their study on direct uranium-lead (U-Pb) dating of Paleocene and Cretaceous dinosaur fossi bones in San Juan Basin, New Mexico. The authors state that their study is independent of past published research work. The authors also mention that their failure to provide detailed dating methodology was due to the limited space provide by the "Geology" journal.

The authors discuss their insights regarding the comments on their study on the direct uranium-lead (U-Pb) dating of Paleocene and Cretaceous dinosaur fossil bones in San Juan Basin, New Mexico. The authors state that the comment by Ludwig was based on the U-Pb data they used to the original fossilization dates for two dinosaur bones. The authors response to the comments by Goodwin and Renne are also cited.

Geochemical studies of komatiite- and basalt-dominated volcanic successions within late Archean supracrustal belts have the potential to yield important insights into the thermal structure and chemical composition of the early Earth's mantle, as well as the dynamics of mantle melting. The high eruption temperatures and degrees of melting that are required to explain the first-order geochemical features displayed by the vast majority of late Archean komatiite, namely their high-MgO contents and pronounced depletion in incompatible elements, have been used to argue that these magmas must have an origin related to mantle plumes. Unfortunately, the near universal acceptance of a mantle plume origin for komatiite is used as an argument (circular) for the existence of plumes, and geochemical observations are routinely interpreted in terms of the modern plume paradigm. The underlying assumptions of the paradigm are seldom questioned or why, if they exist, potential plumes in the modern mantle might be similar to those in the Archean. Currently, no consensus exists on the nature or very existence of modern or ancient mantle plumes and their relationship with respect to the ambient mantle, a situation that requires remediation. This study presents geochemical data from a unique and exceptionally well-preserved ca. 2.73 Ga volcanic succession, dominated by large volumes of chemically diverse komatiite and basalt recently discovered in the Canadian Arctic. This succession constitutes the basal sequence to a vast network of co-genetic clastic-dominated supracrustal belts (~1400 km long and ~400 km wide) formed between ca. 2.73-2.69 Ga. Modeling the geochemical data with constraints from high- and low-pressure peridotite melting experiments has yielded the following conclusions: (1) high-MgO komatiite magmas were derived from thermally anomalous mantle, which was ~150°C hotter than the ambient mantle; (2) the ambient ca. 2.7 Ga mantle was ~200°C hotter than the modern mantle; (3) ambient and thermally anomalous mantle are the same composition and both are similar to the modern depleted upper mantle; (4) Archean mantle plumes are discrete "parcels" of thermally anomalous mantle; (5) primary magmas within the late Archean were komatiitic not basaltic; (6) plumes have an origin within the upper mantle; and (7) the source of thermal energy required to heat the upper mantle may ultimately have originated from the core. [ABSTRACT FROM AUTHOR]

A correlation chart showing the evolution of the Manitoba–Saskatchewan segment of the Paleoproterozoic Trans-Hudson Orogen (THO) has been constructed based on available geochronological and isotopic data. Geological, petrological, geochemical, and structural data have also been incorporated into the correlation chart to provide finer detail and to allow comparison of specific tectonic events. The chart also emphasizes that the bounding Archean cratons (Superior, Hearne, Sask) have been significantly affected by Paleoproterozoic events, and it is hoped that this chart will also facilitate comparison with other Paleoproterozoic orogens in North America and beyond. [ABSTRACT FROM AUTHOR]

We report the first high-precision ID-TIMS U–Pb baddeleyite/zircon and 40 Ar/ 39 Ar step-heating phlogopite age data for diabase and lamprophyre dykes and a mafic intrusion (José Fernandes Gabbro) located within the Ponta Grossa Arch, Brazil, in order to constrain the temporal evolution between Early Cretaceous tholeiitic and alkaline magmatism of the Paraná-Etendeka Magmatic Province. U–Pb dates from chemically abraded zircon data yielded the best estimate for the emplacement ages of a high Ti–P–Sr basaltic dyke (133.9 ± 0.2 Ma), a dyke with basaltic andesite composition (133.4 ± 0.2 Ma) and the José Fernandes Gabbro (134.5 ± 0.1 Ma). A 40 Ar/ 39 Ar phlogopite step-heating age of 133.7 ± 0.1 Ma from a lamprophyre dyke is identical within error to the U–Pb age of the diabase dykes, indicating that tholeiitic and alkaline magmatism were coeval in the Ponta Grossa Arch. Although nearly all analysed fractions are concordant and show low analytical uncertainties (± 0.3–0.9 Ma for baddeleyite; 0.1–0.4 Ma for zircon; 2σ), Pb loss is observed in all baddeleyite fractions and in some initial zircon fractions not submitted to the most extreme chemical abrasion treatment. The resulting age spread may reflect intense and continued magmatic activity in the Ponta Grossa Arch. [ABSTRACT FROM AUTHOR]

We report the discovery of one of the largest ancient (>3.0 Ga) crustal terranes on Earth. Granitoids with crystallization ages >3.0 Ga and/or Sm-Nd depleted mantle model ages ≥3.2 Ga define a -1000 x 100 km belt on the western margin of the Rae craton, Canada, referred to herein as the Perry River terrane (PRT). Zircon U-Pb-Hf-O isotope and whole-rock geochemical data from granitoids show that the PRT is a predominantly juvenile 3.3--3.2 Ga terrane that was partially reworked by more evolved ca. 3.1 Ga magmatism. These findings call for a reassessment of the timing and extent of ancient continental growth on Earth. A global compilation of zircon Hf isotope data from 3.6 to 3.0 Ga igneous rocks reveals clusters of relatively juvenile (initial εHf -2 to +3) rocks at ca. 3.31 and ca. 3.23 Ga, which include samples from the PRT and 13 other terranes worldwide. Other global zircon data sets also document age peaks between 3.3 and 3.2 Ga, and a cluster of broadly chondritic initial εHf values around 3.2 Ga. The 3.3--3.2 Ga period may therefore have been a time of enhanced net continental growth on Earth, and the PRT is one of the largest terranes preserved from that time. Furthermore, zircon Hf isotope data from 3.3--3.1 Ga PRT granitoids and 3.5--3.0 Ga igneous rocks worldwide yield little evidence for parent magmas that interacted with or derived from pre--3.6 Ga continental material. Contrary to some continental-growth models, this latter observation suggests that the volume of continental crust established by 3.6 Ga was relatively small. [ABSTRACT FROM AUTHOR]

Two temporally distinct Neoarchean turbidite packages are known to occur in the Slave craton. The older is a greywacke-mudstone succession that includes the renowned Burwash Formation (ca. 2661 Ma). In this study, a previously undated tuff bed is demonstrated to have crystallized at ca. 2650.5 ± 1.0 Ma refining the deposition age of these turbidites between ca. 2661 and 2650 Ma. The younger turbidites are locally distinctive as they contain interstratified banded iron formation (BIF). Previous work demonstrated that the younger turbidites were deposited between ca. 2640 and 2615 Ma, based entirely on maximum depositional ages from detrital zircons. A ∼3 cm thick felsic to intermediate tuff bed was discovered interbedded with these BIF-bearing turbidites. The tuff bed contains a single age population of zircon with a crystallization age of 2620 ± 6 Ma defining the depositional timing of these BIF-bearing turbidites. New U-Pb detrital zircon dates from extensive turbidite sequences in the eastern and central part of the Slave craton are also presented. We use the new and previously published results to recommend nomenclature for these extensive sedimentary rocks in the Slave craton. The ca. 2661-2650 Ma turbidites remain part of the previously ascribed Duncan Lake Group. The younger ca. 2620 Ma turbidites are assigned to the new Slemon Group. Where robust age-data exist, we recommend formation names and include type localities for each. [ABSTRACT FROM AUTHOR]

A correction to the article "The First Integrated Use of in Situ UPb Geochronology and Geochemical Analyses to Determine Long-Distance Transport of Glacial Erratics From Mainland Canada Into the Western Arctic Archipelago" that was published in the 2009 issue is presented.

The Neoproterozoic volcano-sedimentary successions of Uruguay have been the subject of several sedimentologic, chrono-stratigraphic and tectonic interpretation studies. Recent studies have shown, however, that the stratigraphy, age and tectonic evolution of these units remain uncertain. Here we use new U Pb detrital zircon ages, combined with previously published geochronologic and stratigraphic data in order to provide more precise temporal constraints on their depositional age and to establish a more solid framework for the stratigraphic and tectonic evolution of these units. The sequence of events begins with a period of tectonic quiescence and deposition of extensive mixed siliciclastic-carbonate sedimentary successions. This is followed by the development of small fault-bounded siliciclastic and volcaniclastic basins and the emplacement of voluminous granites associated with episodic terrane accretion. According to our model, the Arroyo del Soldado Group and the Piedras de Afilar Formation were deposited sometime between ∼1000 and 650 Ma, and represent passive continental margin deposits of the Nico Pérez and Piedra Alta terranes, respectively. In contrast, the Ediacaran San Carlos (<552 ± 3 Ma) and Barriga Negra (<581 ± 6 Ma) formations, and the Maldonado Group (<580–566 Ma) were deposited in tectonically active basins developed on the Nico Pérez and Cuchilla Dionisio terranes, and the herein defined Edén Terrane. The Edén and the Nico Pérez terranes likely accreted at ∼650–620 Ma (Edén Accretionary Event), followed by their accretion to the Piedra Alta Terrane at ∼620–600 Ma (Piedra Alta Accretionary Event), and culminating with the accretion of the Cuchilla Dionisio Terrane at ∼600–560 Ma (Cuchilla Dionisio Accretionary Event). Although existing models consider all the Ediacaran granites as a result of a single orogenic event, recently published age constraints point to the existence of at least two distinct stages of granite generation, which are spatially and temporally associated with the Edén and Cuchilla Dionisio accretionary events. [ABSTRACT FROM AUTHOR]